Introduction
What are smart pointers? The answer is fairly simple; a smart pointer is a pointer which is smart. What does that mean? Actually smart pointers are objects which behave like pointers, but do more than a pointer. These objects are flexible as pointers and have the advantage of being an object (like constructor and destructors called automatically). A smart pointer is designed to handle the problems caused by using normal pointers (hence called smart).
Problems with Pointers
What are the common problems we face in C++ programs while using pointers? The answer is memory management. Have a look at the following code.
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Copy Code char* pName = new char[1024];
…
SetName(pName);
…
…
if(null != pName)
{
delete[] pName;
}
How many times we found out a bug which was caused because we forgot deleting ‘pName’. It would be great if somebody takes care of releasing the memory when the pointer is not useful (we are not talking about the garbage collector here). What if the pointer itself takes care of that, yes that’s exactly what smart pointer is intended to do. Let us write a smart pointer and see how we can handle a pointer better.
We shall start with a realistic example. Let’s say we have a class called Person which is defined as listed below.
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class Person
{
int age;
char* pName;
public:
Person(): pName(0),age(0)
{
}
Person(char* pName, int age): pName(pName), age(age)
{
}
~Person()
{
}
void Display()
{
printf("Name = %s Age = %d \n", pName, age);
}
void Shout()
{
printf("Ooooooooooooooooo",);
}
};
Now we shall write the client code to use Person
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void main()
{
Person* pPerson = new Person("Scott", 25);
pPerson->Display();
delete pPerson;
}
Now look at this code, every time I create a pointer I need to take care of deleting it, this is exactly what I want to avoid. I need some automatic mechanism which deletes the pointer. One thing which strikes to me is a destructor. But pointers do not have destructors, so what our smart pointer can have one. So we will create a class called SP which can hold a pointer to the person class and will delete the pointer when its destructor is called. Hence my client code will change to something like this.
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void main()
{
SP p(new Person("Scott", 25));
p->Display();
}
Note the following things
We have created an object of class SP which holds our Person class pointer. Since the destructor of SP class will be called when this object goes out of scope, it will delete the Person class pointer (as its main responsibility); hence we don’t have the pain of deleting the pointer.
One more thing of major importance is that we should be able to call the Display method using the SP class object the way we used to call using the Person class pointer, i.e. the class should behave exactly like a pointer.
Interface for a smart pointer:
Since the smart pointer should behave like a pointer, it should support the same interface as the pointers do; i.e. they should support the following operations.
Dereferencing (operator *)
Indirection (operator ->)
Let us write the SP class now
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class SP
{
private:
Person* pData; public:
SP(Person* pValue) : pData(pValue)
{
}
~SP()
{
delete pData;
}
Person& operator* ()
{
return *pData;
}
Person* operator-> ()
{
return pData;
}
};
This class is our smart pointer class. The main responsibility of this class is to hold a pointer to Person class, and then delete it when its destructor is called. It should also support the interface of the pointer.
Generic smart pointer class
One problem which we see here is that we can use this smart pointer class for pointer of Person class only. This means that we have to create each smart pointer class for each type, that’s not easy. We can solve this problem by making use of templates and make this smart pointer class generic. So let us change the code like this.
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template < typename T > class SP
{
private:
T* pData; public:
SP(T* pValue) : pData(pValue)
{
}
~SP()
{
delete pData;
}
T& operator* ()
{
return *pData;
}
T* operator-> ()
{
return pData;
}
};
void main()
{
SP
p(new Person("Scott", 25));
p->Display();
}
Now we can use our smart pointer class for any type of pointers. So is our smart pointer really smart? Check the following code segment.
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void main()
{
SP
p(new Person("Scott", 25));
p->Display();
{
SP
q = p;
q->Display();
}
p->Display();
}
Look what happens here p and q are referring to the same Person class pointer, now when q goes out of scope the destructor of q will be called which deletes the Person class pointer. Now we cannot call p->Display(); since p will be left with a dangling pointer and this call will fail. (Note that this problem would have existed even if we were using normal pointers instead of smart pointers) We should not delete the Person class pointer unless no body is using it. How do we do that? Implementing reference counting mechanism in our smart pointer class will solve this problem.
