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Overview
Important Note
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Background
C++ is the main development language
used by many of Google's open-source
projects.
As every C++ programmer knows, the language has many powerful features,
but this power brings with it complexity, which in turn can make code
more bug-prone and harder to read and maintain.
The goal of this guide is to manage this complexity by describing
in detail the dos and don'ts of writing C++
code. These rules exist to
keep
the
code base manageable while still allowing coders to use C++ language
features productively.
Style , also known as readability, is what we call the
conventions that govern our C++ code. The term Style is a bit of a
misnomer, since these conventions cover far more than just source
file formatting.
One way in which we keep the code base manageable is by enforcing
consistency .
It is very important that any
programmer
be able to look at another's code and quickly understand it.
Maintaining a uniform style and following conventions means that we can
more easily use "pattern-matching" to infer what various symbols are
and what invariants are true about them. Creating common, required
idioms and patterns makes code much easier to understand. In some
cases there might be good arguments for changing certain style
rules, but we nonetheless keep things as they are in order to
preserve consistency.
Another issue this guide addresses is that of C++ feature bloat.
C++ is a huge language with many advanced features. In some cases
we constrain, or even ban, use of certain features. We do this to
keep code simple and to avoid the various common errors and
problems that these features can cause. This guide lists these
features and explains why their use is restricted.
Open-source projects developed by Google
conform to the requirements in this guide.
Note that this guide is not a C++ tutorial: we assume that the
reader is familiar with the language.
In general, every .cc file should have an associated
.h file. There are some common exceptions, such as
unittests
and small .cc files containing just a main()
function.
Correct use of header files can make a huge difference to the
readability, size and performance of your code.
The following rules will guide you through the various pitfalls of
using header files.
▶
All header files should have
#define guards to
prevent multiple inclusion. The format of the symbol name
should be
<PROJECT>_<PATH>_<FILE>_H_.
link
To guarantee uniqueness, they should be based on the full path
in a project's source tree. For example, the file
foo/src/bar/baz.h in project foo should
have the following guard:
#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_
...
#endif // FOO_BAR_BAZ_H_
▶
Use forward declarations to minimize use of
#include in
.h files.
link
When you include a header file you introduce a dependency that
will cause your code to be recompiled whenever the header file
changes. If your header file includes other header files, any
change to those files will cause any code that includes your
header to be recompiled. Therefore, we prefer to minimize
includes, particularly includes of header files in other
header files.
You can significantly minimize the number of header files you
need to include in your own header files by using forward
declarations. For example, if your header file uses the
File class in ways that do not require access to
the declaration of the File class, your header
file can just forward declare class File; instead
of having to #include "file/base/file.h".
How can we use a class Foo in a header file
without access to its definition?
- We can declare data members of type
Foo* or
Foo&.
- We can declare (but not define) functions with arguments,
and/or return values, of type
Foo.
- We can declare static data members of type
Foo. This is because static data members
are defined outside the class definition.
On the other hand, you must include the header file for
Foo if your class subclasses Foo or
has a data member of type Foo.
Sometimes it makes sense to have pointer (or better,
scoped_ptr)
members instead of object members. However, this complicates code
readability and imposes a performance penalty, so avoid doing
this transformation if the only purpose is to minimize includes
in header files.
Of course, .cc files typically do require the
definitions of the classes they use, and usually have to
include several header files.
▶
Define functions inline only when they are small, say, 10 lines
or less.
link
Definition:
You can declare functions in a way that allows the compiler to
expand them inline rather than calling them through the usual
function call mechanism.
Pros:
Inlining a function can generate more efficient object code,
as long as the inlined function is small. Feel free to inline
accessors and mutators, and other short, performance-critical
functions.
Cons:
Overuse of inlining can actually make programs slower.
Depending on a function's size, inlining it can cause the code
size to increase or decrease. Inlining a very small accessor
function will usually decrease code size while inlining a very
large function can dramatically increase code size. On modern
processors smaller code usually runs faster due to better use
of the instruction cache.
Decision:
A decent rule of thumb is to not inline a function if it is
more than 10 lines long. Beware of destructors, which are
often longer than they appear because of implicit member-
and base-destructor calls!
Another useful rule of thumb: it's typically not cost
effective to inline functions with loops or switch
statements (unless, in the common case, the loop or switch
statement is never executed).
It is important to know that functions are not always
inlined even if they are declared as such; for example,
virtual and recursive functions are not normally inlined.
Usually recursive functions should not be inline. The main
reason for making a virtual function inline is to place its
definition in the class, either for convenience or to
document its behavior, e.g., for accessors and mutators.
▶
You may use file names with a
-inl.h suffix to define
complex inline functions when needed.
link
The definition of an inline function needs to be in a header
file, so that the compiler has the definition available for
inlining at the call sites. However, implementation code
properly belongs in .cc files, and we do not like
to have much actual code in .h files unless there
is a readability or performance advantage.
If an inline function definition is short, with very little,
if any, logic in it, you should put the code in your
.h file. For example, accessors and mutators
should certainly be inside a class definition. More complex
inline functions may also be put in a .h file for
the convenience of the implementer and callers, though if this
makes the .h file too unwieldy you can instead
put that code in a separate -inl.h file.
This separates the implementation from the class definition,
while still allowing the implementation to be included where
necessary.
Another use of -inl.h files is for definitions of
function templates. This can be used to keep your template
definitions easy to read.
Do not forget that a -inl.h file requires a
#define guard just
like any other header file.
▶
When defining a function, parameter order is: inputs,
then outputs.
link
Parameters to C/C++ functions are either input to the
function, output from the function, or both. Input parameters
are usually values or const references, while output
and input/output parameters will be non-const
pointers. When ordering function parameters, put all input-only
parameters before any output parameters. In particular, do not add
new parameters to the end of the function just because they are
new; place new input-only parameters before the output
parameters.
This is not a hard-and-fast rule. Parameters that are both
input and output (often classes/structs) muddy the waters,
and, as always, consistency with related functions may require
you to bend the rule.
▶
Use standard order for readability and to avoid hidden
dependencies: C library, C++ library,
other libraries'
.h, your
project's
.h.
link
All of a project's header files should be
listed as descentants of the project's source directory
without use of UNIX directory shortcuts . (the current
directory) or .. (the parent directory). For
example,
google-awesome-project/src/base/logging.h
should be included as
#include "base/logging.h"
In dir/foo.cc, whose main purpose is
to implement or test the stuff in
dir2/foo2.h, order your includes as
follows:
-
dir2/foo2.h (preferred location
— see details below).
- C system files.
- C++ system files.
- Other libraries'
.h files.
-
Your project's
.h files.
The preferred ordering reduces hidden dependencies. We want
every header file to be compilable on its own. The easiest
way to achieve this is to make sure that every one of them is
the first .h file #included in some
.cc.
dir/foo.cc and
dir2/foo2.h are often in the same
directory (e.g. base/basictypes_unittest.cc and
base/basictypes.h), but can be in different
directories too.
Within each section it is nice to order the includes
alphabetically.
For example, the includes in
google-awesome-project/src/foo/internal/fooserver.cc
might look like this:
#include "foo/public/fooserver.h" // Preferred location.
#include <sys/types.h>
#include <unistd.h>
#include <hash_map>
#include <vector>
#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "foo/public/bar.h"
Scoping
▶
Unnamed namespaces in
.cc files are encouraged. With
named namespaces, choose the name based on the
project, and possibly its path.
Do not use a
using-directive.
link
Definition:
Namespaces subdivide the global scope into distinct, named
scopes, and so are useful for preventing name collisions in
the global scope.
Pros:
Namespaces provide a (hierarchical) axis of naming, in
addition to the (also hierarchical) name axis provided by
classes.
For example, if two different projects have a class
Foo in the global scope, these symbols may
collide at compile time or at runtime. If each project
places their code in a namespace, project1::Foo
and project2::Foo are now distinct symbols that
do not collide.
Cons:
Namespaces can be confusing, because they provide an
additional (hierarchical) axis of naming, in addition to the
(also hierarchical) name axis provided by classes.
Use of unnamed spaces in header files can easily cause
violations of the C++ One Definition Rule (ODR).
Decision:
Use namespaces according to the policy described below.
Unnamed Namespaces
- Unnamed namespaces are allowed and even encouraged in
.cc files, to avoid runtime naming
conflicts:
namespace { // This is in a .cc file.
// The content of a namespace is not indented
enum { UNUSED, EOF, ERROR }; // Commonly used tokens.
bool AtEof() { return pos_ == EOF; } // Uses our namespace's EOF.
} // namespace
However, file-scope declarations that are
associated with a particular class may be declared
in that class as types, static data members or
static member functions rather than as members of
an unnamed namespace. Terminate the unnamed
namespace as shown, with a comment //
namespace.
- Do not use unnamed namespaces in
.h
files.
Named Namespaces
Named namespaces should be used as follows:
- Namespaces wrap the entire source file after includes,
gflags
definitions/declarations, and forward declarations of classes
from other namespaces:
// In the .h file
namespace mynamespace {
// All declarations are within the namespace scope.
// Notice the lack of indentation.
class MyClass {
public:
...
void Foo();
};
} // namespace mynamespace
// In the .cc file
namespace mynamespace {
// Definition of functions is within scope of the namespace.
void MyClass::Foo() {
...
}
} // namespace mynamespace
The typical .cc file might have more
complex detail, including the need to reference classes
in other namespaces.
#include "a.h"
DEFINE_bool(someflag, false, "dummy flag");
class C; // Forward declaration of class C in the global namespace.
namespace a { class A; } // Forward declaration of a::A.
namespace b {
...code for b... // Code goes against the left margin.
} // namespace b
- Do not declare anything in namespace
std, not even forward declarations of
standard library classes. Declaring entities in
namespace std is undefined behavior,
i.e., not portable. To declare entities from the
standard library, include the appropriate header
file.
- You may not use a using-directive to
make all names from a namespace available.
// Forbidden -- This pollutes the namespace.
using namespace foo;
- You may use a using-declaration
anywhere in a
.cc file, and in functions,
methods or classes in .h files.
// OK in .cc files.
// Must be in a function, method or class in .h files.
using ::foo::bar;
- Namespace aliases are allowed anywhere in a
.cc file, and in functions and methods in
.h files.
// OK in .cc files.
// Must be in a function or method in .h files.
namespace fbz = ::foo::bar::baz;
▶
Although you may use public nested classes when they are part of
an interface, consider a
namespace to
keep declarations out of the global scope.
link
Definition:
A class can define another class within it; this is also
called a member class.
class Foo {
private:
// Bar is a member class, nested within Foo.
class Bar {
...
