条款49：了解new-handler的行为

       我们知道new操作符私底下通过调用operator new来实现内存分配的。当operator new无法满足内存申请时,它会不断调用new-handler函数，直到找到足够内存。而客户是通过set_new_handler将自己的new-handler传递给它的。

struct Widget{ static std::new_handler set_new_handler( std::new_handler p )throw(); static void* operator new( std::size_t size) throw(std::bad_alloc); private: static std::new_handler st_current_handler_; }; std::new_handler Widget::set_new_handler( std::new_handler p ) throw (){ std::new_handler old_handler = st_current_handler_; st_current_handler_ = p; return old_handler; } void* Widget::operator new(std::size_t size) throw(std::bad_alloc) { NewHandlerHolder alloc_handler( std::set_new_handler( st_current_handler_ ) ); return ::operator new( size ); } //构造一个资源处理类来操作new-handler,在构造过程中获得资源,在析构过程中释放 struct NewHandlerHolder{ explicit NewHandlerHolder(std::new_handler handler) :handler_( handler ){} ~NewHandlerHolder(){ std::set_new_handler( handler_ ); } private: NewHandlerHolder(const NewHandlerHolder&); NewHandlerHolder& operator=(const NewHandlerHolder&); std::new_handler handler_; }; //我们可以用base class将new-handler操作独立出来,让Widget继承自base class template<typename T> struct NewHandlerSupport{ static std::new_handler set_new_handler( std::new_handler p ) throw (){ std::new_handler old_handler = st_current_handler_; st_current_handler_ = p; return old_handler; } static void* operator new( std::size_t size ) throw ( std::bad_alloc ){ NewHandlerHolder h( std::set_new_handler( st_current_handler_ ) ); return ::operator new( size ); } ... private: static std::new_handler st_current_handler_; };

      直到 1993 年，C++ 还要求 operator new 不能分配被请求的内存时要返回 null。operator new 现在则被指定抛出一个 bad_alloc exception。提供传统的 failure-yields-null（失败导致 null）的行为：

class Widget { ... }; Widget *pw1 = new Widget; // throws bad_alloc if // allocation fails if (pw1 == 0) ... // this test must fail Widget *pw2 =new (std::nothrow) Widget; // returns 0 if allocation for // the Widget fails if (pw2 == 0) ... // this test may succeed