Scott Meyers: Item 7, new operator exception handling

[biddlesby]

Hi Guys,

I'm working through Scott Meyer's Effective C++ and I am on Item 7, which is exception handling for the new operator.

I've implemented his example of using a base class to provide custom exception handling functionality to any class, but my code doesn't do what I'd expect it to. Could anybody help me?

I would expect that this should call myfun() and print my message, but it's not doing so.

#include <iostream>
#include <new>

/******************************************************************************************/
template <class X>
class NewHandlerSupport
{
public:

	/// new operater using this class
	static void * operator new(size_t size);

	/// set a new_handler for this class
	static std::new_handler set_new_handler(std::new_handler);

private:

	static std::new_handler m_handler;

};

template <class X>
std::new_handler NewHandlerSupport<X>::m_handler(0); // default value

template <class X>
void * NewHandlerSupport<X>::operator new(size_t size)
{
	// temporarily switch in m_handler

	std::new_handler current_handler = std::set_new_handler(m_handler);

	// allocate

	void* memory;
	try
	{
		memory = ::operator new(size);
	}
	catch (const std::bad_alloc&)
	{
		// restore handler and propogate exception
		std::set_new_handler (current_handler);
		throw;
	}

	// restore handler
	std::set_new_handler (current_handler);

	return memory;
}

template <class X>
std::new_handler NewHandlerSupport<X>::set_new_handler(std::new_handler h)
{
	std::new_handler old = m_handler;
	m_handler = h;
	return old;
}
/******************************************************************************************/

/// An example of a class using this

class X : public NewHandlerSupport<X>
{

public:

	X() { throw std::bad_alloc(); }

};

void myfun() { std::cerr << "Handler for X" << std::endl; }

int main()
{
	X::set_new_handler(myfun);

	X* x = new X;
}

Thanks!

[cire]

Why would it call myfun? new didn't fail to allocate memory. You just threw an arbitrary exception.

[biddlesby]

Cool, understood. Cheers

[JLBorges]

To test your code, just force an ::operator new() failure. For instance:

struct X
{
    static void* operator new( std::size_t sz ) { return ::operator new(-1) ; }
};

void my_new_handler()
{
    static int n = 0 ;
    std::clog << '\t' << ++n << ". my_new_handler was called\n" ;
    if( n == 5 )
    {
        std::set_new_handler(nullptr) ;
        std::clog << "*** my_new_handler removed\n" ;
    }
}

int main()
{
    std::set_new_handler(my_new_handler) ;
    std::clog << "*** my_new_handler installed\n" ;
    try { new X ; }
    catch( const std::exception& e ) { std::cerr << "error: " << e.what() << '\n' ; }
}