在哪种情况下是C ++复制构造函数调用？ [英] In which situations is the C++ copy constructor called?

问题描述

我知道在c ++中将调用复制构造函数的以下情况：

1. 它自己类的对象

     MyClass A，B; 
    A = new MyClass（）; 
    B = A; //复制构造函数称为
     




2. 如果函数接收到作为参数传递的值，的类

     void foo（MyClass a）; 
    foo（a）; //复制构造函数调用
     




3.   MyClass foo（）
    {
    MyClass temp; 
    .... 
    return temp; // copy constructor called 
   } 
     

请随时更正我所做的任何错误;但我更好奇，如果有任何其他情况下，其中复制构造函数被调用。

解决方案

，但是这个类允许你看到什么叫什么时候：

  class a {
 public：
a（）{
 printf（constructor called\\\
）; 
}; 
 a（const a& other）{
 printf（copy constructor called\\\
）; 
}; 
 a& operator =（const a& other）{
 printf（copy assignment operator called\\\
）; 
 return * this; 
}; 
}; 
  

那么这段代码：

  ab; // constructor 
 a c; // constructor 
 b = c; // copy assignment 
 c = a（b）; //复制构造函数，然后复制赋值
  

产生结果：

构造函数
构造函数称为
复制赋值运算符
复制构造函数称为
复制赋值运算符


另一个有趣的事情，说你有以下代码：

  a * b = new a（）; // constructor called 
 a * c; //什么都不叫
 c = b; //仍然没有调用
 c = new a（* b）; //复制构造函数被调用
  

这是因为当你分配一个指针时，实际对象。

I know of the following situations in c++ where the copy constructor would be invoked:

1. when an existing object is assigned an object of it own class

   MyClass A,B;
   A = new MyClass();
   B=A; //copy constructor called 
   

2. if a functions receives as argument, passed by value, an object of a class

   void foo(MyClass a);
   foo(a); //copy constructor invoked
   

3. when a function returns (by value) an object of the class

   MyClass foo ()
      {
         MyClass temp;
         ....
         return temp; //copy constructor called
      } 
   

Please feel free to correct any mistakes I've made; but I am more curious if there are any other situations in which the copy constructor is called.

解决方案

I might be wrong about this, but this class allows you to see what is called and when:

class a {
public:
    a() {
        printf("constructor called\n");
    };  
    a(const a& other) { 
        printf("copy constructor called\n");
    };    
    a& operator=(const a& other) {
        printf("copy assignment operator called\n");
        return *this; 
    };
};

So then this code:

a b; //constructor
a c; //constructor
b = c; //copy assignment
c = a(b); //copy constructor, then copy assignment

produces this as the result:

constructor called
constructor called
copy assignment operator called
copy constructor called
copy assignment operator called

Another interesting thing, say you have the following code:

a* b = new a(); //constructor called
a* c; //nothing is called
c = b; //still nothing is called
c = new a(*b); //copy constructor is called

This occurs because when you when you assign a pointer, that does nothing to the actual object.

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