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Re: Deprecating arithmetic on std::atomic<void*>


On 20/04/17 11:57 +0200, Florian Weimer wrote:
On 04/20/2017 11:52 AM, Jonathan Wakely wrote:
On 20/04/17 11:43 +0200, Florian Weimer wrote:
On 04/20/2017 11:25 AM, Jonathan Wakely wrote:

I mean, with -pedantic-errors we already error on void * arighmetics
or function pointer arithmetics.  If std::atomic<void*> would use
the void * arithmetics, it would also reject it. Or does it use integer
arithmetics instead?

No, it does it on void*, but the __atomic built-ins still perform that
arithmetic even with -pedantic-errors.

sizeof is not defined for the relevant types, either, and already triggers a warning by default:

t.cc:25:58: warning: invalid application of ‘sizeof’ to a void type [-Wpointer-arith]
 _M_type_size(ptrdiff_t __d) const { return __d * sizeof(_PTp); }

That could be easily avoided if we care about supporting arithmetic on
std::atomic<void*>.

I think the actual problem here is the suppression of warnings from system headers.

The atomic built-ins still don't warn even with -Wsystem-headers

Sorry, this is what I meant: -pedantic-errors does not catch this case due to the system header warning suppression. Otherwise, with the current std::atomic implementation, -pedantic-errors would already disable the extension on std::atomic alongside with the extension on built-in pointer types, due to the use of sizeof in the definition of _M_type_size.

Ah yes, you're right.

Yet another case where warning suppression in system headers hurts the
library's ability to give diagnostics. We can't warn about using
incomplete types in std::unique_ptr because the -Wdelete-incomplete
warning gets suppressed, and the interaction of #pragma GCC diagnostic
with -Wsystem-headers is ... interesting. We need a way to enable
warnings for specific blocks of code, to say "do not suppress warnings
here, even though it's in a system header".



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