operator new and delete

Effective C++的笔记

#include <tr1/memory>
#include <iostream>

class NewHandlerHolder {
public:
    explicit NewHandlerHolder(std::new_handler nh)
        :handler(nh){ }
    ~NewHandlerHolder() { std::set_new_handler(handler); }
private:
    std::new_handler handler;
    NewHandlerHolder(const NewHandlerHolder&);
    NewHandlerHolder & operator=(const NewHandlerHolder&);
};

template <typename T>
class NewHandlerSupport {
public:
    static std::new_handler set_new_handler(std::new_handler p) throw();
    static void *operator new(std::size_t size) throw(std::bad_alloc);
    static void *operator new(std::size_t size, std::nothrow_t &) throw();

private:
    static std::new_handler currentHandler;
};

template <typename T>
std::new_handler NewHandlerSupport<T>::currentHandler = NULL;

template <typename T>
std::new_handler 
NewHandlerSupport<T>::set_new_handler(std::new_handler p) throw()
{
    //throw()指出，该函数不允许抛出异常，否则属于严重错误
    std::new_handler oldHandler = currentHandler;
    currentHandler = p;
    return oldHandler;
}

template <typename T>
void *
NewHandlerSupport<T>::operator new(std::size_t size, std::nothrow_t &nt) throw()
{
    NewHandlerHolder h(std::set_new_handler(currentHandler));
    return ::operator new(size, nt);
}

template <typename T>
void *
NewHandlerSupport<T>::operator new(std::size_t size) throw(std::bad_alloc)
{
    NewHandlerHolder h(std::set_new_handler(currentHandler));

    return ::operator new(size);
}

class Widget : public NewHandlerSupport<Widget>{
public:
    Widget(){ }
    ~Widget(){ std::cout << "Widget Destructor Enter" << std::endl; }
};

int main(void)
{
    std::cout << "Hello World!" << std::endl;

    Widget::set_new_handler(NULL);

    std::tr1::shared_ptr<Widget> pW(new Widget);

    Widget *pw2 = ::new (std::nothrow) Widget;
    if(pw2)
        delete pw2;

    return 0;
}