casting problem

[l2u]

Hello..

What am I doing wrong with casting?

class someclass {
public:
	char member;
};

int main() {
	someclass *sc = new someclass();
	ULONG_PTR sc_pointer = static_cast<ULONG_PTR>(sc);	
	return 0;
}

Thanks for help

[Salem]

> What am I doing wrong with casting?
At the moment, everything.

C++ has 4 different kinds of cast, maybe read up on what each kind is really meant for.

[hk_mp5kpdw]

Yeah, you're using the wrong cast. Try the one that starts out reint...

[l2u]

Thanks guys.. I used reinterpret_cast and works now..

One more question..

People often use c style cast in c++ code, is this a 'smart way' to do it? Do you use c style casts? If so, when and when not?

[hk_mp5kpdw]

You should definitely read this very recent post.

[dwks]

Or the version that got added to the FAQ: http://cboard.cprogramming.com/showthread.php?t=86924

[l2u]

I have another question.
What is the right way to cast someclass (sc) so somefunc can change the pointer?

class someclass {
public:
	char member;
};

void somefunc(ULONG_PTR *ptr) { }

int main() {
	someclass *sc;
	somefunc(reinterpret_cast<ULONG_PTR*>(sc));
	return 0;
}

[CornedBee]

What do you mean? If you want to change the pointer, you need to pass a pointer to it. In other words:

void somefunc(ULONG_PTR ptr)
{
  someclass **p = reinterpret_cast<someclass **>(ptr);
  *p = new someclass;
}

int main() {
	someclass *sc;
	somefunc(reinterpret_cast<ULONG_PTR>(&sc));
	return 0;
}

The other option would be to use your prototypes, but that code is 1) ugly and 2) useless. It would look like this:

void somefunc(ULONG_PTR *ptr)
{
  someclass *p = new someclass;
  *ptr = reinterpret_cast<ULONG_PTR>(p);
}

int main() {
	someclass *sc;
	ULONG_PTR sccast = reinterpret_cast<ULONG_PTR>(sc);
	somefunc(&sccast);
	return 0;
}

Edit: Just realized this won't actually change sc. Just sccast.

However, as I said before, this is useless. The only valid reason to cast a pointer this way in the first place is if you use a WinAPI function that requires the ULONG_PTR or similar type: callbacks, SetWindowLongPtr, SendMessage. None of these takes an ULONG_PTR, so this approach is out of the question.

In all cases where you have the choice to use a ULONG_PTR*, you also have the choice to use a someclass* or someclass** or someclass*&. And all of those are far better alternatives. (Although not as good as a smart pointer.)

[l2u]

I use WinAPI function GetQueuedCompletionStatus thats why I need this.

[vart]

GetQueuedCompletionStatus want to receive the pointer to ULONG_PTR variable to be filled with the completion key value. Why do you want instead pass the pointer to your class?

[l2u]

Because completion key is address of client structure? I want to change the pointer so it would point to this address.

[vart]

Why do you want to cast outside the function? I think better to hide the cast inside the function

someclass* getCompl()
{
	ULONG_PTR temp;
	if(GetQueuedCompletionStatus(...,&temp,...))
	{
		return reinterpret_cast<someclass*>(temp);
	}
	return 0;
}

[l2u]

The problem is I would have to return OVERLAPPED structure too, so it would be better to pass both as argument and then set it.