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Copy

哪些情况会发生copy？

在下面文章中，对此进行了介绍:

1、cplusplus When a copy constructor is called

2、ariya C++ Class and Preventing Object Copy

cplusplus When a copy constructor is called

The following are the cases when copy constructor is called.

1、When instantiating one object and initializing it with values from another object.

2、When passing an object by value.

3、When an object is returned from a function by value.

demo:

#include <iostream>

struct A
{
    A() {}
    A( const A& ) { std::cout << "A::copy_constructor is called to make a copy\n" ; }
    // other stuff
    A( A&& ) { std::cout << "A::move_constructor is called to make a copy\n" ; }
};

void foo( A object ) 
{ 
    std::cout << "foo has recieved a copy of an object\n";
}

A bar() 
{ 
    std::cout << "bar - about to return an object by value\n" ; 
    static A a ; 
    return a ; 

}

int main()
{
    std::cout << "about to call bar\n" ;
    A object = bar() ;
    std::cout << "bar has returned\n" ;

    std::cout << "\n\nabout to call foo\n" ;
    foo(object) ;
}

上面两个函数演示了2和3；下面这段解释了为什么A object = bar()没有触发move constructor。

A( A&& ) 
{ 
    std::cout << "A::move_constructor is called to make a copy\n" ; 
}
A object = bar() ;

In practice, the move constructor will never be used in this particular program.

bar() can't use the move constructor to make a copy. And RVO enables copy-elision of the anonymous return value.

http://ideone.com/iWp77h

Requiring and Prohibiting、copyable and non-copyable

1、non-copyable: 参见 Non-copyable 章节