[Bug c/25733] missed diagnostic about assignment used as truth value.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
--- Comment #13 from Vincent Lefèvre ---
Another example:
int f1 (int a, int b, int c, int d)
{
if (a = c && b == d)
return 0;
else
return 1;
}
int f2 (int a, int b, int c, int d)
{
if (a == b || (a = c && b == d))
return 0;
else
return 1;
}
int f3 (int a, int b, int c, int d)
{
if (a == b && (a = c && b == d))
return 0;
else
return 1;
}
GCC emits a warning for f1 (this is the usual case), but neither for f2, nor
for f3. Note that for f2, the parentheses around what should be a == c && b ==
d are optional, but strongly recommended (GCC has a warning for that), so one
would really expect
if (a == b || (a == c && b == d))
or
if (a == b || ((a = c && b == d)))
For f3, the parentheses around what should be a == c && b == d are not
specially recommended (but may be useful to emphasize on some symmetry in some
context), but I think that there should be the same rule as f2.
FYI, MPFR had an undetected bug corresponding to f3 and a warning would have
really been useful:
https://gforge.inria.fr/scm/viewvc.php/mpfr/trunk/src/sqrt.c?r1=11021&r2=11022
[Bug c/25733] missed diagnostic about assignment used as truth value.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733 Manuel López-Ibáñez changed: What|Removed |Added CC||aravindvijayan224185 at ||gmail dot com --- Comment #12 from Manuel López-Ibáñez 2012-01-19 13:48:48 UTC --- *** Bug 51894 has been marked as a duplicate of this bug. ***
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #11 from manu at gcc dot gnu dot org 2010-02-21 01:43 --- I am not saying that the warning is not useful. I am saying that the current behaviour of not warning if there are parenthesis is not ideal. So either: if ((bool) a = 0) or if ( (a=0) != 0) seem much better choices for avoiding warning. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #10 from bangerth at gmail dot com 2010-02-21 01:25 --- (In reply to comment #7) > (In reply to comment #3) > > As another data-point, > > > > if ( (a=10) ) ; > > > > also doesn't warn. I'm not sure what the standard says on that, but other > > contemporary compilers do give the an "assignment used as truth value" > > warning > > for the example above. > > > > How do other compilers deal with false positives? That is, how can a > programmer > specify that they really want to do an assignment? > > We could use a cast to bool. I think at least in C++ the warning is useful. Conditions in if(...) statements have type bool, and things like if (a=10) use the implicit conversion from int to bool. If a programmer wants to avoid the warning, one can always be explicit and write if ( (a=10) != 0) W. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #9 from manu at gcc dot gnu dot org 2010-02-21 00:34 ---
Another testcase:
int bar(int a, int b, int c)
{
if ((b = c) && (a == 0))
{
return 0;
}
return 1;
}
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #8 from manu at gcc dot gnu dot org 2010-02-21 00:33 --- *** Bug 32587 has been marked as a duplicate of this bug. *** -- manu at gcc dot gnu dot org changed: What|Removed |Added CC||fritz at intrinsity dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #7 from manu at gcc dot gnu dot org 2010-02-21 00:30 --- (In reply to comment #3) > As another data-point, > > if ( (a=10) ) ; > > also doesn't warn. I'm not sure what the standard says on that, but other > contemporary compilers do give the an "assignment used as truth value" warning > for the example above. > How do other compilers deal with false positives? That is, how can a programmer specify that they really want to do an assignment? We could use a cast to bool. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #6 from ianw at vmware dot com 2008-04-28 22:28 ---
(In reply to comment #5)
> (In reply to comment #4)
> > Oh, just to be clear, my point was if (a=10) warns, but the extra
> > parenthesis
> > hide the warning.
>
> That is by design.
Ok, I did try looking but is that documented anywhere?
It would seem to have ramifications for people that do things like make an
ASSERT statement something like
#define ASSERT(v) {if (!(v))}
A quick search on google codesearch showed this was a very common idiom...
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #5 from pinskia at gcc dot gnu dot org 2008-04-28 22:17 --- (In reply to comment #4) > Oh, just to be clear, my point was if (a=10) warns, but the extra parenthesis > hide the warning. That is by design. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #4 from ianw at vmware dot com 2008-04-28 22:16 --- Oh, just to be clear, my point was if (a=10) warns, but the extra parenthesis hide the warning. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #3 from ianw at vmware dot com 2008-04-28 22:14 --- As another data-point, if ( (a=10) ) ; also doesn't warn. I'm not sure what the standard says on that, but other contemporary compilers do give the an "assignment used as truth value" warning for the example above. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #2 from manu at gcc dot gnu dot org 2008-04-27 22:13 --- *** Bug 36050 has been marked as a duplicate of this bug. *** -- manu at gcc dot gnu dot org changed: What|Removed |Added CC||ianw at vmware dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
[Bug c/25733] missed diagnostic about assignment used as truth value.
--- Comment #1 from rguenth at gcc dot gnu dot org 2006-01-10 10:17 --- Confirmed. Same in the C++ frontend. -- rguenth at gcc dot gnu dot org changed: What|Removed |Added Severity|normal |enhancement Status|UNCONFIRMED |NEW Ever Confirmed|0 |1 GCC build triplet|i686-pld-linux | GCC host triplet|i686-pld-linux | GCC target triplet|i686-pld-linux | Keywords||diagnostic Known to fail|3.4.5 4.1.0 |2.95.3 3.3.6 3.4.5 4.0.2 ||4.1.0 Last reconfirmed|-00-00 00:00:00 |2006-01-10 10:17:07 date|| http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25733
