copy constructor & operator =

// copyconstructor.cpp : 定义控制台应用程序的入口点。

//

#include "stdafx.h"

#include <iostream>

using namespace std;

class A

{

public:

 A(int num)

 {

  cout<<"the constructor of A is called"<<endl;

  m_num = num;

 }

 A(A& source)

 {

  cout<<"the copy constructor of A is called"<<endl;

  m_num = source.m_num;

  ++source.m_num;

 }

 A operator = (A source)

 {

  //在进入函数之前,会调用一次copy constructor,情况是以object作为参数传入,此时因为是传值

  //所以会调用,如果参数是以引用传入时A& source,就不会调用copy constructor,因为初值已有了

  cout<<"the operator = of A is called"<<endl;

  m_num = source.m_num;

  //此时因为是以object作为返回值,所以会调用copy constructor

  cout<<"the operator = of A calling is finished"<<endl;

  return source;  

 }

private:

 int m_num;

};

int _tmain(int argc, _TCHAR* argv[])

{

 A a(1);

 A b(2) ;

 b = a;  //此时不是以object为另一个object赋初值,所以不会调用copy constructor,而是调用operator =

 A c = a; //在这里是以一个object为另一个object赋初值,所以会调用copy constructor

 return 0;

}