};
};
Pros:
This is useful when the nested (or member) class is only used
by the enclosing class; making it a member puts it in the
enclosing class scope rather than polluting the outer scope
with the class name. Nested classes can be forward declared
within the enclosing class and then defined in the
.cc file to avoid including the nested class
definition in the enclosing class declaration, since the
nested class definition is usually only relevant to the
implementation.
Cons:
Nested classes can be forward-declared only within the
definition of the enclosing class. Thus, any header file
manipulating a Foo::Bar* pointer will have to
include the full class declaration for Foo.
Decision:
Do not make nested classes public unless they are actually
part of the interface, e.g., a class that holds a set of
options for some method.
▶
Prefer nonmember functions within a namespace or static member
functions to global functions; use completely global functions
rarely.
link
Pros:
Nonmember and static member functions can be useful in some
situations. Putting nonmember functions in a namespace avoids
polluting the global namespace.
Cons:
Nonmember and static member functions may make more sense as
members of a new class, especially if they access external
resources or have significant dependencies.
Decision:
Sometimes it is useful, or even necessary, to define a
function not bound to a class instance. Such a function can
be either a static member or a nonmember function.
Nonmember functions should not depend on external variables,
and should nearly always exist in a namespace. Rather than
creating classes only to group static member functions which
do not share static data, use
namespaces instead.
Functions defined in the same compilation unit as production
classes may introduce unnecessary coupling and link-time
dependencies when directly called from other compilation
units; static member functions are particularly susceptible
to this. Consider extracting a new class, or placing the
functions in a namespace possibly in a separate library.
If you must define a nonmember function and it is only
needed in its .cc file, use an unnamed
namespace or static
linkage (eg static int Foo() {...}) to limit
its scope.
▶
Place a function's variables in the narrowest scope possible,
and initialize variables in the declaration.
link
C++ allows you to declare variables anywhere in a function.
We encourage you to declare them in as local a scope as
possible, and as close to the first use as possible. This
makes it easier for the reader to find the declaration and see
what type the variable is and what it was initialized to. In
particular, initialization should be used instead of
declaration and assignment, e.g.
int i;
i = f(); // Bad -- initialization separate from declaration.
int j = g(); // Good -- declaration has initialization.
Note that gcc implements for (int i = 0; i
< 10; ++i) correctly (the scope of i is
only the scope of the for loop), so you can then
reuse i in another for loop in the
same scope. It also correctly scopes declarations in
if and while statements, e.g.
while (const char* p = strchr(str, '/')) str = p + 1;
There is one caveat: if the variable is an object, its
constructor is invoked every time it enters scope and is
created, and its destructor is invoked every time it goes
out of scope.
// Inefficient implementation:
for (int i = 0; i < 1000000; ++i) {
Foo f; // My ctor and dtor get called 1000000 times each.
f.DoSomething(i);
}
It may be more efficient to declare such a variable used in a
loop outside that loop:
Foo f; // My ctor and dtor get called once each.
for (int i = 0; i < 1000000; ++i) {
f.DoSomething(i);
}
▶
Global variables of class types are forbidden. Global variables
of built-in types are allowed, although non-
const
globals are forbidden in threaded code. Global variables should
never be initialized with the return value of a function.
link
Unfortunately the order in which constructors, destructors,
and initializers for global variables are called is only
partially specified and can change from build to build. This
can cause bugs that are very difficult to find.
Therefore we forbid global variables of class types (which
includes STL string, vector, etc.) because initialization
order might matter for their constructor, now or in the
future. Built-in types and structs of built-in types without
constructors are okay.
If you need a global variable of a class
type, use the
singleton
pattern.
For global string constants, use C style strings, not
STL strings:
const char kFrogSays[] = "ribbet";
Although we permit global variables in the global scope,
please be judicious in your use of them. Most global variables
should either be static data members of some class, or, if only
needed in one .cc file, defined in an unnamed
namespace. (As an alternative to using
an unnamed namespace, you can use static linkage to
limit the variable's scope.)
Please note that static class member variables
count as global variables, and should not be of class types!
Classes
Classes are the fundamental unit of code in C++. Naturally, we use
them extensively. This section lists the main dos and don'ts you
should follow when writing a class.
▶
Do only trivial initialization in a constructor. If at all
possible, use an
Init() method for non-trivial
initialization.
link
Definition:
It is possible to perform initialization in the body of the
constructor.
Pros:
Convenience in typing. No need to worry about whether the
class has been initialized or not.
Cons:
The problems with doing work in constructors are:
- There is no easy way for constructors to signal errors,
short of using exceptions (which are
forbidden).
- If the work fails, we now have an object whose
initialization code failed, so it may be an
indeterminate state.
- If the work calls virtual functions, these calls will
not get dispatched to the subclass implementations.
Future modification to your class can quietly introduce
this problem even if your class is not currently
subclassed, causing much confusion.
- If someone creates a global variable of this type
(which is against the rules, but still), the
constructor code will be called before
main(), possibly breaking some implicit
assumptions in the constructor code. For instance,
gflags
will not yet have been initialized.
Decision:
If your object requires non-trivial initialization, consider
having an explicit Init() method and/or adding a
member flag that indicates whether the object was successfully
initialized.
▶
You must define a default constructor if your class defines
member variables and has no other constructors. Otherwise the
compiler will do it for you, badly.
link
Definition:
The default constructor is called when we new a
class object with no arguments. It is always called when
calling new[] (for arrays).
Pros:
Initializing structures by default, to hold "impossible"
values, makes debugging much easier.
Cons:
Extra work for you, the code writer.
Decision:
If your class defines member variables has no other
constructors you must define a default constructor (one that
takes no arguments). It should preferably initialize the
object in such a way that its internal state is consistent
and valid.
The reason for this is that if you have no other
constructors and do not define a default constructor, the
compiler will generate one for you. This compiler
generated constructor may not initialize your object
sensibly.
If your class inherits from an existing class but you add no
new member variables, you are not required to have a default
constructor.
▶
Use the C++ keyword
explicit for constructors with
one argument.
link
Definition:
Normally, if a constructor takes one argument, it can be used
as a conversion. For instance, if you define
Foo::Foo(string name) and then pass a string to a
function that expects a Foo, the constructor will
be called to convert the string into a Foo and
will pass the Foo to your function for you. This
can be convenient but is also a source of trouble when things
get converted and new objects created without you meaning them
to. Declaring a constructor explicit prevents it
from being invoked implicitly as a conversion.
Pros:
Avoids undesirable conversions.
Cons:
None.
Decision:
We require all single argument constructors to be
explicit. Always put explicit in front of
one-argument constructors in the class definition:
explicit Foo(string name);
The exception is copy constructors, which, in the rare
cases when we allow them, should probably not be
explicit.
Classes that are intended to be
transparent wrappers around other classes are also
exceptions.
Such exceptions should be clearly marked with comments.
▶
Use copy constructors only when your code needs to copy a class;
most do not need to be copied and so should use
DISALLOW_COPY_AND_ASSIGN.
link
Definition:
The copy constructor is used when copying one object into a
new one (especially when passing objects by value).
Pros:
Copy constructors make it easy to copy objects. STL
containers require that all contents be copyable and
assignable.
Cons:
Implicit copying of objects in C++ is a rich source of bugs
and of performance problems. It also reduces readability, as
it becomes hard to track which objects are being passed around
by value as opposed to by reference, and therefore where
changes to an object are reflected.
Decision:
Most classes do not need to be copyable, and should not have a
copy constructor or an assignment operator. Unfortunately, the
compiler generates these for you, and makes them public, if
you do not declare them yourself.
Consider adding dummy declarations for the copy constructor and
assignment operator in the class' private: section,
without providing definitions. With these dummy routines marked
private, a compilation error will be raised if other code
attempts to use them. For convenience, a
DISALLOW_COPY_AND_ASSIGN macro can be used:
// A macro to disallow the copy constructor and operator= functions
// This should be used in the private: declarations for a class
#define DISALLOW_COPY_AND_ASSIGN(TypeName) \
TypeName(const TypeName&); \
void operator=(const TypeName&)
Then, in class Foo:
class Foo {
public:
Foo(int f);
~Foo();
private:
DISALLOW_COPY_AND_ASSIGN(Foo);
};
In almost all cases your class should use the
DISALLOW_COPY_AND_ASSIGN
macro as described above. If your class is one of the rare
classes that does need to be copyable, you should document why
this is so in the header file for that class, and you should
define the copy constructor and assignment operator
appropriately. Remember to check for self-assignment in
operator=.
You may be tempted to make your class copyable so that you
can use it as a value in STL containers. In almost all such
cases you should really put pointers to your
objects in the STL container. You may also want to consider
using
std::tr1::shared_ptr.
▶
Use a
struct only for passive objects that carry data;
everything else is a
class.
link
The struct and class keywords behave
almost identically in C++. We add our own semantic meanings
to each keyword, so you should use the appropriate keyword for
the data-type you're defining.
structs should be used for passive objects that carry
data, and may have associated constants, but lack any functionality
other than access/setting the data members. The
accessing/setting of fields is done by directly accessing the
fields rather than through method invocations. Methods should
not provide behavior but should only be used to set up the
data members, e.g., constructor, destructor,
Initialize(), Reset(),
Validate().
If more functionality is required, a class is more
appropriate. If in doubt, make it a class.
For consistency with STL, you can use struct
instead of class for functors and traits.
Note that member variables in structs and classes have
different naming rules.
▶
Composition is often more appropriate than inheritance. When
using inheritance, make it
public.
link
Definition:
When a sub-class inherits from a base class, it includes the
definitions of all the data and operations that the parent
base class defines. In practice, inheritance is used in two
major ways in C++: implementation inheritance, in which
actual code is inherited by the child, and interface inheritance, in which only
method names are inherited.
Pros:
Implementation inheritance reduces code size by re-using the
base class code as it specializes an existing type. Because
inheritance is a compile-time declaration, you and the
compiler can understand the operation and detect errors.
Interface inheritance can be used to programmatically enforce
that a class expose a particular API. Again, the compiler
can detect errors, in this case, when a class does not define
a necessary method of the API.
Cons:
For implementation inheritance, because the code implementing
a sub-class is spread between the base and the sub-class, it
can be more difficult to understand an implementation. The
sub-class cannot override functions that are not virtual, so
the sub-class cannot change implementation. The base class
may also define some data members, so that specifies physical
layout of the base class.
Decision:
All inheritance should be public. If you want to
do private inheritance, you should be including an instance of
the base class as a member instead.
