Is it Ok to call method on NULL pointer instance of a class?

Question

This C++ code works for my platform and compiler (Windows, GCC 4.7):

#include <stdio.h>

class A {
public:

    /* ... */

    int size() const
    {
        if ( this == NULL ) {
            return 0;
        }
        return m_size;
    }

private:
    int m_size;
};

int main()
{
    A* a = NULL;

    printf( "%d\n", a->size() );
}

But is this code valid standard C++ and portable? Is it Ok for method to accept this == NULL?

I didn't read the standard but I can't imagine something that says "It's OK to call a method from a null variable". — Unda, Jun 25 '14 at 12:35

score 5 · Accepted Answer · answered Jun 25 '14 at 12:34

5

No, that's not OK. Any dereference of a NULL pointer is undefined behavior. It happens to work because it's not a virtual function (therefore there's no jump through the object's vtable), but that's not an excuse for doing what you're doing.

answered Jun 25 '14 at 12:34

Michael Kohne

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Actually it's a little more complex than that, but the end result is that — for the time being at least — we can more or less conclude that it's _probably_ UB. – Lightness Races in Orbit Jun 25 '14 at 12:36
It doesn't need to be virtual to crash and burn. – Cat Plus Plus Jun 25 '14 at 12:39
@CatPlusPlus: In practice, with no member accesses, it pretty much does. – Lightness Races in Orbit Jun 25 '14 at 12:40

Is it Ok to call method on NULL pointer instance of a class?

1 Answers1