avoiding incorrect code

[Jed Brown]

On Wed, Jan 14, 2009 at 13:56, Barry Smith <bsmith at mcs.anl.gov> wrote:
> All of this demonstrates that it would be legal that that (!ptr) is not
> correct and one should always use if (ptr == NULL).

FWIW, C99, 6.5.3.3.5 reads

| The result of the logical negation operator ! is 0 if the value of
its operand compares
| unequal to 0, 1 if the value of its operand compares equal to 0. The
result has type int.
| The expression !E is equivalent to (0==E).

In (0 == ptr) the 0 is a null pointer constant (as per 6.5.9.2 `one
operand is a pointer and the other is a null pointer constant') so
there is no reason to prefer (ptr == NULL).  Note that in C++, NULL is
a macro which expands to 0, not (void*)0 as is common in C, so that it
doesn't need an explicit cast.

Jed