Do not overuse implementation inheritance. Composition is
often more appropriate. Try to restrict use of inheritance
to the "is-a" case: Bar subclasses
Foo if it can reasonably be said that
Bar "is a kind of" Foo.
Make your destructor virtual if necessary. If
your class has virtual methods, its destructor
should be virtual.
Limit the use of protected to those member
functions that might need to be accessed from subclasses.
Note that data members must always
be private.
When redefining an inherited virtual function, explicitly
declare it virtual in the declaration of the
derived class. Rationale: If virtual is
omitted, the reader has to check all ancestors of the
class in question to determine if the function is virtual
or not.
▶
Only very rarely is multiple implementation inheritance actually
useful. We allow multiple inheritance only when at most one of
the base classes has an implementation; all other base classes
must be
pure interface classes tagged
with the
Interface suffix.
link
Definition:
Multiple inheritance allows a sub-class to have more than one
base class. We distinguish between base classes that are
pure interfaces and those that have an
implementation .
Pros:
Multiple implementation inheritance may let you re-use even more code
than single inheritance (see Inheritance).
Cons:
Only very rarely is multiple implementation
inheritance actually useful. When multiple implementation
inheritance seems like the solution, you can usually find a
different, more explicit, and cleaner solution.
Decision:
Multiple inheritance is allowed only when all superclasses, with the
possible exception of the first one, are pure
interfaces. In order to ensure that they remain pure interfaces,
they must end with the Interface suffix.
Note:
There is an exception to this
rule on Windows.
▶
Classes that satisfy certain conditions are allowed, but not required, to
end with an
Interface suffix.
link
Definition:
A class is a pure interface if it meets the following requirements:
- It has only public pure virtual ("
= 0") methods
and static methods (but see below for destructor).
- It may not have non-static data members.
- It need not have any constructors defined. If a constructor is
provided, it must take no arguments and it must be protected.
- If it is a subclass, it may only be derived from classes
that satisfy these conditions and are tagged with the
Interface suffix.
An interface class can never be directly instantiated
because of the pure virtual method(s) it declares. To make
sure all implementations of the interface can be destroyed
correctly, they must also declare a virtual destructor (in
an exception to the first rule, this should not be pure). See
Stroustrup, The C++ Programming Language, 3rd
edition, section 12.4 for details.
Pros:
Tagging a class with the Interface suffix lets
others know that they must not add implemented methods or non
static data members. This is particularly important in the case of
multiple inheritance.
Additionally, the interface concept is already well-understood by
Java programmers.
Cons:
The Interface suffix lengthens the class name, which
can make it harder to read and understand. Also, the interface
property may be considered an implementation detail that shouldn't
be exposed to clients.
Decision:
A class may end with Interface only if it meets the
above requirements. We do not require the converse, however:
classes that meet the above requirements are not required to end
with Interface.
▶
Do not overload operators except in rare, special circumstances.
link
Definition:
A class can define that operators such as + and
/ operate on the class as if it were a built-in
type.
Pros:
Can make code appear more intuitive because a class will
behave in the same way as built-in types (such as
int). Overloaded operators are more playful
names for functions that are less-colorfully named, such as
Equals() or Add(). For some
template functions to work correctly, you may need to define
operators.
Cons:
While operator overloading can make code more intuitive, it
has several drawbacks:
- It can fool our intuition into thinking that expensive
operations are cheap, built-in operations.
- It is much harder to find the call sites for overloaded
operators. Searching for
Equals() is much
easier than searching for relevant invocations of
==.
- Some operators work on pointers too, making it easy to
introduce bugs.
Foo + 4 may do one thing,
while &Foo + 4 does something totally
different. The compiler does not complain for either of
these, making this very hard to debug.
Overloading also has surprising ramifications. For instance,
you can't forward declare classes that overload
operator&.
Decision:
In general, do not overload operators. The assignment operator
(operator=), in particular, is insidious and
should be avoided. You can define functions like
Equals() and CopyFrom() if you need
them.
However, there may be rare cases where you need to overload
an operator to interoperate with templates or "standard" C++
classes (such as operator<<(ostream&, const
T&) for logging). These are acceptable if fully
justified, but you should try to avoid these whenever
possible. In particular, do not overload operator==
or operator< just so that your class can be
used as a key in an STL container; instead, you should
create equality and comparison functor types when declaring
the container.
Some of the STL algorithms do require you to overload
operator==, and you may do so in these cases,
provided you document why.
See also Copy Constructors
and Function
Overloading.
▶
Make
all data members
private, and provide
access to them through accessor functions as needed. Typically
a variable would be called
foo_ and the accessor
function
foo(). You may also want a mutator
function
set_foo().
link
The definitions of accessors are usually inlined in the header
file.
See also Inheritance and Function Names.
▶
Use the specified order of declarations within a class:
public: before
private:, methods
before data members (variables), etc.
link
Your class definition should start with its public:
section, followed by its protected: section and
then its private: section. If any of these sections
are empty, omit them.
Within each section, the declarations generally should be in
the following order:
- Typedefs and Enums
- Constants
- Constructors
- Destructor
- Methods, including static methods
- Data Members, including static data members
The DISALLOW_COPY_AND_ASSIGN macro invocation
should be at the end of the private: section. It
should be the last thing in the class. See Copy Constructors.
Method definitions in the corresponding .cc file
should be the same as the declaration order, as much as possible.
Do not put large method definitions inline in the class
definition. Usually, only trivial or performance-critical,
and very short, methods may be defined inline. See Inline Functions for more
details.
▶
Prefer small and focused functions.
link
We recognize that long functions are sometimes appropriate, so
no hard limit is placed on functions length. If a function
exceeds about 40 lines, think about whether it can be broken
up without harming the structure of the program.
Even if your long function works perfectly now, someone
modifying it in a few months may add new behavior. This could
result in bugs that are hard to find. Keeping your functions
short and simple makes it easier for other people to read and
modify your code.
You could find long and complicated functions when working
with
some
code. Do not be intimidated by modifying existing
code: if working with such a function proves to be difficult,
you find that errors are hard to debug, or you want to use a
piece of it in several different contexts, consider breaking
up the function into smaller and more manageable pieces.
Google-Specific Magic
There are various tricks and utilities that we use to make C++
code more robust, and various ways we use C++ that may differ from
what you see elsewhere.
▶
If you actually need pointer semantics,
scoped_ptr
is great. You should only use
std::tr1::shared_ptr
under very specific conditions, such as when objects need to be
held by STL containers. You should never use
auto_ptr.
link
"Smart" pointers are objects that act like pointers but have
added semantics. When a scoped_ptr is
destroyed, for instance, it deletes the object it's pointing
to. shared_ptr is the same way, but implements
reference-counting so only the last pointer to an object
deletes it.
Generally speaking, we prefer that we design code with clear
object ownership. The clearest object ownership is obtained by
using an object directly as a field or local variable, without
using pointers at all. On the other extreme, by their very definition,
reference counted pointers are owned by nobody. The problem with
this design is that it is easy to create circular references or other
strange conditions that cause an object to never be deleted.
It is also slow to perform atomic operations every time a value is
copied or assigned.
Although they are not recommended, reference counted pointers are
sometimes the simplest and most elegant way to solve a problem.
Other C++ Features
▶
All parameters passed by reference must be labeled
const.
link
Definition:
In C, if a function needs to modify a variable, the
parameter must use a pointer, eg int foo(int
*pval). In C++, the function can alternatively
declare a reference parameter: int foo(int
&val).
Pros:
Defining a parameter as reference avoids ugly code like
(*pval)++. Necessary for some applications like
copy constructors. Makes it clear, unlike with pointers, that
NULL is not a possible value.
Cons:
References can be confusing, as they have value syntax but
pointer semantics.
Decision:
Within function parameter lists all references must be
const:
void Foo(const string &in, string *out);
In fact it is a very strong convention that input
arguments are values or const references while
output arguments are pointers. Input parameters may be
const pointers, but we never allow
non-const reference parameters.
One case when you might want an input parameter to be a
const pointer is if you want to emphasize that the
argument is not copied, so it must exist for the lifetime of the
object; it is usually best to document this in comments as
well. STL adapters such as bind2nd and
mem_fun do not permit reference parameters, so
you must declare functions with pointer parameters in these
cases, too.
▶
Use overloaded functions (including constructors) only in cases
where input can be specified in different types that contain the
same information. Do not use function overloading to simulate
default function parameters.
link
Definition:
You may write a function that takes a
const string& and overload it with another that
takes const char*.
class MyClass {
public:
void Analyze(const string &text);
void Analyze(const char *text, size_t textlen);
};
Pros:
Overloading can make code more intuitive by allowing an
identically-named function to take different arguments. It
may be necessary for templatized code, and it can be
convenient for Visitors.
Cons:
One reason to minimize function overloading is that
overloading can make it hard to tell which function is being
called at a particular call site. Another one is that most
people are confused by the semantics of inheritance if a
deriving class overrides only some of the variants of a
function. Moreover, reading client code of a library may
become unnecessarily hard because of all the reasons against
default function parameters.
Decision:
If you want to overload a function, consider qualifying the
name with some information about the arguments, e.g.,
AppendString(), AppendInt() rather
than just Append().
▶
We do not allow default function parameters.
link
Pros:
Often you have a function that uses lots of default values,
but occasionally you want to override the defaults. Default
parameters allow an easy way to do this without having to
define many functions for the rare exceptions.
Cons:
People often figure out how to use an
API by looking at existing code that uses it.
Default parameters are more difficult to maintain because
copy-and-paste from previous code may not reveal all the
parameters. Copy-and-pasting of code segments can cause major
problems when the default arguments are not appropriate for
the new code.
Decision:
We require all arguments to be explicitly specified, to
force programmers to consider the API and the values they are
passing for each argument rather than silently accepting
defaults they may not be aware of.
▶
We do not allow variable-length arrays or
alloca().
link
Pros:
Variable-length arrays have natural-looking syntax. Both
variable-length arrays and alloca() are very
efficient.
Cons:
Variable-length arrays and alloca are not part of Standard
C++. More importantly, they allocate a data-dependent amount
of stack space that can trigger difficult-to-find memory
overwriting bugs: "It ran fine on my machine, but dies
mysteriously in production".
Decision:
Use a safe allocator instead, such as
scoped_ptr/scoped_array.
▶
We allow use of
friend classes and functions,
within reason.
link
Friends should usually be defined in the same file so that the
reader does not have to look in another file to find uses of
the private members of a class. A common use of
friend is to have a FooBuilder class
be a friend of Foo so that it can construct the
inner state of Foo correctly, without exposing
this state to the world. In some cases it may be useful to
make a unittest class a friend of the class it tests.
