C/C++中的拷贝构造函数和赋值构造函数

代码：

 #include <iostream>
 #include <cstdio>

 using namespace std;

 class A{
     public:
         A(){
             cout<<"construct"<<endl;
         }
         ~A(){
             cout<<"destory"<<endl;
         }
         A(const A& a){
             cout<<"copy construct"<<endl;
         }
         A& operator=(const A& a){
             cout<<"assert construct"<<endl;
         }
 };

 //const A& func(const A& a){ //返回值也是引用，不调用拷贝构造函数
 A func(const A& a){ //返回的时候需要调用拷贝构造函数
     cout<<"in the func"<<endl;
     return a;
 }

 int main(){

     A a1,a2;
     cout<<"function called"<<endl;
     a2 = func(a1);
     cout<<"function finished"<<endl;

     return ;
 }

输出：

construct
construct
function called
in the func
copy construct
assert construct
destory
function finished
destory
destory

分析：

如果将代码23行注释掉并改为22行，输出变为

construct
construct
function called
in the func
assert construct
function finished
destory
destory