C++使用返回的對象來初始化一個對象

Question

我對C++的返回值機制感到困惑，並且我編寫了以下代碼來證明我的opion和代碼的結果（在它後面有「？」，輸出是大膽）混淆了我，任何人都可以解釋爲什麼它輸出，或者只是因爲我的編譯器供應商（MS Visual C++）爲我優化？

#include <iostream> 

class WInt 
{ 
public: 
    WInt(int a) : a(a){ std::cout << a << " " << "A constructor" << std::endl; } 
    WInt(const WInt& a) 
    { 
     std::cout << "copy constructor run" << std::endl; 
     this->a = a.a; 
    } 
    ~WInt(){ std::cout << "WInt destructor" << std::endl; } 

    WInt& operator=(const WInt& v) 
    { 
     std::cout << "assignment operator" << std::endl; 
     this->a = v.a; 
     return *this; 
    } 

    friend const WInt operator+(const WInt& v1, const WInt& v2) 
    { 
     return WInt(v1.a + v2.a); 
    } 

private: 
    int a; 
}; 

int main(int argc, char* argv[]) 
{ 
    std::cout << "-----------" << std::endl; 
    WInt a(1); // run constructor 
    WInt b(2); // run constructor 

    std::cout << "-----------" << std::endl; 
    WInt c = a + b; // ??????????????????? 

    std::cout << "-----------" << std::endl; 
    WInt d(a + b); // ??????????????????? 

    std::cout << "-----------" << std::endl; 
    c = a + b + c; // run the +, +, =, ~, ~ 

    std::cout << "-----------" << std::endl; 
    WInt e = c; // run copy constructor 

    std::cout << "-----------" << std::endl; 

    return 0; 
} 

，輸出是：

----------- 

1 A constructor 

2 A constructor 

----------- 

**3 A constructor** 

----------- 

**3 A constructor** 

----------- 

3 A constructor 

6 A constructor 

assignment operator 

WInt destructor 

WInt destructor 

----------- 

copy constructor run 

----------- 

WInt destructor 

WInt destructor 

WInt destructor 

WInt destructor 

WInt destructor 

Answer 1

這是return value optimization。您的編譯器正在優化不必要的副本（儘可能最佳）。

編輯：檢查this問題的進一步解釋。