Friends extend, but do not break, the encapsulation
boundary of a class. In some cases this is better than making
a member public when you want to give only one other class
access to it. However, most classes should interact with
other classes solely through their public members.
▶
We do not use C++ exceptions.
link
Pros:
- Exceptions allow higher levels of an application to
decide how to handle "can't happen" failures in deeply
nested functions, without the obscuring and error-prone
bookkeeping of error codes.
- Exceptions are used by most other modern
languages. Using them in C++ would make it more consistent with
Python, Java, and the C++ that others are familiar with.
- Some third-party C++ libraries use exceptions, and turning
them off internally makes it harder to integrate with those
libraries.
- Exceptions are the only way for a constructor to fail.
We can simulate this with a factory function or an
Init() method, but these require heap
allocation or a new "invalid" state, respectively.
- Exceptions are really handy in testing frameworks.
Cons:
- When you add a
throw statement to an existing
function, you must examine all of its transitive callers. Either
they must make at least the basic exception safety guarantee, or
they must never catch the exception and be happy with the
program terminating as a result. For instance, if
f() calls g() calls
h(), and h throws an exception
that f catches, g has to be
careful or it may not clean up properly.
- More generally, exceptions make the control flow of
programs difficult to evaluate by looking at code: functions
may return in places you don't expect. This results
maintainability and debugging difficulties. You can minimize
this cost via some rules on how and where exceptions can be
used, but at the cost of more that a developer needs to know
and understand.
- Exception safety requires both RAII and different coding
practices. Lots of supporting machinery is needed to make
writing correct exception-safe code easy. Further, to avoid
requiring readers to understand the entire call graph,
exception-safe code must isolate logic that writes to
persistent state into a "commit" phase. This will have both
benefits and costs (perhaps where you're forced to obfuscate
code to isolate the commit). Allowing exceptions would force
us to always pay those costs even when they're not worth
it.
- Turning on exceptions adds data to each binary produced,
increasing compile time (probably slightly) and possibly
increasing address space pressure.
- The availability of exceptions may encourage developers
to throw them when they are not appropriate or recover from
them when it's not safe to do so. For example, invalid user
input should not cause exceptions to be thrown. We would
need to make the style guide even longer to document these
restrictions!
Decision:
On their face, the benefits of using exceptions outweigh the
costs, especially in new projects. However, for existing code,
the introduction of exceptions has implications on all dependent
code. If exceptions can be propagated beyond a new project, it
also becomes problematic to integrate the new project into
existing exception-free code. Because most existing C++ code at
Google is not prepared to deal with exceptions, it is
comparatively difficult to adopt new code that generates
exceptions.
Given that Google's existing code is not exception-tolerant, the
costs of using exceptions are somewhat greater than the costs in
in a new project. The conversion process would be slow and
error-prone. We don't believe that the available alternatives to
exceptions, such as error codes and assertions, introduce a
significant burden.
Our advice against using exceptions is not predicated on
philosophical or moral grounds, but practical ones.
Because we'd like to use our open-source
projects at Google and it's difficult to do so if those projects
use exceptions, we need to advise against exceptions in Google
open-source projects as well.
Things would probably be different if we had to do it all over
again from scratch.
There is an exception to this
rule (no pun intended) for Windows code.
▶
We do not use Run Time Type Information (RTTI).
link
Definition:
RTTI allows a programmer to query the C++ class of an
object at run time.
Pros:
It is useful in some unittests. For example, it is useful in
tests of factory classes where the test has to verify that a
newly created object has the expected dynamic type.
In rare circumstances, it is useful even outside of
tests.
Cons:
A query of type during run-time typically means a
design problem. If you need to know the type of an
object at runtime, that is often an indication that
you should reconsider the design of your class.
Decision:
Do not use RTTI, except in unittests. If you find yourself
in need of writing code that behaves differently based on
the class of an object, consider one of the alternatives to
querying the type.
Virtual methods are the preferred way of executing different
code paths depending on a specific subclass type. This puts
the work within the object itself.
If the work belongs outside the object and instead in some
processing code, consider a double-dispatch solution, such
as the Visitor design pattern. This allows a facility
outside the object itself to determine the type of class
using the built-in type system.
If you think you truly cannot use those ideas,
you may use RTTI. But think twice
about it. :-) Then think twice again.
Do not hand-implement an RTTI-like workaround. The arguments
against RTTI apply just as much to workarounds like class
hierarchies with type tags.
▶
Use C++ casts like
static_cast<>(). Do not use
other cast formats like
int y = (int)x; or
int y = int(x);.
link
Definition:
C++ introduced a different cast system from C that
distinguishes the types of cast operations.
Pros:
The problem with C casts is the ambiguity of the operation;
sometimes you are doing a conversion (e.g.,
(int)3.5) and sometimes you are doing a
cast (e.g., (int)"hello"); C++ casts
avoid this. Additionally C++ casts are more visible when
searching for them.
Cons:
The syntax is nasty.
Decision:
Do not use C-style casts. Instead, use these C++-style
casts.
- Use
static_cast as the equivalent of a
C-style cast that does value conversion, or when you need to explicitly up-cast
a pointer from a class to its superclass.
- Use
const_cast to remove the const
qualifier (see const).
- Use
reinterpret_cast to do unsafe
conversions of pointer types to and from integer and
other pointer types. Use this only if you know what you are
doing and you understand the aliasing issues.
- Do not use
dynamic_cast except in test code.
If you need to know type information at runtime in this way
outside of a unittest, you probably have a design
flaw.
▶
Use streams only for logging.
link
Definition:
Streams are a replacement for printf() and
scanf().
Pros:
With streams, you do not need to know the type of the object
you are printing. You do not have problems with format
strings not matching the argument list. (Though with gcc, you
do not have that problem with printf either.) Streams
have automatic constructors and destructors that open and close the
relevant files.
Cons:
Streams make it difficult to do functionality like
pread(). Some formatting (particularly the common
format string idiom %.*s) is difficult if not
impossible to do efficiently using streams without using
printf-like hacks. Streams do not support operator
reordering (the %1s directive), which is helpful for
internationalization.
Decision:
Do not use streams, except where required by a logging interface.
Use printf-like routines instead.
There are various pros and cons to using streams, but in
this case, as in many other cases, consistency trumps the
debate. Do not use streams in your code.
Extended Discussion
There has been debate on this issue, so this explains the
reasoning in greater depth. Recall the Only One Way
guiding principle: we want to make sure that whenever we
do a certain type of I/O, the code looks the same in all
those places. Because of this, we do not want to allow
users to decide between using streams or using
printf plus Read/Write/etc. Instead, we should
settle on one or the other. We made an exception for logging
because it is a pretty specialized application, and for
historical reasons.
Proponents of streams have argued that streams are the obvious
choice of the two, but the issue is not actually so clear. For
every advantage of streams they point out, there is an
equivalent disadvantage. The biggest advantage is that
you do not need to know the type of the object to be
printing. This is a fair point. But, there is a
downside: you can easily use the wrong type, and the
compiler will not warn you. It is easy to make this
kind of mistake without knowing when using streams.
cout << this; // Prints the address
cout << *this; // Prints the contents
The compiler does not generate an error because
<< has been overloaded. We discourage
overloading for just this reason.
Some say printf formatting is ugly and hard to
read, but streams are often no better. Consider the following
two fragments, both with the same typo. Which is easier to
discover?
cerr << "Error connecting to '" << foo->bar()->hostname.first
<< ":" << foo->bar()->hostname.second << ": " << strerror(errno);
fprintf(stderr, "Error connecting to '%s:%u: %s",
foo->bar()->hostname.first, foo->bar()->hostname.second,
strerror(errno));
And so on and so forth for any issue you might bring up.
(You could argue, "Things would be better with the right
wrappers," but if it is true for one scheme, is it not
also true for the other? Also, remember the goal is to
make the language smaller, not add yet more machinery that
someone has to learn.)
Either path would yield different advantages and
disadvantages, and there is not a clearly superior
solution. The simplicity doctrine mandates we settle on
one of them though, and the majority decision was on
printf + read/write.
▶
Use prefix form (
++i) of the increment and
decrement operators with iterators and other template objects.
link
Definition:
When a variable is incremented (++i or
i++) or decremented (--i or
i--) and the value of the expression is not used,
one must decide whether to preincrement (decrement) or
postincrement (decrement).
Pros:
When the return value is ignored, the "pre" form
(++i) is never less efficient than the "post"
form (i++), and is often more efficient. This is
because post-increment (or decrement) requires a copy of
i to be made, which is the value of the
expression. If i is an iterator or other
non-scalar type, copying i could be expensive.
Since the two types of increment behave the same when the
value is ignored, why not just always pre-increment?
Cons:
The tradition developed, in C, of using post-increment when
the expression value is not used, especially in for
loops. Some find post-increment easier to read, since the
"subject" (i) precedes the "verb" (++),
just like in English.
Decision:
For simple scalar (non-object) values there is no reason to
prefer one form and we allow either. For iterators and other
template types, use pre-increment.
▶
We strongly recommend that you use
const whenever
it makes sense to do so.
link
Definition:
Declared variables and parameters can be preceded by the
keyword const to indicate the variables are not
changed (e.g., const int foo). Class functions
can have the const qualifier to indicate the
function does not change the state of the class member
variables (e.g., class Foo { int Bar(char c) const;
};).
Pros:
Easier for people to understand how variables are being used.
Allows the compiler to do better type checking, and,
conceivably, generate better code. Helps people convince
themselves of program correctness because they know the
functions they call are limited in how they can modify your
variables. Helps people know what functions are safe to use
without locks in multi-threaded programs.
Cons:
const is viral: if you pass a const
variable to a function, that function must have const
in its prototype (or the variable will need a
const_cast). This can be a particular problem
when calling library functions.
Decision:
const variables, data members, methods and
arguments add a level of compile-time type checking; it
is better to detect errors as soon as possible.
Therefore we strongly recommend that you use
const whenever it makes sense to do so:
- If a function does not modify an argument passed by
reference or by pointer, that argument should be
const.
- Declare methods to be
const whenever
possible. Accessors should almost always be
const. Other methods should be const if they do
not modify any data members, do not call any
non-const methods, and do not return a
non-const pointer or non-const
reference to a data member.
- Consider making data members
const
whenever they do not need to be modified after
construction.
However, do not go crazy with const. Something like
const int * const * const x; is likely
overkill, even if it accurately describes how const x is.
