Item 51 new和delete的规则

● 全局new/delete重载

void * operator new(std::size_t size) throw(std::bad_alloc)
{
	using namespace std;
	if (size == 0) {				// C++规定：零申请也要给一个字节
		size = 1;
	}
	void* mem = NULL;
	while (true) {
		mem = malloc(size);
		if (mem) {
		   return mem;				// 退出死循环的条件1：申请成功
		}
		new_handler globalHandler = set_new_handler(0);
		set_new_handler(globalHandler);
		if (globalHandler)
			(*globalHandler)();		// 条件2：让new_handler处理
		else
			throw std::bad_alloc();	// 条件3：异常
	}
}
void operator delete(void* mem) throw()
{
	if (mem == 0) return;			// C++规定，对NULL指针delete应该是安全的
	free(mem);
}

● 某个类的new重载

class Base {
public:
	static void * operator new(std::size_t size) throw(std::bad_alloc);
	static void * operator new[](std::size_t size) throw(std::bad_alloc);
	static void operator delete(void *mem, std::size_t size) throw();
	...
};
void * Base::operator new(std::size_t size) throw(std::bad_alloc)
{
	if (size != sizeof(Base))             // 1. Base是个freestanding类，尺寸永不为零！--C++规定
		return ::operator new(size);	  // 2. 如果是派生类调用，那么交给全局new去处理
	...
}
void * Base::operator new[](std::size_t size) throw(std::bad_alloc)
{
	return malloc(size);                  // 3. 只需申请内存，不能处理那些尚不存在的对象，类型也未知
}
void Base::operator delete(void *mem, std::size_t size) throw()
{
	if (mem == 0) return;
	if (size != sizeof(Base)) {			  // 4. 检查是否来自派生类的调用
		::operator delete(mem);
		return;
	}
	free(mem);
}
class Derived: public Base                // 5. 没有重载new的派生类
{...}									  // 6. 必须有虚拟的dtor，否则delete调用Base::operator delete时size会传错
Derived *p = new Derived;                 // 6. 调用 Base::operator new