请教一个构造函数的问题

bxb_koala

1. #include <iostream>
   
2. 
   
3. using namespace std;
   
4. 
   
5. class A
   
6. {
   
7. public:
   
8.     A(int)
   
9.     {
   
10.         cout << "11111" << endl;;
    
11.     };
    
12.     A(const A&)
    
13.     {
    
14.         cout << "22222" << endl;;
    
15.     };
    
16. };
    
17. 
    
18. 
    
19. int main()
    
20. {
    
21.     A a2 = A(47);
    
22.     return 0;
    
23. }

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以上代码，我认为应该输出两行“11111”和“22222”，但运行结果只有一行“11111”。1 A(47)调用构造函数生成临时对象输出“11111” 2 这个临时对象初始化a2对象，此时应该输出“22222”。
谁能解释一下原因呢？

chary8088

you havn't understood what the copy construct function is at all!
see the codes I just modified, which is your expected!

1. #include <iostream>
   
2. 
   
3. using namespace std;
   
4. 
   
5. class A
   
6. {
   
7. public:
   
8.    A(int)
   
9.    {
   
10.      cout << "11111" << endl;;
    
11.    };
    
12.    A(const A&)
    
13.   {
    
14.      cout << "22222" << endl;;
    
15.   };
    
16. };
    
17. 
    
18. int main()
    
19. {
    
20.     A a1(47);
    
21.     A a2 = a1;
    
22.     return 0;
    
23. }

复制代码

namelij

去搜一下 RNV，你就明白了

namelij

汗
NRV name return value 优化

int-main

c++对象模型

chary8088

because the gcc use the NRVO option to optimaze the codes for improving the performance, but the MS Studio doesn't use the NRVO;

The Visual C++ 8.0 compiler makes use of the flexibility that the standard provides and adds a new feature: Named Return Value Optimization (NRVO). NRVO eliminates the copy constructor and destructor of a stack-based return value. This optimizes out the redundant copy constructor and destructor calls and thus improves overall performance. It is to be noted that this could lead to different behavior between optimized and non-optimized programs (see the Optimization Side Effects section).

supermegaboy

以上代码，我认为应该输出两行“11111”和“22222”，但运行结果只有一行“11111”。1 A(47)调用构造函数生 ...
bxb_koala 发表于 2011-06-14 17:03

    这是命名返回值优化，为了消除复制构造函数对性能的影响，在符合条件的情况下，编译器可以将函数的返回值隐式修改为引用，因此A(47)是直接在a2上构造的，无需经过复制构造函数。

bxb_koala

这是命名返回值优化，为了消除复制构造函数对性能的影响，在符合条件的情况下，编译器可以将函数 ...
supermegaboy 发表于 2011-06-14 19:27

It makes sense, Thank you all guys.I found a useful article about NRV written by Stan Lippman.Refer to the below link:
http://blogs.msdn.com/b/slippman/archive/2004/02/03/66739.aspx