Focus on what's really useful to know: in this case,
const int** x is probably sufficient.
The mutable keyword is allowed but is unsafe
when used with threads, so thread safety should be carefully
considered first.
Where to put the const
Some people favor the form int const *foo to
const int* foo. They argue that this is more
readable because it's more consistent: it keeps the rule
that const always follows the object it's
describing. However, this consistency argument doesn't
apply in this case, because the "don't go crazy" dictum
eliminates most of the uses you'd have to be consistent with.
Putting the const first is arguably more readable,
since it follows English in putting the "adjective"
(const) before the "noun" (int).
That said, while we encourage putting const first,
we do not require it. But be consistent with the code around
you!
▶
Of the built-in C++ integer types, the only one used
is
int. If a program needs a variable of a different
size, use
a precise-width integer type from
<stdint.h>, such as
int16_t.
link
Definition:
C++ does not specify the sizes of its integer types. Typically
people assume that short is 16 bits,
int is 32 bits, long is 32 bits and
long long is 64 bits.
Pros:
Uniformity of declaration.
Cons:
The sizes of integral types in C++ can vary based on compiler
and architecture.
Decision:
<stdint.h> defines
types like int16_t, uint32_t,
int64_t, etc.
You should always use those in preference to
short, unsigned long long and the
like, when you need a guarantee on the size of an integer.
Of the C integer types, only int should be
used. When appropriate, you are welcome to use standard
types like size_t and ptrdiff_t.
We use int very often, for integers we know are not
going to be too big, e.g., loop counters. Use plain old
int for such things. You should assume that an
int is
at least 32 bits,
but don't assume that it has more than 32 bits.
If you need a 64-bit integer type, use
int64_t or
uint64_t.
For integers we know can be "big",
use
int64_t.
You should not use the unsigned integer types such as
uint32_t,
unless the quantity you are representing is really a bit pattern
rather than a number. In particular, do not use unsigned types to
say a number will never be negative. Instead, use
assertions for this.
On Unsigned Integers
Some people, including some textbook authors, recommend
using unsigned types to represent numbers that are never
negative. This is intended as a form of self-documentation.
However, in C, the advantages of such documentation are
outweighed by the real bugs it can introduce. Consider:
for (unsigned int i = foo.Length()-1; i >= 0; --i) ...
This code will never terminate! Sometimes gcc will notice
this bug and warn you, but often it will not. Equally bad
bugs can occur when comparing signed and unsigned
variables. Basically, C's type-promotion scheme causes
unsigned types to behave differently than one might expect.
So, document that a variable is non-negative using
assertions.
Don't use an unsigned type.
▶
Code should be 64-bit and 32-bit friendly. Bear in mind problems of
printing, comparisons, and structure alignment.
link
-
printf() specifiers for some types are
not cleanly portable between 32-bit and 64-bit
systems. C99 defines some portable format
specifiers. Unfortunately, MSVC 7.1 does not
understand some of these specifiers and the
standard is missing a few, so we have to define our
own ugly versions in some cases (in the style of the
standard include file inttypes.h):
// printf macros for size_t, in the style of inttypes.h
#ifdef _LP64
#define __PRIS_PREFIX "z"
#else
#define __PRIS_PREFIX
#endif
// Use these macros after a % in a printf format string
// to get correct 32/64 bit behavior, like this:
// size_t size = records.size();
// printf("%"PRIuS"\n", size);
#define PRIdS __PRIS_PREFIX "d"
#define PRIxS __PRIS_PREFIX "x"
#define PRIuS __PRIS_PREFIX "u"
#define PRIXS __PRIS_PREFIX "X"
#define PRIoS __PRIS_PREFIX "o"
Type
|
DO NOT use
|
DO use
|
Notes
|
void * (or any pointer)
|
%lx
|
%p
|
|
int64_t
|
%qd,
%lld
|
%"PRId64"
|
|
uint64_t
|
%qu,
%llu,
%llx
|
%"PRIu64",
%"PRIx64"
|
|
size_t
|
%u
|
%"PRIuS",
%"PRIxS"
|
C99 specifies %zu
|
ptrdiff_t
|
%d
|
%"PRIdS"
|
C99 specifies %zd
|
Note that the PRI* macros expand to independent
strings which are concatenated by the compiler. Hence
if you are using a non-constant format string, you
need to insert the value of the macro into the format,
rather than the name. It is still possible, as usual,
to include length specifiers, etc., after the
% when using the PRI*
macros. So, e.g. printf("x = %30"PRIuS"\n",
x) would expand on 32-bit Linux to
printf("x = %30" "u" "\n", x), which the
compiler will treat as printf("x = %30u\n",
x).
- Remember that
sizeof(void *) !=
sizeof(int). Use intptr_t if
you want a pointer-sized integer.
- You may need to be careful with structure alignments,
particularly for structures being stored on disk. Any
class/structure with a
int64_t/uint64_t
member will by default end up being 8-byte aligned on a 64-bit
system. If you have such structures being shared on disk
between 32-bit and 64-bit code, you will need to ensure
that they are packed the same on both architectures.
Most compilers offer a way to alter
structure alignment. For gcc, you can use
__attribute__((packed)). MSVC offers
#pragma pack() and
__declspec(align()).
-
Use the
LL or ULL suffixes as
needed to create 64-bit constants. For example:
int64_t my_value = 0x123456789LL;
uint64_t my_mask = 3ULL << 48;
- If you really need different code on 32-bit and 64-bit
systems, use
#ifdef _LP64 to choose between
the code variants. (But please avoid this if
possible, and keep any such changes localized.)
▶
Be very cautious with macros. Prefer inline functions, enums,
and
const variables to macros.
link
Macros mean that the code you see is not the same as the code
the compiler sees. This can introduce unexpected behavior,
especially since macros have global scope.
Luckily, macros are not nearly as necessary in C++ as they are
in C. Instead of using a macro to inline performance-critical
code, use an inline function. Instead of using a macro to
store a constant, use a const variable. Instead of
using a macro to "abbreviate" a long variable name, use a
reference. Instead of using a macro to conditionally compile code
... well, don't do that at all (except, of course, for the
#define guards to prevent double inclusion of
header files). It makes testing much more difficult.
Macros can do things these other techniques cannot, and you do
see them in the codebase, especially in the lower-level
libraries. And some of their special features (like
stringifying, concatenation, and so forth) are not available
through the language proper. But before using a macro,
consider carefully whether there's a non-macro way to achieve
the same result.
The following usage pattern will avoid many problems with
macros; if you use macros, follow it whenever possible:
- Don't define macros in a
.h file.
-
#define macros right before you use them,
and #undef them right after.
- Do not just
#undef an existing macro before
replacing it with your own; instead, pick a name that's
likely to be unique.
- Try not to use macros that expand to unbalanced C++
constructs, or at least document that behavior well.
▶
Use
0 for integers,
0.0 for reals,
NULL for pointers, and
'\0' for chars.
link
Use 0 for integers and 0.0 for reals.
This is not controversial.
For pointers (address values), there is a choice between 0
and NULL. Bjarne Stroustrup prefers an unadorned
0. We prefer NULL because it looks like a
pointer. In fact, some C++ compilers, such as gcc 4.1.0, provide special
definitions of NULL which enable them to give useful
warnings, particularly in situations where sizeof(NULL)
is not equal to sizeof(0).
Use '\0' for chars.
This is the correct type and also makes code more readable.
▶
Use
sizeof(varname) instead of
sizeof(type) whenever possible.
link
Use sizeof(varname) because it will update
appropriately if the type of the variable changes.
sizeof(type) may make sense in some cases,
but should generally be avoided because it can fall out of sync if
the variable's type changes.
Struct data;
memset(&data, 0, sizeof(data));
memset(&data, 0, sizeof(Struct));
▶
Use only approved libraries from the Boost library collection.
link
Definition:
The Boost library collection is
a popular collection of peer-reviewed, free, open-source C++ libraries.
Pros:
Boost code is generally very high-quality, is widely portable, and fills
many important gaps in the C++ standard library, such as type traits,
better binders, and better smart pointers. It also provides an
implementation of the TR1 extension to the standard library.
Cons:
Some Boost libraries encourage coding practices which can hamper
readability, such as metaprogramming and other advanced template
techniques, and an excessively "functional" style of programming.
Decision:
In order to maintain a high level of readability for all contributors
who might read and maintain code, we only allow an approved subset of
Boost features. Currently, the following libraries are permitted:
We are actively considering adding other Boost features to the list, so
this rule may be relaxed in the future.
Naming
The most important consistency rules are those that govern
naming. The style of a name immediately informs us what sort of
thing the named entity is: a type, a variable, a function, a
constant, a macro, etc., without requiring us to search for the
declaration of that entity. The pattern-matching engine in our
brains relies a great deal on these naming rules.
Naming rules are pretty arbitrary, but
we feel that consistency is more important than individual preferences
in this area, so regardless of whether you find them sensible or not,
the rules are the rules.
▶
Function names, variable names, and filenames should be
descriptive; eschew abbreviation. Types and variables should be
nouns, while functions should be "command" verbs.
link
How to Name
Give as descriptive a name as possible, within reason. Do
not worry about saving horizontal space as it is far more
important to make your code immediately understandable by a
new reader. Examples of well-chosen names:
int num_errors; // Good.
int num_completed_connections; // Good.
Poorly-chosen names use ambiguous abbreviations or arbitrary
characters that do not convey meaning:
int n; // Bad - meaningless.
int nerr; // Bad - ambiguous abbreviation.
int n_comp_conns; // Bad - ambiguous abbreviation.
Type and variable names should typically be nouns: e.g.,
FileOpener,
num_errors.
Function names should typically be imperative (that is they
should be commands): e.g., OpenFile(),
set_num_errors(). There is an exception for
accessors, which, described more completely in Function Names, should be named
the same as the variable they access.
Abbreviations
Do not use abbreviations unless they are extremely well
known outside your project. For example:
// Good
// These show proper names with no abbreviations.
int num_dns_connections; // Most people know what "DNS" stands for.
int price_count_reader; // OK, price count. Makes sense.
// Bad!
// Abbreviations can be confusing or ambiguous outside a small group.
int wgc_connections; // Only your group knows what this stands for.
int pc_reader; // Lots of things can be abbreviated "pc".
Never abbreviate by leaving out letters:
int error_count; // Good.
int error_cnt; // Bad.
▶
Filenames should be all lowercase and can include underscores
(
_) or dashes (
-). Follow the
convention that your
project
uses.
link
Examples of acceptable file names:
my_useful_class.cc
my-useful-class.cc
myusefulclass.cc
C++ files should end in .cc and header files
should end in .h.
