tommy

《windows图形编程》有讲：

KTimer.h

#pragma once
inline unsigned __int64 GetCycleCount( void )
{
    _asm _emit  0x0F
    _asm _emit  0x31
}

class  KTimer  
{
    unsigned __int64 m_startcycle;
public :
    unsigned __int64 m_overhead;     // RTSC指令的运行时间

    KTimer()
     {
        m_overhead  =   0 ;
        Start();
        m_overhead  =  Stop();
    }
     void  Start();
    unsigned __int64 Stop();
    unsigned unsigned GetCPUSpeed();

} ;

#include "KTimer.h"

#include <iostream>
#include <windows.h>


void KTimer::Start()
{
    m_startcycle = GetCycleCount();
}
unsigned __int64 KTimer::Stop()
{
    return GetCycleCount() - m_startcycle - m_overhead;
}
unsigned unsigned KTimer::GetCPUSpeed()
{
    cout << "开始测试 cpu速度.." << endl;
    Start();
    Sleep(1000);
    unsigned cputime = Stop();
    unsigned cpuspeed10 = (unsigned)(cputime/100000);
    cout << "CPU速度 每秒:" << cputime << " clocks" << endl;
    return cpuspeed10 == 0 ? 1 : cpuspeed10;
}

#include "stdafx.h"
#include <tchar.h>
#include <windows.h>
#include <iostream>

#include "KTimer.h"

int main(int argc, char* argv[])
{    
    KTimer timer;

    unsigned cpuspeed10 = timer.GetCPUSpeed();

    timer.Start();
    
//做耗时操作
    
    unsigned time = timer.Stop();

    TCHAR mess[128];
    wsprintf(mess,_T("耗时:%d ns"), time * 10000 / cpuspeed10);
    cout << mess << endl;

    return 0;
}

书里面说的这个词：
型别计算的边界标记

NullType只有声明没有定义。

class NullType;

struct EmptyType {};

简而言之：explicit修饰的构造函数不能担任转换函数

这个 《ANSI/ISO C++ Professional Programmer's Handbook 》是这样说的

explicit Constructors
A constructor that takes a single argument is, by default, an implicit conversion operator, which converts its argument to
an object of its class (see also Chapter 3, "Operator Overloading"). Examine the following concrete example:
class string
{
private:
int size;
int capacity;
char *buff;
public:
string();
string(int size); // constructor and implicit conversion operator
string(const char *); // constructor and implicit conversion operator
~string();
};
Class string has three constructors: a default constructor, a constructor that takes int, and a constructor that
constructs a string from const char *. The second constructor is used to create an empty string object with an
initial preallocated buffer at the specified size. However, in the case of class string, the automatic conversion is
dubious. Converting an int into a string object doesn't make sense, although this is exactly what this constructor does.

Consider the following:
int main()
{
string s = "hello"; //OK, convert a C-string into a string object
int ns = 0;
s = 1; // 1 oops, programmer intended to write ns = 1,
}
In the expression s= 1;, the programmer simply mistyped the name of the variable ns, typing s instead. Normally,
the compiler detects the incompatible types and issues an error message. However, before ruling it out, the compiler first
searches for a user-defined conversion that allows this expression; indeed, it finds the constructor that takes int.
Consequently, the compiler interprets the expression s= 1; as if the programmer had written
s = string(1);
You might encounter a similar problem when calling a function that takes a string argument. The following example
can either be a cryptic coding style or simply a programmer's typographical error. However, due to the implicit
conversion constructor of class string, it will pass unnoticed:
int f(string s);
int main()
{
f(1); // without a an explicit constructor,
//this call is expanded into: f ( string(1) );
//was that intentional or merely a programmer's typo?
}
'In order to avoid such implicit conversions, a constructor that takes one argument needs to be declared explicit:
class string
{
//...
public:
explicit string(int size); // block implicit conversion
string(const char *); //implicit conversion
~string();
};
An explicit constructor does not behave as an implicit conversion operator, which enables the compiler to catch the
typographical error this time:
int main()
{
string s = "hello"; //OK, convert a C-string into a string object
int ns = 0;
s = 1; // compile time error ; this time the compiler catches the typo
}
Why aren't all constructors automatically declared explicit? Under some conditions, the automatic type conversion is
useful and well behaved. A good example of this is the third constructor of string:
string(const char *);

The implicit type conversion of const char * to a string object enables its users to write the following:
string s;
s = "Hello";
The compiler implicitly transforms this into
string s;
//pseudo C++ code:
s = string ("Hello"); //create a temporary and assign it to s
On the other hand, if you declare this constructor explicit, you have to use explicit type conversion:
class string
{
//...
public:
explicit string(const char *);
};
int main()
{
string s;
s = string("Hello"); //explicit conversion now required
return 0;
}
Extensive amounts of legacy C++ code rely on the implicit conversion of constructors. The C++ Standardization
committee was aware of that. In order to not make existing code break, the implicit conversion was retained. However, a
new keyword, explicit, was introduced to the languageto enable the programmer to block the implicit conversion
when it is undesirable. As a rule, a constructor that can be invoked with a single argument needs to be declared
explicit. When the implicit type conversion is intentional and well behaved, the constructor can be used as an
implicit conversion operator.

就是这样一个结构：

template <typename T>
struct Type2Type
{
    typedef T OriginalType;    
};

template <class T,class U>
T* Create(const U& arg)
{
    return new T(arg);
}

//错误的代码
template <class U>
Widget* Create<Widget,U>(const U& arg)
{
    return new Widget(arg,-1);
}

template <class T,class U>
T* Create(const U&arg,T /**//*虚拟*/)
{
   return new T(arg);
}

template <class U>
Widget * Create(const U& arg, Widget /**//*虚拟*/)
{
    return new Widget(arg,-1);
}

  这样是可以解决问题，但最大的毛病在于运行时构造了 未被使用的对象这个开销（虚拟的Widget参数）。这时 Type2Type 这个咚咚出场了，按照书的说法，这是“一个型别代表物、一个可以让你传给重载函数的轻量级ID”，如下：

template <class T,class U>
T* Create(const U& arg,Type2Type<T>)
{
    return new T(arg);
}

template <class U>
Widget * Create(const U& arg,Type2Type<Widget>)
{
    return new Widget(arg,-1);
}

调用方：
String *pStr = Create("hello",Type2Type<String>());
Widget *pW = Create(100,Type2Type<Widget>());

关键是，这个东西也是给编译器看的，妙