对复制构造函数和析构函数的无关调用 [英] extraneous calls to copy-constructor and destructor

问题描述

[a跟踪此问题]

class A
{
    public:
         A()          {cout<<"A Construction"     <<endl;}
         A(A const& a){cout<<"A Copy Construction"<<endl;}
        ~A()          {cout<<"A Destruction"      <<endl;}
};

int main() {
    {
        vector<A> t;
        t.push_back(A());
        t.push_back(A());   // once more
    }
}

输出为：

A Construction        // 1
A Copy Construction   // 1
A Destruction         // 1
A Construction        // 2
A Copy Construction   // 2
A Copy Construction   // WHY THIS?
A Destruction         // 2
A Destruction         // deleting element from t
A Destruction         // deleting element from t
A Destruction         // WHY THIS?

推荐答案

为清楚地了解发生了什么，我建议将在输出中使用 this 指针来标识哪个A正在调用该方法。

To clearly see what's going on, I recommend include the this pointer in the output to identify which A is calling the method.

     A()          {cout<<"A (" << this << ") Construction"     <<endl;}
     A(A const& a){cout<<"A (" << &a << "->" << this << ") Copy Construction"<<endl;}
    ~A()          {cout<<"A (" << this << ") Destruction"      <<endl;}

我得到的输出是

A (0xbffff8cf) Construction
A (0xbffff8cf->0x100160) Copy Construction
A (0xbffff8cf) Destruction
A (0xbffff8ce) Construction
A (0x100160->0x100170) Copy Construction
A (0xbffff8ce->0x100171) Copy Construction
A (0x100160) Destruction
A (0xbffff8ce) Destruction
A (0x100170) Destruction
A (0x100171) Destruction

所以流程可以解释为：

1. 创建了临时A（…cf）。


2. 将临时A（…cf）复制到向量（…60）。


3. 临时A（…cf）被破坏。


4. 另一个临时A（…ce）被创建。


5. 展开向量，并将该向量中的旧A（…60）复制到新位置（…70）


6. 另一个临时A（ …ce）被复制到向量（…71）中。


7. 所有不必要的A副本（... 60，…ce）现在都被销毁。


8. 向量被破坏了，所以里面的A（…70，…71）也被破坏了。

1. The temporary A (…cf) is created.
2. The temporary A (…cf) is copied into the vector (…60).
3. The temporary A (…cf) is destroyed.
4. Another temporary A (…ce) is created.
5. The vector is expanded, and the old A (…60) in that vector is copied to the new place (…70)
6. The other temporary A (…ce) is copied into the vector (…71).
7. All unnecessary copies of A (…60, …ce) are now destroyed.
8. The vector is destroyed, so the A's (…70, …71) inside are also destroyed.

如果您

    vector<A> t;
    t.reserve(2); // <-- reserve space for 2 items.
    t.push_back(A());
    t.push_back(A());

输出将变为：

A (0xbffff8cf) Construction
A (0xbffff8cf->0x100160) Copy Construction
A (0xbffff8cf) Destruction
A (0xbffff8ce) Construction
A (0xbffff8ce->0x100161) Copy Construction
A (0xbffff8ce) Destruction
A (0x100160) Destruction
A (0x100161) Destruction

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