Do not use filenames that already exist
in /usr/include, such as db.h.
In general, make your filenames very specific. For example,
use http_server_logs.h rather
than logs.h. A very common case is to have a
pair of files called, e.g., foo_bar.h
and foo_bar.cc, defining a class
called FooBar.
Inline functions must be in a .h file. If your
inline functions are very short, they should go directly into your
.h file. However, if your inline functions
include a lot of code, they may go into a third file that
ends in -inl.h. In a class with a lot of inline
code, your class could have three files:
url_table.h // The class declaration.
url_table.cc // The class definition.
url_table-inl.h // Inline functions that include lots of code.
See also the section -inl.h Files
▶
Type names start with a capital letter and have a capital
letter for each new word, with no underscores:
MyExcitingClass,
MyExcitingEnum.
link
The names of all types — classes, structs, typedefs, and enums
— have the same naming convention. Type names should start
with a capital letter and have a capital letter for each new
word. No underscores. For example:
// classes and structs
class UrlTable { ...
class UrlTableTester { ...
struct UrlTableProperties { ...
// typedefs
typedef hash_map<UrlTableProperties *, string> PropertiesMap;
// enums
enum UrlTableErrors { ...
▶
Variable names are all lowercase, with underscores between
words. Class member variables have trailing underscores. For
instance:
my_exciting_local_variable,
my_exciting_member_variable_.
link
Common Variable names
For example:
string table_name; // OK - uses underscore.
string tablename; // OK - all lowercase.
string tableName; // Bad - mixed case.
Class Data Members
Data members (also called instance variables or member
variables) are lowercase with optional underscores like
regular variable names, but always end with a trailing
underscore.
string table_name_; // OK - underscore at end.
string tablename_; // OK.
Struct Variables
Data members in structs should be named like regular
variables without the trailing underscores that data members
in classes have.
struct UrlTableProperties {
string name;
int num_entries;
}
See Structs vs. Classes for a
discussion of when to use a struct versus a class.
Global Variables
There are no special requirements for global variables,
which should be rare in any case, but if you use one,
consider prefixing it with g_ or some other
marker to easily distinguish it from local variables.
▶
Use a
k followed by mixed case:
kDaysInAWeek.
link
All compile-time constants, whether they are declared locally,
globally, or as part of a class, follow a slightly different
naming convention from other variables. Use a k
followed by words with uppercase first letters:
const int kDaysInAWeek = 7;
▶
Regular functions have mixed case; accessors and mutators match
the name of the variable:
MyExcitingFunction(),
MyExcitingMethod(),
my_exciting_member_variable(),
set_my_exciting_member_variable().
link
Regular Functions
Functions should start with a capital letter and have a
capital letter for each new word. No underscores:
AddTableEntry()
DeleteUrl()
Accessors and Mutators
Accessors and mutators (get and set functions) should match
the name of the variable they are getting and setting. This
shows an excerpt of a class whose instance variable is
num_entries_.
class MyClass {
public:
...
int num_entries() const { return num_entries_; }
void set_num_entries(int num_entries) { num_entries_ = num_entries; }
private:
int num_entries_;
};
You may also use lowercase letters for other very short
inlined functions. For example if a function were so cheap
you would not cache the value if you were calling it in a
loop, then lowercase naming would be acceptable.
▶
Namespace names are all lower-case, and based on project names and
possibly their directory structure:
google_awesome_project.
link
See Namespaces for a discussion of
namespaces and how to name them.
▶
Enumerators should be all uppercase with underscores between
words:
MY_EXCITING_ENUM_VALUE.
link
The individual enumerators should be all uppercase. The
enumeration name, UrlTableErrors, is a type, and
therefore mixed case.
enum UrlTableErrors {
OK = 0,
ERROR_OUT_OF_MEMORY,
ERROR_MALFORMED_INPUT,
};
▶
You're not really going to
define
a macro, are you? If you do, they're like this:
MY_MACRO_THAT_SCARES_SMALL_CHILDREN.
link
Please see the description of
macros; in general macros should not be used.
However, if they are absolutely needed, then they should be
named like enum value names with all capitals and underscores.
#define ROUND(x) ...
#define PI_ROUNDED 3.0
▶
If you are naming something that is analogous to an existing C
or C++ entity then you can follow the existing naming convention
scheme.
link
-
bigopen()
- function name, follows form of
open()
-
uint
-
typedef
-
bigpos
-
struct or class, follows form of
pos
-
sparse_hash_map
- STL-like entity; follows STL naming conventions
-
LONGLONG_MAX
- a constant, as in
INT_MAX
Though a pain to write, comments are absolutely vital to keeping our
code readable. The following rules describe what you should
comment and where. But remember: while comments are very
important, the best code is self-documenting. Giving sensible
names to types and variables is much better than using obscure
names that you must then explain through comments.
When writing your comments, write for your audience: the next
contributor
who will need to understand your code. Be generous — the next
one may be you!
▶
Use either the
// or
/* */ syntax, as long
as you are consistent.
link
You can use either the // or the /* */
syntax; however, // is much more common.
Be consistent with how you comment and what style you use where.
▶
Start each file with a copyright notice, followed by a
description of the contents of the file.
link
Legal Notice and Author Line
Every file should contain the following items, in order:
- a copyright statement (for example,
Copyright 2008 Google Inc.)
- a license boilerplate. Choose the appropriate boilerplate
for the license used by the project (for example,
Apache 2.0, BSD, LGPL, GPL)
- an author line to identify the original author of the
file
If you make significant changes to a file that someone else
originally wrote, add yourself to the author line. This can
be very helpful when another
contributor
has questions about the file and needs to know whom to contact
about it.
File Contents
Every file should have a comment at the top, below the
and author line, that describes the contents of the file.
Generally a .h file will describe the classes
that are declared in the file with an overview of what they
are for and how they are used. A .cc file
should contain more information about implementation details
or discussions of tricky algorithms. If you feel the
implementation details or a discussion of the algorithms
would be useful for someone reading the .h,
feel free to put it there instead, but mention in the
.cc that the documentation is in the
.h file.
Do not duplicate comments in both the .h and
the .cc. Duplicated comments diverge.
▶
Every class definition should have an accompanying comment that
describes what it is for and how it should be used.
link
// Iterates over the contents of a GargantuanTable. Sample usage:
// GargantuanTable_Iterator* iter = table->NewIterator();
// for (iter->Seek("foo"); !iter->done(); iter->Next()) {
// process(iter->key(), iter->value());
// }
// delete iter;
class GargantuanTable_Iterator {
...
};
If you have already described a class in detail in the
comments at the top of your file feel free to simply state
"See comment at top of file for a complete description", but
be sure to have some sort of comment.
Document the synchronization assumptions the class makes, if
any. If an instance of the class can be accessed by multiple
threads, take extra care to document the rules and invariants
surrounding multithreaded use.
▶
Declaration comments describe use of the function; comments at
the definition of a function describe operation.
link
Function Declarations
Every function declaration should have comments immediately
preceding it that describe what the function does and how to
use it. These comments should be descriptive ("Opens the
file") rather than imperative ("Open the file"); the comment
describes the function, it does not tell the function what
to do. In general, these comments do not describe how the
function performs its task. Instead, that should be left to
comments in the function definition.
Types of things to mention in comments at the function
declaration:
- What the inputs and outputs are.
- For class member functions: whether the object
remembers reference arguments beyond the
duration of the method call, and whether it will
free them or not.
- If the function allocates memory that the caller
must free.
- Whether any of the arguments can be
NULL.
- If there are any performance implications of how a
function is used.
- If the function is re-entrant. What are its
synchronization assumptions?
Here is an example:
// Returns an iterator for this table. It is the client's
// responsibility to delete the iterator when it is done with it,
// and it must not use the iterator once the GargantuanTable object
// on which the iterator was created has been deleted.
//
// The iterator is initially positioned at the beginning of the table.
//
// This method is equivalent to:
// Iterator* iter = table->NewIterator();
// iter->Seek("");
// return iter;
// If you are going to immediately seek to another place in the
// returned iterator, it will be faster to use NewIterator()
// and avoid the extra seek.
Iterator* GetIterator() const;
However, do not be unnecessarily verbose or state the
completely obvious. Notice below that it is not necessary
to say "returns false otherwise" because this is implied.
// Returns true if the table cannot hold any more entries.
bool IsTableFull();
When commenting constructors and destructors, remember that
the person reading your code knows what constructors and
destructors are for, so comments that just say something like
"destroys this object" are not useful. Document what
constructors do with their arguments (for example, if they
take ownership of pointers), and what cleanup the destructor
does. If this is trivial, just skip the comment. It is
quite common for destructors not to have a header comment.
Function Definitions
Each function definition should have a comment describing
what the function does and anything tricky about how it does
its job. For example, in the definition comment you might
describe any coding tricks you use, give an overview of the
steps you go through, or explain why you chose to implement
the function in the way you did rather than using a viable
alternative. For instance, you might mention why it must
acquire a lock for the first half of the function but why it
is not needed for the second half.
Note you should not just repeat the comments given
with the function declaration, in the .h file or
wherever. It's okay to recapitulate briefly what the function
does, but the focus of the comments should be on how it does it.
▶
In general the actual name of the variable should be descriptive
enough to give a good idea of what the variable is used for. In
certain cases, more comments are required.
link
Class Data Members
Each class data member (also called an instance variable or
member variable) should have a comment describing what it is
used for. If the variable can take sentinel values with
special meanings, such as NULL or -1, document this.
For example:
private:
// Keeps track of the total number of entries in the table.
// Used to ensure we do not go over the limit. -1 means
// that we don't yet know how many entries the table has.
int num_total_entries_;
Global Variables
As with data members, all global variables should have a
comment describing what they are and what they are used for.
For example:
// The total number of tests cases that we run through in this regression test.
const int kNumTestCases = 6;
▶
In your implementation you should have comments in tricky,
non-obvious, interesting, or important parts of your code.
link
Class Data Members
Tricky or complicated code blocks should have comments
before them. Example:
// Divide result by two, taking into account that x
// contains the carry from the add.
for (int i = 0; i < result->size(); i++) {
x = (x << 8) + (*result)[i];
(*result)[i] = x >> 1;
x &= 1;
}
Line Comments
Also, lines that are non-obvious should get a comment at the
end of the line. These end-of-line comments should be
separated from the code by 2 spaces. Example:
// If we have enough memory, mmap the data portion too.
mmap_budget = max<int64>(0, mmap_budget - index_->length());
if (mmap_budget >= data_size_ && !MmapData(mmap_chunk_bytes, mlock))
return; // Error already logged.
