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elsethrow 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