定位new

常规的new是分配内存，然后调用相应的构造函数，而定位new是在已经分配内存的上面调用构造函数:

// ConsoleApplication7.cpp : 定义控制台应用程序的入口点。
#include "stdafx.h"
#include<iostream>
using namespace std;

class A
{
public:
    A()
    {
        cout << "A()被调用了" << endl;
    }
    ~A()
    {
        cout << "~A()被调用了" << endl;
    }
private:
    int x;
    int y;
};

int main()
{
    void* p = (void*)new char[sizeof(A)];
    A* s = new(p) A();
    s->~A();
    delete (void*)p;

    return 0;
}