复制construtor称为额外 [英] copy construtor called extra

问题描述

我有一个程序显示奇怪的行为

I have a program which is showing weird behaviour

#include <cstdlib>
#include <iostream>

using namespace std;
class man{
           int i ;
      public:
           man(){
             i=23;
             cout << "\n defaul constructir called\t"<< i<<"\n";
           }
           man (const man & X) {
               i = 24;
           cout << "\n COPY constructir called\t"<< i<<"\n";
           }       
           man &  operator = (man x ) {
               i = 25;
               cout << "\n = operator  called\t"<< i<<"\n"; 
               return *this;
           }      
      };

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

     man x;
     cout <<"\n ----------\n";
     man y = x;
     cout <<"\n ----------\n";
     x=y;


    return 0;
}

 defaul constructir called  23

 ----------

 COPY constructir called    24

 ----------

 COPY constructir called    24

 = operator  called 25

这个输出对x = y的第三次调用很奇怪;

This output is weird for the third call of x=y ;

为什么当我没有创建一个新对象而是使用旧对象时，会调用一个额外的副本打印。

Why is there an extra print of copy construtor called when I did not made a new object but am working with old objects .

这是因为两者之间的临时对象，如果是，我可以在这里停止....

Is it becuase of temporary objects in between and if yes can i stop them here ....

推荐答案

man& operator =(man x);

您的参数通过 - 值接受其参数，将调用复制构造函数。这就是造成额外不必要的电话。通过引用传递参数将避免一个副本，但是你将不能通过临时（通常称为右值）：

Your parameter takes its argument by-value, and when that happens it will invoke the copy-constructor. That's what's causing the extra unwanted call. Passing your argument by reference will avoid a copy, but then you will not be able to pass temporaries (commonly referred to as rvalues):

struct man
{
    man& operator =(man& x) { return *this; };
};

int main()
{
    man a, b;

    a = b;     // works
    a = man(); // error: no viable overloaded '=',
               // expected an l-value for 1st argument
}

通过引用 const 将允许复制lvalue和rvalues：

Passing by reference to const will allow the copy-construction of both lvalues and rvalues:

man& operator =(man const& x);
//                  ^^^^^

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