Difference in behavior while using dynamic_cast with reference and pointers

I was checking the behavior of dynamic_cast and found that when it fails, std::bad_cast exception is thrown only if the destination is a reference type. If the destination is a pointer type then no exception is thrown from the cast. This is my sample code:

class A
{
	public:
		virtual ~A()
		{
		}
};

class B : public A
{
};

int  main()
{
    A* p = new A;
    
    //Using reference
    try
    {
	B& b = dynamic_cast<B&>(*p);
    }
    catch(std::bad_cast exp)
    {
	std::cout<<"Caught bad cast\n";
    }
    
    //Using pointer
      try
    {
	B* pB = dynamic_cast<B*>(p);
	
	if( pB == NULL)
	{
		std::cout<<"NULL Pointer\n";
	}
    }
    catch(std::bad_cast exp)
    {
	std::cout<<"Caught bad cast\n";
    }
    
    return 0;
}

Output is "Caught bad cast" and "NULL pointer". Code is compiled using VS2008. Is this the correct behavior ? If yes, then why there is a difference?

Yes, this is correct behaviour. The reason is that you can have a null pointer, but not a null reference - any reference has to be bound to an object.

So when dynamic_cast for a pointer type fails it returns a null pointer and the caller can check for that, but when it fails for a reference type it can't return a null reference, so an exception is the only reasonable way to signal a problem.

See the C++ Standard, section 5.2.7/9:

> 9 The value of a failed cast to > pointer type is the null pointer value > of the required result type. A failed > cast to reference type throws bad_cast > (18.5.2).

As to why - these are Stroustrup's words from the D & E book, section 14.2.2:

> I use a reference cast when I want an > assumption about a reference type > checked and consider it a failure for > my assumption to be wrong. If instead > I want to select among plausible > alternatives, I use a pointer cast and > test the result.

Yes, 5.2.7/9

> The value of a failed cast to pointer type is the null pointer value of the required result type. A failed cast to reference type throws bad_cast (18.5.2).

Yes, it is. Because dynamic_cast can't return NULL for a failed reference cast, an exception is the only way out.

That is, a reference can't be NULL, so there is nothing suitable to return.