void *p...; if (p > 0) .... Is this undefined behavior?

Question

I ran into a new warning message after a compiler upgrade.

warning: ordered comparison of pointer with integer zero [-Wextra]

    if (inx > 0)

As it turns out inx is a pointer. Normally I would expect to see this old code compared against 0, or NULL. This got me to thinking about signed and unsigned values, and possible risk.

A bit of research suggests:

• A pointer greater than zero in c++, what does mean?
• Can a pointer (address) ever be negative?
• memory address positive or negative value in c?
• malloc returns negative value

These seem to suggest that an address (returned by malloc) can never be zero

Which took me to my old copy of the standard.

4.10 Pointer conversions

1 A null pointer constant is an integral constant expression (5.19) prvalue of integer type that evaluates to zero or a prvalue of type std::nullptr_t. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of pointer to object or pointer to function type. Such a conversion is called a null pointer conversion. Two null pointer values of the same type shall compare equal. The conversion of a null pointer constant to a pointer to cv-qualified type is a single conversion, and not the sequence of a pointer conversion followed by a qualification conversion (4.4). A null pointer constant of integral type can be converted to a prvalue of type std::nullptr_t.

It specifically states that two null pointers compare equal.

With that in mind, is that little piece of code undefined behavior? or is there another piece to the puzzle I am missing?

Answer 1

It's not undefined behaviour, but the result is unspecified if inx is not null.

C++11 5.9/2: If two pointers p and q of the same type point to different objects that are not members of the same object or elements of the same array or to different functions, or if only one of them is null, the results of p<q, p>q, p<=q, and p>=q are unspecified.

So you can be sure that the the conditional code won't execute if inx is null - but not that it will if it's not null. The comparison should probably be inx != 0, which is well defined to be true if and only if inx is non-null.