Reference counting
What we are going to do is we will have a reference counting class RC. This class will maintain an integer value which represents the reference count. We will have methods to increment and decrement the reference count.
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class RC
{
private:
int count;
public:
void AddRef()
{
count++;
}
int Release()
{
return --count;
}
};
Now we have a reference counting class, we will introduce this to our smart pointer class. We will maintain a pointer to class RC in our SP class and this pointer will be shared for all instance of the smart pointer which refers to the same pointer. For this to happen we need to have an assignment operator and copy constructor in our SP class.
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template < typename T > class SP
{
private:
T* pData; RC* reference;
public:
SP() : pData(0), reference(0)
{
reference = new RC();
reference->AddRef();
}
SP(T* pValue) : pData(pValue), reference(0)
{
reference = new RC();
reference->AddRef();
}
SP(const SP<T>& sp) : pData(sp.pData), reference(sp.reference)
{
reference->AddRef();
}
~SP()
{
if(reference->Release() == 0)
{
delete pData;
delete reference;
}
}
T& operator* ()
{
return *pData;
}
T* operator-> ()
{
return pData;
}
SP<T>& operator = (const SP<T>& sp)
{
if (this != &sp) {
if(reference->Release() == 0)
{
delete pData;
delete reference;
}
pData = sp.pData;
reference = sp.reference;
reference->AddRef();
}
return *this;
}
};
Let us have a look at the client code.
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void main()
{
SP
p(new Person("Scott", 25));
p->Display();
{
SP
q = p;
q->Display();
SP
r;
r = p;
r->Display();
}
p->Display();
}
When we create a smart pointer p of type person, the constructor of SP will be called, the data will be stored and a new RC pointer will be created. The AddRef method of RC is called to increment the reference count to 1. Now SP
q = p; will create a new smart pointer q using the copy constructor, here the data will be copied and reference will again incremented to 2. Now r = p; will call the assignment operator to assign the value of p to q, here also we copy the data and increment the reference count, thus making the count 3. When r and q goes out of scope the destructors of respective objects will be called, here the reference count will be decremented, but data will not be deleted unless the reference count becomes zero, this happens only when destructor of p is called. Hence our data will be deleted only when no body is referring to it.
Applications
Memory leaks: Using smart pointers reduces work of managing pointers for memory leaks. Now you could create a pointer and forget about deleting it, the smart pointer will do that for you. This is the simplest garbage collector we could think off.
Exceptions: Smart pointers are very useful where exceptions are used. For example look at the following code.
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void MakeNoise()
{
Person* p = new Person("Scott", 25);
p->Shout();
delete p;
}
We are using a normal pointer here and deleting it after using, so every thing looks okay here. But what if our Shout function thows some exception delete p; will never be called. So we have a memory leak, let us handle that.
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void MakeNoise()
{
Person* p = new Person("Scott", 25);
try
{
p->Shout();
}
catch(...)
{
delete p;
throw;
}
delete p;
}
Don't you think this is a over head of catching an exception and rethrowing it? This code becomes cumbersome if you have many pointes created. How will a smart pointer help here, lets have a look at the same code if smart pointer is used.
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void MakeNoise()
{
SP<Person>
p(new Person("Scott", 25));
p->Shout();
}
We are making use of a smart pointer here; yes we don’t need to catch exception here. If the Shout method throws an exception stack unwinding will happen for the function and during this the destructor of all local object will be called, hence destructor of p will be called which will release the memory hence we are safe. So this makes it very useful to use smart pointers here.
Conclusion
Smart pointers are useful for writing safe and efficient code in C++. Make use of smart pointers and take the advantage of garbage collection. Take a look at Scott Meyers' auto_ptr implementation in STL.