c++常规new和定位(placement)new

定位放置new操作的语法形式不同于普通的new操作。例如，一般都用如下语句A* p=new A;申请空间，而定位放置new操作则使用如下语句A* p=new (ptr) A;申请空间，其中ptr就是程序员指定的内存首地址。

通常，定位new负责在堆中找到一个足以能满足要求的内存块。new运算符还有另外一种变体，被称为定位(placement)new运算符，他让您能够制定要使用的位置。

例子如下:

#include<iostream>
#include<string>
#include<new>

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\n";
}
~JustTesting()
{
cout << words << " destroyed\n";
}

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 addresses:\n" << "buffer: " << (void *)buffer << " heap: " << pc2 << endl;
cout << "Memory contents:\n";
cout << pc1 << ": ";
pc1->Show();
cout << pc2 << ": ";
pc2->Show();

JustTesting *pc3, *pc4;
pc3 = new(buffer) JustTesting("Bad Idea", 6);
pc4 = new JustTesting("Heap2", 10);

cout << "Memory contents:\n";
cout << pc3 << ": ";
pc3->Show();
cout << pc4 << ": ";
pc4->Show();

delete pc2;
delete pc4;
delete []buffer;

cout << "Done\n";

return 0;
}这个程序出现两个问题。第一：在创建第二个对象时，定位new运算符使用第一个新对象来覆盖用于第一个对象的内存单元。显然，如果类动态地为其成员分配内存，将引发问题。
第二：将delete用于pc2和pc4时，将自动调用为pc2和pc4指向的对象调用析构函数；然而，将delete[]用于buffer时，不会使用定位new运算符创建的对象调用析构函数。

不能使用delete pc1;delete pc3。原因在于delete可以和常规new运算符配合使用，但是不能和定位new运算符配合使用。

代码二:

#include<iostream>
#include<string>
#include<new>
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\n";
}

~JustTesting()
{
cout << words << " destroyed\n";
}

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 addresses:\n" << " buffer: " << (void*)buffer << " heap: " << pc2 << endl;
cout << "Memory contents:\n";
cout << pc1 << ": ";
pc1->Show();
cout << pc2 << ": ";
pc2->Show();

JustTesting *pc3, *pc4;
pc3 = new(buffer + sizeof(JustTesting)) JustTesting("Better Idea", 6);
pc4 = new JustTesting("Heap2", 10);

cout << "Memory contents:\n";
cout << pc3 << ":";
pc3->Show();
cout << pc4 << ": ";
pc4->Show();

delete pc2;
delete pc4;

pc3->~JustTesting();
pc1->~JustTesting();

delete[]buffer;
cout << "Done\n";

while (1);
return 0;
}解决这种问题方案是，显式地调用析构函数，必须指定要销毁的对象。需要注意一点是，正确的删除顺序。应以与创建顺序相反的顺序进行删除。原因在于，晚创建的对象可以依赖于早创建的对象。对象创建完成过后，才能释放用于存储这些对象的缓冲区。