[英]extraneous calls to copy-constructor and destructor
[跟進這個問題 ]
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
}
}
輸出是:
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?
為了清楚地看到發生了什么,我建議在輸出中包含this
指針以識別哪個A正在調用該方法。
A() {cout<<"A (" << this << ") Construction" <<endl;}
A(A const& a){cout<<"A (" << &a << "->" << this << ") Copy Construction"<<endl;}
~A() {cout<<"A (" << this << ") Destruction" <<endl;}
我得到的輸出是
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
所以流程可以解釋為:
如果你這樣做,第5步將會消失
vector<A> t;
t.reserve(2); // <-- reserve space for 2 items.
t.push_back(A());
t.push_back(A());
輸出將變為:
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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