按值返回時複製構造函數奇怪的行爲

Question

我測試了複製構造函數的功能，通過值返回一個對象的行爲，我遇到了複製構造函數確實被調用的情況下，而不是它的情況。

請考慮下面的代碼：

class A { 
public: 
    A() {} 
    A(const A& a) { 
     cout << "Copy CTOR: " << "This address is " << this 
      << " input address is "<< &a <<  "\n"; 
    } 
}; 

A returnMyself(A& a) { 
    cout<<"Myself address is: "<< &a << "\n"; 
    return a; 
} 

A returnLocal(A& a) { 
    A local; 
    cout<<"local address in returnLocal is "<< &local << "\n"; 
    return local; 
} 

int main() { 
    A a; 
    cout<<"Before returnMyself\n"; 
    returnMyself(a); 
    cout<<"After returnMyself\n\n"; 

    cout<<"Before returnLocal\n"; 
    returnLocal(a); 
    cout<<"After returnLocal\n"; 
} 

的main輸出是：

 
Before returnMyself. 
Myself address is: 0x7fff6afd88f0. 
Copy CTOR Invoked: This address is 0x7fff6afd88d8. Input address is 0x7fff6afd88f0. 
After returnMyself. 

Before returnLocal. 
Local address in returnLocal is 0x7fff6afd88d0. 
After returnLocal. 

正如你所看到的，當我宣佈一個本地對象並返回它，拷貝構造函數不被調用，而不是返回給定的引用對象，它調用複製構造函數。

沒有人有這個解釋？一般情況下，從函數返回值調用複製構造函數的情況是什麼？

謝謝！

Answer 1

這是copy elision。編譯器非常聰明，可以優化該拷貝。