Question

Is this code correct?

void foo ( int* p )
{
  if ( int* p2 = p ) // single "="
  {
    *p2++;
  }
}

I always been thinking it’s not, but recently i’ve saw such code in a colleague’s of mine sources.

What if “p” is NULL? MS VS 2008 works correct but shows “warning C4706: assignment within conditional expression”.

Thank you.

Answer 1

By raising warning C4706, the compiler is simply questioning whether you actually meant to write if ( int* p2 == p ) instead of if ( int* p2 = p ) as shown.

Per 6.4 of the 2003 C++ Standard, if ( int* p2 = p ) is legal:

The value of a condition [if (condition)] that is an initialized
declaration in a statement other than a switch statement is the
value of the declared variable implicitly converted to type bool.

If p is NULL, condition (int* p2 = p) fails because the value of p2 is 0, and thus implicitly false, and *p2 is not incremented.