Re: Undefined behaviour with Non-static, non-polymorphic + null pointer?

"Victor Bazarov" <>
Wed, 21 Nov 2007 10:03:31 -0500
Alan Woodland wrote:

I'm fairly sure this is undefined behaviour, despite the fact that
it compiles and 'runs' (prints "this doesn't exist") on all my

#include <iostream>

class foo {
  void bar() {
     std::cout << "hello evil world!" << std::endl;
     if (this) {
        std::cout << "this exists" << std::endl;
     else {
        std::cout << "this doesn't exist!" << std::endl;

int main() {
  foo *inst = 0;

Here you're "using" the pointer that has an invalid value (does not
point to any object). That's undefined behavour. I could not quickly
locate the exact passage in the Standard that says that it is, but I
am sure you can find a mention of it in the archives, just search for
"dereference null pointer".

  return 0;

Can someone please quote chapter and verse on this one and help me win
my current "this is a very bad idea" argument I'm having? I'd have
expected it to be forbidden under some general rule, and no exceptions
to have been made for it? Or is it actually really legal and defined
because nothing ever dereferences the this pointer?

The expression


is in fact


which already dereferences the null pointer 'inst'.

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