placement new 的使用实例

代码

#include<iostream>using namespace std;class P {public:    virtual void print() {        cout << "P";    }};class Q : public P {public:    virtual void print() {        cout << "Q";    }};int main() {    P *p = new P;    Q *q = static_cast<Q *>(p);    q->print();    delete p;    cout << endl;    q = new Q;    p = q;    p->print();    q->print();    cout << endl;    p = new(q)P;    q->print();    p->print();    cout << endl;    p->~P();    delete  q;    return 0;}