How to prevent the passing of a NULL-pointer ?

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Hello.

Is there a way
to get a compiler error message
by passing of NULL (explicitly) to a pointer parameter, please ? :)

void f(CWnd* pcTest)
{
}

void test()
{
  // would like to get an "error" here
  f(NULL);
}

The cases of f(const CWnd&) and f(CWnd*&)
are already checked.

Thank you ! :)

Posted 1-Mar-10 0:41am

Eugen Podsypalnikov

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Rajesh R Subramanian · Accepted Answer · 2010-03-01T00:56:00

The best way is to change the function prototype such that it accepts a reference to a CWnd object:

void f(CWnd &obj)
{
 //blah
}

void test()
{
 //f(NULL); //compiler error!
 CWnd wnd;
 f(wnd); //will succeed.
}

If you're writing a multithreaded program, you might want to take thread safety into account.

Richard MacCutchan · Accepted Answer · 2010-03-01T00:58:00

I don't think that is possible, since NULL is a valid pointer, as far as the compiler is concerned. It just means that the pointer currently does not point to any object, which may be what the programmer wants. You could redefine NULL to some illegal value, but that would probably break your code elsewhere.

peterchen · Accepted Answer · 2010-03-01T07:09:00

One trick is to provide a separate overload:

void f(char c);

which makes calls with a literal 0 ambiguous. The overload doesn't need an implementation. You'd have to specify f((CWnd*)0) to pick the correct overload if needed.

However, I would not recommend to use that. It's of questionable value, since it wouldn't even catch:

CWnd * wnd = 0;
f(wnd);

(which can't be caught - at least not easily - at compile time).

A simple ASSERT within f() should do the trick - or pass a reference, as suggested.

How to prevent the passing of a NULL-pointer ?

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