https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88738
--- Comment #6 from Ulrich Drepper <drepper.fsp+rhbz at gmail dot com> ---
(In reply to Martin Sebor from comment #5)
> If it did we could have GCC apply it implicitly to
> all such functions or operators defined in namespace std, and provide a new
> attribute to disable it in the cases where it might not be appropriate (say
> no_warn_unused_result).
I looked at what clang does and it's very simplistic. The code below produces
the output shows below. I.e., they use the warning for all comparison
operators regardless of
- namespace
- return value
- member function or not
- const-ness of operators
- visible side effects
Nevertheless people use the compiler without complaining, at least visibly.
$ clang++ -c -O -Wall u.cc
u.cc:26:5: warning: equality comparison result unused [-Wunused-comparison]
l == r;
~~^~~~
u.cc:26:5: note: use '=' to turn this equality comparison into an assignment
l == r;
^~
=
u.cc:31:5: warning: inequality comparison result unused [-Wunused-comparison]
l != r;
~~^~~~
u.cc:31:5: note: use '|=' to turn this inequality comparison into an
or-assignment
l != r;
^~
|=
u.cc:36:5: warning: relational comparison result unused [-Wunused-comparison]
l <= r;
~~^~~~
u.cc:41:5: warning: relational comparison result unused [-Wunused-comparison]
l >= r;
~~^~~~
u.cc:46:5: warning: relational comparison result unused [-Wunused-comparison]
l < r;
~~^~~
5 warnings generated.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
struct foo {
int a;
foo(int) : a(42) {}
auto operator==(const foo& o) const { return a == o.a; }
auto operator!=(foo& o) { return a == o.a; }
};
auto operator<=(const foo& l, const foo& r)
{
return l.a == r.a;
}
auto operator>=(foo& l, foo& r)
{
l.a |= 1;
return l.a == r.a;
}
int operator<(foo& l, foo& r)
{
return l.a == r.a ? 0 : l.a < r.a ? -1 : 1;
}
auto f1(foo& l, foo& r)
{
l == r;
}
auto f2(foo& l, foo& r)
{
l != r;
}
auto f3(foo& l, foo& r)
{
l <= r;
}
auto f4(foo& l, foo& r)
{
l >= r;
}
auto f5(foo& l, foo& r)
{
l < r;
}
