Item 52 new/delete成对儿出现

● 通用概念
Widget *pw = new Widget;
其实分为两步：
1> 调用operator new分配内存
2> 调用ctor初始化对象

如果第2步抛出异常，系统会自动调用
1> void operator delete(void *mem) throw();
2> void Widget::operator delete(void *mem, std::size_t size) throw();
二者之一来回收内存。

系统查找和new对应的delete时是一一对应。如果对于一个placement new没有给出placement delete，则无法回收内存。
所以，自定义new/delete时要保持一一对应：

class Widget {
public:
	static void* operator new(std::size_t size, std::ostream& log) throw(std::bad_alloc);
	static void operator delete(void *pMemory, std::ostream& log) throw();
	...
};
Widget *pw = new (std::cerr) Widget;

此时若出现异常，系统会自动找到带ostream的delete。

● STL中有一个placement new：
void* operator new(std::size_t, void *pMemory) throw();
该函数用于vector在内部给新对象寻找存储位置。placement new因此而得名。placement是名词“安置、寻找合适位置”的意思。
现在一般意义上，只要带有多余参数的new都叫placement new。系统在匹配placement new和placement delete时，也是靠比较那些多余的参数而完成的。

● Item 33讲了类在派生的时候，如果重名就会发生定义遮挡的问题：

class Base {
public:
	static void* operator new(std::size_t size, std::ostream& log) throw(std::bad_alloc);
	...
};
Base *pb = new Base;              // 错误！默认的new被遮挡了
Base *pb = new (std::cerr) Base;  // 正确
class Derived: public Base {
public:
	// 这个厉害，把基类的也给挡了
	static void* operator new(std::size_t size) throw(std::bad_alloc);
	...
};
Derived *pd = new (std::clog) Derived; // 错误
Derived *pd = new Derived;             // OK

先要了解C++预定义的new：

void* operator new(std::size_t) throw(std::bad_alloc);          // normal new
void* operator new(std::size_t, void*) throw();                 // placement new
void* operator new(std::size_t, const std::nothrow_t&) throw(); // no-throw new

这些都是你要在你的类里重定义的。

简单做法如下：

class StandardNewDeleteForms {
public:
	// normal new/delete
	static void* operator new(std::size_t size) throw(std::bad_alloc)
	{ return ::operator new(size); }
	static void operator delete(void *pMemory) throw()
	{ ::operator delete(pMemory); }
	// placement new/delete
	static void* operator new(std::size_t size, void *ptr) throw()
	{ return ::operator new(size, ptr); }
	static void operator delete(void *pMemory, void *ptr) throw()
	{ return ::operator delete(pMemory, ptr); }
	// nothrow new/delete
	static void* operator new(std::size_t size, const std::nothrow_t& nt) throw()
	{ return ::operator new(size, nt); }
	static void operator delete(void *pMemory, const std::nothrow_t&) throw()
	{ ::operator delete(pMemory); }
};
// 谁想自定义new，就要从StandardNewDeleteForms里派生
class Widget: public StandardNewDeleteForms {
public:
	// 然后用using声明一下标准的
	using StandardNewDeleteForms::operator new;
	using StandardNewDeleteForms::operator delete;
	// 最后定义自己的
	static void* operator new(std::size_t size, std::ostream& log) throw(std::bad_alloc);
	static void operator delete(void *pMemory, std::ostream& log) throw();
	...
};