C++ 定位new运算符

#include<iostream>
#include<string>

using namespace std;

const int BUF = 512;

class JustTesting
{
private:
    string words;
    int number;
public:
    JustTesting(const string& s = "Just Testing", int n = 0)
    {
        words = s;
        number = n;
        cout << words << " constructed" << endl;
    }
    ~JustTesting()
    {
        cout << words << " destroyed" << endl;
    }
    void show() const
    {
        cout << words << " ," << number << endl;
    }
};

int main()
{
    char* buffer = new char[BUF];
    JustTesting *pc1, *pc2;
    pc1 = new(buffer)JustTesting;
    pc2 = new JustTesting("Heap1", 20);
    cout << "Memory block address" << endl << "buffer: " << (void*)buffer << "     heap: " << pc2 << endl;
    cout << pc1 << ": ";
    pc1->show();
    cout << pc2 << ": ";
    pc2->show();

    JustTesting *pc3, *pc4;
    //pc3 = new (buffer)JustTesting("Bad Idea", 6); //pc3直接占用了pc1的内存区域，这样一来如果pc1中有new分配的成员变量，将造成内存泄漏，
    //因此更好的方式是
    pc3 = new (buffer+sizeof(JustTesting)) JustTesting("Better Idea",6);
    pc4 = new JustTesting("Heap2", 10);

    cout << "Memory Contents" << endl;
    cout << pc3 << ": ";
    pc3->show();
    cout << pc4 << ": ";
    pc4->show();
        
    delete pc2;
    delete pc4;
    //由于delete buffer并不会出发buffer内对象的析构函数，所以只能手动调用其析构函数
    //需要注意调用析构函数的顺序，由于晚创建的对象可能依赖早创建的对象，因此析构的顺序应该与创建顺序相反，另外，仅当析构了所有对象之后，才能释放用于存储这些对象的缓冲区
    pc3->~JustTesting();
    pc1->~JustTesting();
    delete[] buffer;
    //delete pc1; 因为buffer和pc1实际上指向同一个地址，因此如果注释掉上面一行，打开这里，也可以正常运行，但逻辑上却是没有道理的
    cout << "done" << endl;
    cin.get();
    return 0;
}