Note that there are both comments that describe what the
code is doing, and comments that mention that an error has
already been logged when the function returns.
If you have several comments on subsequent lines, it can
often be more readable to line them up:
...
DoSomething(); // Comment here so the comments line up.
DoSomethingElseThatIsLonger(); // Comment here so there are two spaces between
// the code and the comment.
...
NULL, true/false, 1, 2, 3...
When you pass in NULL, boolean, or literal integer
values to functions, you should consider adding a comment about
what they are, or make your code self-documenting by using
constants. For example, compare:
bool success = CalculateSomething(interesting_value,
10,
false,
NULL); // What are these arguments??
versus:
bool success = CalculateSomething(interesting_value,
10, // Default base value.
false, // Not the first time we're calling this.
NULL); // No callback.
Or alternatively, constants or self-describing variables:
const int kDefaultBaseValue = 10;
const bool kFirstTimeCalling = false;
Callback *null_callback = NULL;
bool success = CalculateSomething(interesting_value,
kDefaultBaseValue,
kFirstTimeCalling,
null_callback);
Don'ts
Note that you should never describe the code
itself. Assume that the person reading the code knows C++
better than you do, even though he or she does not know what
you are trying to do:
// Now go through the b array and make sure that if i occurs,
// the next element is i+1.
... // Geez. What a useless comment.
▶
Pay attention to punctuation, spelling, and grammar; it is
easier to read well-written comments than badly written ones.
link
Comments should usually be written as complete
sentences with proper capitalization and periods at the end.
Shorter comments, such as comments at the end of a line of
code, can sometimes be less formal, but you should be
consistent with your style. Complete sentences are more
readable, and they provide some assurance that the comment is
complete and not an unfinished thought.
Although it can be frustrating to have a code reviewer point
out that you are using a comma when you should be using a
semicolon, it is very important that source code maintain a
high level of clarity and readability. Proper punctuation,
spelling, and grammar help with that goal.
▶
Use
TODO comments for code that is temporary, a
short-term solution, or good-enough but not perfect.
link
TODOs should include the string TODO in
all caps, followed by your
name, e-mail address, or other
identifier
in parentheses. A colon is optional. The main purpose is to have
a consistent TODO format searchable by the person
adding the comment (who can provide more details upon request). A
TODO is not a commitment to provide the fix yourself.
// TODO(kl@gmail.com): Use a "*" here for concatenation operator.
// TODO(Zeke) change this to use relations.
If your TODO is of the form "At a future date do
something" make sure that you either include a very specific
date ("Fix by November 2005") or a very specific event
("Remove this code when all clients can handle XML responses.").
Coding style and formatting are pretty arbitrary, but a
project
is much easier to follow if everyone uses the same style. Individuals
may not agree with every aspect of the formatting rules, and some of
the rules may take some getting used to, but it is important that all
project contributors
follow the style rules so that
they
can all read and understand everyone's code easily.
▶
Each line of text in your code should be at most 80 characters
long.
link
We recognize that this rule is controversial, but so much existing
code already adheres to it, and we feel that consistency is
important.
Pros:
Those who favor
this rule argue
that it is rude to force them to resize their windows and there
is no need for anything longer. Some folks are used to having
several code windows side-by-side, and thus don't have room to
widen their windows in any case. People set up their work
environment assuming a particular maximum window width, and 80
columns has been the traditional standard. Why change it?
Cons:
Proponents of change argue that a wider line can make code
more readable. The 80-column limit is an hidebound
throwback to 1960s mainframes;
modern equipment has
wide screens that can easily show longer lines.
Decision:
80 characters is the maximum.
Exception: if a comment line contains an example command or
a literal URL longer than 80 characters, that line may be
longer than 80 characters for ease of cut and paste.
Exception: an #include statement with a long
path may exceed 80 columns. Try to avoid situations where this
becomes necessary.
Exception: you needn't be concerned about
header guards
that exceed the maximum length.
▶
Non-ASCII characters should be rare, and must use UTF-8 formatting.
link
You shouldn't hard-code user-facing text in source, even English,
so use of non-ASCII characters should be rare. However, in certain
cases it is appropriate to include such words in your code. For
example, if your code parses data files from foreign
it may be appropriate to hard-code the non-ASCII string(s) used in
those data files as delimiters. More commonly, unittest code
(which does not
need to be localized) might contain non-ASCII strings. In such
cases, you should use UTF-8, since that is
an encoding understood by most tools able
to handle more than just ASCII.
Hex encoding is also OK, and encouraged where it enhances
readability — for example, "\xEF\xBB\xBF" is the
Unicode zero-width no-break space character, which would be
invisible if included in the source as straight UTF-8.
▶
Use only spaces, and indent 2 spaces at a time.
link
We use spaces for indentation. Do not use tabs in your code.
You should set your editor to emit spaces when you hit the tab
key.
▶
Return type on the same line as function name, parameters on the
same line if they fit.
link
Functions look like this:
ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {
DoSomething();
...
}
If you have too much text to fit on one line:
ReturnType ClassName::ReallyLongFunctionName(Type par_name1,
Type par_name2,
Type par_name3) {
DoSomething();
...
}
or if you cannot fit even the first parameter:
ReturnType LongClassName::ReallyReallyReallyLongFunctionName(
Type par_name1, // 4 space indent
Type par_name2,
Type par_name3) {
DoSomething(); // 2 space indent
...
}
Some points to note:
- The return type is always on the same line as the
function name.
- The open parenthesis is always on the same line as the
function name.
- There is never a space between the function name and the
open parenthesis.
- There is never a space between the parentheses and the
parameters.
- The open curly brace is always at the end of the same
line as the last parameter.
- The close curly brace is always on the last line by
itself.
- There should be a space between the close parenthesis and
the open curly brace.
- All parameters should be named, with identical names in the
declaration and implementation.
- All parameters should be aligned if possible.
- Default indentation is 2 spaces.
- Wrapped parameters have a 4 space indent.
If your function is const, the const
keyword should be on the same line as the last parameter:
// Everything in this function signature fits on a single line
ReturnType FunctionName(Type par) const {
...
}
// This function signature requires multiple lines, but
// the const keyword is on the line with the last parameter.
ReturnType ReallyLongFunctionName(Type par1,
Type par2) const {
...
}
If some parameters are unused, comment out the variable name in the
function definition:
// Always have named parameters in interfaces.
class Shape {
public:
virtual void Rotate(double radians) = 0;
}
// Always have named parameters in the declaration.
class Circle : public Shape {
public:
virtual void Rotate(double radians);
}
// Comment out unused named parameters in definitions.
void Circle::Rotate(double /*radians*/) {}
// Bad - if someone wants to implement later, it's not clear what the
// variable means.
void Circle::Rotate(double) {}
▶
On one line if it fits; otherwise, wrap arguments at the
parenthesis.
link
Function calls have the following format:
bool retval = DoSomething(argument1, argument2, argument3);
If the arguments do not all fit on one line, they should be
broken up onto multiple lines, with each subsequent line
aligned with the first argument. Do not add spaces after the
open paren or before the close paren:
bool retval = DoSomething(averyveryveryverylongargument1,
argument2, argument3);
If the function has many arguments, consider having one per
line if this makes the code more readable:
bool retval = DoSomething(argument1,
argument2,
argument3,
argument4);
If the function signature is so long that it cannot fit within
the maximum line length, you may
place all arguments on subsequent lines:
if (...) {
...
...
if (...) {
DoSomethingThatRequiresALongFunctionName(
very_long_argument1, // 4 space indent
argument2,
argument3,
argument4);
}
▶
Prefer no spaces inside parentheses. The
else
keyword belongs on a new line.
link
There are two acceptable formats for a basic conditional
statement. One includes spaces between the parentheses and the
condition, and one does not.
The most common form is without spaces. Either is fine, but
be consistent . If you are modifying a file, use the
format that is already present. If you are writing new code,
use the format that the other files in that directory or
project use. If in doubt and you have no personal preference,
do not add the spaces.
if (condition) { // no spaces inside parentheses
... // 2 space indent.
} else { // The else goes on the same line as the closing brace.
...
}
If you prefer you may add spaces inside the
parentheses:
if ( condition ) { // spaces inside parentheses - rare
... // 2 space indent.
} else { // The else goes on the same line as the closing brace.
...
}
Note that in all cases you must have a space between the
if and the open parenthesis. You must also have
a space between the close parenthesis and the curly brace, if
you're using one.
if(condition) // Bad - space missing after IF.
if (condition){ // Bad - space missing before {.
if(condition){ // Doubly bad.
if (condition) { // Good - proper space after IF and before {.
Short conditional statements may be written on one line if
this enhances readability. You may use this only when the
line is brief and the statement does not use the
else clause.
if (x == kFoo) return new Foo();
if (x == kBar) return new Bar();
This is not allowed if the if statement has an
else:
// Not allowed - IF statement on one line when there is an ELSE clause
if (x) DoThis();
else DoThat();
In general, curly braces are not required for single-line
statements, but they are allowed if you like them. Some
require that an if must always always have an
accompanying brace.
if (condition)
DoSomething(); // 2 space indent.
if (condition) {
DoSomething(); // 2 space indent.
}
However, if one part of an if-else
statement uses curly braces, the other part must too:
// Not allowed - curly on IF but not ELSE
if (condition) {
foo;
} else
bar;
// Not allowed - curly on ELSE but not IF
if (condition)
foo;
else {
bar;
}
// Curly braces around both IF and ELSE required because
// one of the clauses used braces.
if (condition) {
foo;
} else {
bar;
}
▶
Switch statements may use braces for blocks. Empty loop bodies should use
{} or
continue.
link
case blocks in switch statements can have
curly braces or not, depending on your preference. If you do
include curly braces they should be placed as shown below.
If not conditional on an enumerated value, switch statements
should always have a default case (in the case of
an enumerated value, the compiler will warn you if any values
are not handled). If the default case should never execute,
simply
assert:
switch (var) {
case 0: { // 2 space indent
... // 4 space indent
break;
}
case 1: {
...
break;
}
default: {
assert(false);
}
}
Empty loop bodies should use {} or
continue, but not a single semicolon.
while (condition) {
// Repeat test until it returns false.
}
for (int i = 0; i < kSomeNumber; ++i) {} // Good - empty body.
while (condition) continue; // Good - continue indicates no logic.
while (condition); // Bad - looks like part of do/while loop.
