[cfe-dev] [RFC] Compiler warning -Wparentheses for code : while (c = *str++)

[Justin Bogner]

Daniel Marjamäki <Daniel.Marjamaki at evidente.se> writes:
> Hello!
>
> This is a RFC.
>
> I wonder what you think about trying to avoid a compiler warning for
> such code:
>
>         while (c = *str++) {
>                 *p++ = (c >= 'A' && c <= 'Z') ? (c - 'A' + 'a') : c;
>         }
>
> Output:
>
> a.c:3:11: warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
>         while (c = *str++) {
>                ~~^~~~~~~~
> a.c:3:11: note: place parentheses around the assignment to silence this warning
>         while (c = *str++) {
>                  ^
>                (         )

This tells you what to do to avoid the warning, spell this like so:

  while ((c = *str++)) {
    *p++ = (c >= 'A' && c <= 'Z') ? (c - 'A' + 'a') : c;
  }

> a.c:3:11: note: use '==' to turn this assignment into an equality comparison
>         while (c = *str++) {
>                  ^
>                  ==
> 1 warning generated.
>
> I would personally recommend that it is avoided in while if there is
> just an assignment and rhs is a nonconstant expression.

The trade off here is between the programmer overhead of adding an extra
set of parentheses vs the likelihood of accidentally typing "=" when you
mean "==". In real code we've seen that this warning finds enough errors
that burdening the programmer to be more explicit when they really want
an assignment is worth it.

To illustrate why a side-effectful RHS isn't sufficient to turn off this
warning, consider:

  char end = lastline ? '\0' : '\n';
  while (end == *str++) { /* do something with *str */ }

This is a bit contrived, but it hints at the complexity of turning off
-Wparentheses for particular cases - it's both easier to get right and
easier to explain if we just say "put extra parens around assignments in
conditions to make it obvious that they're intentional".