▶
No spaces around period or arrow. Pointer operators do not have
trailing spaces.
link
The following are examples of correctly-formatted pointer and
reference expressions:
x = *p;
p = &x;
x = r.y;
x = r->y;
Note that:
- There are no spaces around the period or arrow when
accessing a member.
- Pointer operators have no space after the
* or
&.
When declaring a pointer variable or argument, you may place
the asterisk adjacent to either the type or to the variable
name:
// These are fine, space preceding.
char *c;
const string &str;
// These are fine, space following.
char* c; // but remember to do "char* c, *d, *e, ...;"!
const string& str;
char * c; // Bad - spaces on both sides of *
const string & str; // Bad - spaces on both sides of &
You should do this consistently within a single file or
so when modifying an existing file, use the style in that
file.
▶
When you have a boolean expression that is longer than the
standard line length, be consistent in
how you break up the lines.
link
In this example, the logical AND operator is always at the end
of the lines:
if (this_one_thing > this_other_thing &&
a_third_thing == a_fourth_thing &&
yet_another & last_one) {
...
}
Note that both of the && logical AND
operators are the end of the line. Feel free to insert extra
parentheses judiciously, because they can be very helpful in
increasing readability when used appropriately.
▶
Do not surround the
return expression with parentheses.
link
Return values should not have parentheses:
return x; // not return(x);
▶
Your choice of
= or
().
link
You may choose between = and (); the
following are all correct:
int x = 3;
int x(3);
string name("Some Name");
string name = "Some Name";
▶
Preprocessor directives should not be indented but should
instead start at the beginning of the line.
link
Even when pre-processor directives are within the body of
indented code, the directives should start at the beginning of
the line.
// Good - directives at beginning of line
if (lopsided_score) {
#if DISASTER_PENDING // Correct -- Starts at beginning of line
DropEverything();
#endif
BackToNormal();
}
// Bad - indented directives
if (lopsided_score) {
#if DISASTER_PENDING // Wrong! The "#if" should be at beginning of line
DropEverything();
#endif // Wrong! Do not indent "#endif"
BackToNormal();
}
▶
Sections in
public,
protected and
private order, each indented one space.
link
The basic format for a class declaration (lacking the
comments, see Class Comments for
a discussion of what comments are needed) is:
class MyClass : public OtherClass {
public: // Note the 1 space indent!
MyClass(); // Regular 2 space indent.
explicit MyClass(int var);
~MyClass() {}
void SomeFunction();
void SomeFunctionThatDoesNothing() {
}
void set_some_var(int var) { some_var_ = var; }
int some_var() const { return some_var_; }
private:
bool SomeInternalFunction();
int some_var_;
int some_other_var_;
DISALLOW_COPY_AND_ASSIGN(MyClass);
};
Things to note:
- Any base class name should be on the same line as the
subclass name, subject to the 80-column limit.
- The
public:, protected:, and
private: keywords should be indented one
space.
- Except for the first instance, these keywords should be preceded
by a blank line. This rule is optional in small classes.
- Do not leave a blank line after these keywords.
- The
public section should be first, followed by
the protected and finally the
private section.
- See Declaration Order for
rules on ordering declarations within each of these sections.
▶
Constructor initializer lists can be all on one line or with
subsequent lines indented four spaces.
link
There are two acceptable formats for initializer lists:
// When it all fits on one line:
MyClass::MyClass(int var) : some_var_(var), some_other_var_(var + 1) {}
or
// When it requires multiple lines, indent 4 spaces, putting the colon on
// the first initializer line:
MyClass::MyClass(int var)
: some_var_(var), // 4 space indent
some_other_var_(var + 1) { // lined up
...
DoSomething();
...
}
▶
The contents of namespaces are not indented.
link
Namespaces do not add an extra level of
indentation. For example, use:
namespace {
void foo() { // Correct. No extra indentation within namespace.
...
}
} // namespace
Do not indent within a namespace:
namespace {
// Wrong. Indented when it should not be.
void foo() {
...
}
} // namespace
▶
Use of horizontal whitespace depends on location. Never put trailing
whitespace at the end of a line.
link
General
void f(bool b) { // Open braces should always have a space before them.
...
int i = 0; // Semicolons usually have no space before them.
int x[] = { 0 }; // Spaces inside braces for array initialization are
int x[] = {0}; // optional. If you use them, put them on both sides!
// Spaces around the colon in inheritance and initializer lists.
class Foo : public Bar {
public:
// For inline function implementations, put spaces between the braces
// and the implementation itself.
Foo(int b) : Bar(), baz_(b) {} // No spaces inside empty braces.
void Reset() { baz_ = 0; } // Spaces separating braces from implementation.
...
Adding trailing whitespace can cause extra work for others editing
the same file, when they merge, as can removing existing trailing
whitespace. So: Don't introduce trailing whitespace. Remove it
if you're already changing that line, or do it in a separate
clean-up
operation (preferably when no-one else
is working on the file).
Loops and Conditionals
if (b) { // Space after the keyword in conditions and loops.
} else { // Spaces around else.
}
while (test) {} // There is usually no space inside parentheses.
switch (i) {
for (int i = 0; i < 5; ++i) {
switch ( i ) { // Loops and conditions may have spaces inside
if ( test ) { // parentheses, but this is rare. Be consistent.
for ( int i = 0; i < 5; ++i ) {
for ( ; i < 5 ; ++i) { // For loops always have a space after the
... // semicolon, and may have a space before the
// semicolon.
switch (i) {
case 1: // No space before colon in a switch case.
...
case 2: break; // Use a space after a colon if there's code after it.
Operators
x = 0; // Assignment operators always have spaces around
// them.
x = -5; // No spaces separating unary operators and their
++x; // arguments.
if (x && !y)
...
v = w * x + y / z; // Binary operators usually have spaces around them,
v = w*x + y/z; // but it's okay to remove spaces around factors.
v = w * (x + z); // Parentheses should have no spaces inside them.
Templates and Casts
vector<string> x; // No spaces inside the angle
y = static_cast<char*>(x); // brackets (< and >), before
// <, or between >( in a cast.
vector<char *> x; // Spaces between type and pointer are
// okay, but be consistent.
set<list<string> > x; // C++ requires a space in > >.
set< list<string> > x; // You may optionally make use
// symmetric spacing in < <.
▶
Minimize use of vertical whitespace.
link
This is more a principle than a rule: don't use blank lines
when you don't have to. In particular, don't put more than
one or two blank lines between functions, don't start or end
functions with a blank line, and be discriminating with your
use of blank lines inside functions.
The basic principle is: The more code that fits on one screen,
the easier it is to follow and understand the control flow of
the program. Of course, readability can suffer from code
being too dense as well as too spread out, so use your
judgement. But in general, minimize use of vertical
whitespace.
Don't start or end functions with blank lines:
void Function() {
// Unnecessary blank lines before and after
}
Don't start and end blocks with blank lines either:
while (condition) {
// Unnecessary blank line after
}
if (condition) {
// Unnecessary blank line before
}
However, it's okay to add blank lines between a chain of
if-else blocks:
if (condition) {
// Some lines of code too small to move to another function,
// followed by a blank line.
} else {
// Another block of code
}
Exceptions to the Rules
The coding conventions described above are mandatory. However,
like all good rules, these sometimes have exceptions, which we
discuss here.
▶
You may diverge from the rules when dealing with code that does not
conform to this style guide.
link
If you find yourself modifying code that was written to
specifications other than those presented by this guide, you may
have to diverge from these rules in order to stay consistent with
the local conventions in that code. If you are in doubt about
how to do this, ask the original author or the person currently
responsible for the code. Remember that consistency
includes local consistency, too.
▶
Windows programmers have developed their own set of coding
conventions, mainly derived from the conventions in Windows headers
and other Microsoft code. We want to make it easy for anyone to
understand your code, so we have a single set of guidelines for
everyone writing C++ on any platform.
link
It is worth reiterating a few of the guidelines that you might
forget if you are used to the prevalent Windows style:
- Do not use Hungarian notation (for example, naming an
integer
iNum). Use the Google naming conventions,
including the .cc extension for source files.
- Windows defines many of its own synonyms for primitive
types, such as
DWORD, HANDLE, etc.
It is perfectly acceptable, and encouraged, that you use these
types when calling Windows API functions. Even so, keep as
close as you can to the underlying C++ types. For example, use
const TCHAR * instead of LPCTSTR.
- When compiling with Microsoft Visual C++, set the
compiler to warning level 3 or higher, and treat all
warnings as errors.
- Do not use
#pragma once; instead use the
standard Google include guards. The path in the include
guards should be relative to the top of your project
tree.
- In fact, do not use any nonstandard extensions, like
#pragma and __declspec, unless you
absolutely must. Using __declspec(dllimport) and
__declspec(dllexport) is allowed; however, you
must use them through macros such as DLLIMPORT
and DLLEXPORT, so that someone can easily disable
the extensions if they share the code.
However, there are just a few rules that we occasionally need
to break on Windows:
- Normally we forbid
the use of multiple implementation inheritance; however,
it is required when using COM and some ATL/WTL
classes. You may use multiple implementation inheritance
to implement COM or ATL/WTL classes and interfaces.
- Although you should not use exceptions in your own code,
they are used extensively in the ATL and some STLs,
including the one that comes with Visual C++. When using
the ATL, you should define
_ATL_NO_EXCEPTIONS to
disable exceptions. You should investigate whether you can
also disable exceptions in your STL, but if not, it is OK to
turn on exceptions in the compiler. (Note that this is
only to get the STL to compile. You should still not
write exception handling code yourself.)
- The usual way of working with precompiled headers is to
include a header file at the top of each source file,
typically with a name like
StdAfx.h or
precompile.h. To make your code easier to share
with other projects, avoid including this file explicitly
(except in precompile.cc), and use the
/FI compiler option to include the file
automatically.
- Resource headers, which are usually named
resource.h and contain only macros, do not need
to conform to these style guidelines.
Parting Words
Use common sense and BE CONSISTENT .
If you are editing code, take a few minutes to look at the
code around you and determine its style. If they use spaces
around their if clauses, you should, too. If
their comments have little boxes of stars around them, make
your comments have little boxes of stars around them too.
The point of having style guidelines is to have a common
vocabulary of coding so people can concentrate on what you are
saying, rather than on how you are saying it. We present
global style rules here so people know the vocabulary. But
local style is also important. If code you add to a file
looks drastically different from the existing code around it,
the discontinuity throws readers out of their rhythm when they
go to read it. Try to avoid this.
OK, enough writing about writing code; the code itself is much
more interesting. Have fun!
posted on 2008-07-20 09:17
chatler 阅读(1600)
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