[cfe-dev] Doubt on a couple of warnings on deleted functions

[David Blaikie]

On Mon, Apr 21, 2014 at 1:57 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>
> On 21 Apr 2014 12:43, "David Blaikie" <dblaikie at gmail.com> wrote:
>>
>> On Mon, Apr 21, 2014 at 11:38 AM, Richard Smith <richard at metafoo.co.uk>
>> wrote:
>> > On Mon, Apr 21, 2014 at 7:47 AM, David Blaikie <dblaikie at gmail.com>
>> > wrote:
>> >>
>> >> Yep
>> >> Both of these sound like bugs and should be able to be fixed just by
>> >> checking for a non-deleted definition. I'm not sure if there's a
>> >> particular
>> >> function to test that.
>> >
>> > I think the first warning is entirely appropriate. The point of this
>> > warning
>> > is that every external linkage function should be declared in a header
>> > file;
>>
>> Sure, but how would you declare the deleted function in a header file
>> separate from its delete definition?
>>
>> What you put in the header file is "void func() = delete;" and that's
>> where the warning fires, isn't it?
>
> No, the warning fires if the first declaration of an externally visible
> function is in the main source file.

That doesn't seem to be consistent with what I'm seeing:

$ cat deleted.h
void func() = delete;
$ cat deleted.cpp
#include "deleted.h"
$ clang++-tot deleted.cpp -Wmissing-prototypes -fsyntax-only -std=c++11
In file included from deleted.cpp:1:
./deleted.h:1:6: warning: no previous prototype for function 'func'
[-Wmissing-prototypes]
void func() = delete;
     ^
1 warning generated.


>
>> > it'd help if the warning suggested something like 'mark this function
>> > 'static' if it's not intended to be used in other files' or similar.
>> >>
>> >> The unused variable one might crop up in defaulted functions too, in
>> >> which
>> >> case we'll want to check for a user provided definition.
>> >
>> > I agree on both counts. The same warning is visible here:
>> >
>> > struct S { S(const S&s); }; S::S(const S&s) = default;
>> >
>> >>
>> >> On Apr 21, 2014 3:35 AM, "Nicola Gigante" <nicola.gigante at gmail.com>
>> >> wrote:
>> >>>
>> >>> Hello.
>> >>>
>> >>> Consider this:
>> >>>
>> >>> $ cat example.cpp
>> >>> void func(int x) = delete;
>> >>> $ clang++ -std=c++11 -fsyntax-only -Wunused-parameter
>> >>> -Wmissing-prototypes example.cpp
>> >>> example.cpp:1:6: warning: no previous prototype for function 'func'
>> >>> [-Wmissing-prototypes]
>> >>> void func(int x) = delete;
>> >>>      ^
>> >>> example.cpp:1:15: warning: unused parameter 'x' [-Wunused-parameter]
>> >>> void func(int x) = delete;
>> >>>               ^
>> >>> 2 warnings generated.
>> >>>
>> >>> The two warnings seems false positives:
>> >>>
>> >>> 1) why does it warn about a missing prototype? One can't add a
>> >>> prototype
>> >>> because
>> >>> he'll get this error:
>> >>>
>> >>> error: deleted definition must be first declaration
>> >>> void func(int x) = delete;
>> >>>
>> >>> So, the deleted definition is acting like a prototype itself, doesn't
>> >>> it?
>> >>> I think the warning
>> >>> is wrong.
>> >>>
>> >>> 2) Does it make sense to warn about an unused parameter on a deleted
>> >>> definition?
>> >>> Of course is unused, but it can't be used anyway.
>> >>> This force the programmer to avoid parameters name, like:
>> >>> void func(int) = delete;
>> >>> but this can reduce readability and it seems to me there's no reason
>> >>> to
>> >>> force it.
>> >>>
>> >>> Note that these warnings pop up only on free functions. Deleted member
>> >>> functions
>> >>> don't generate these warnings in these cases.
>> >>>
>> >>> Does it make sense to fix these issues?
>> >>>
>> >>> This mail is a part of a more general effort of mine to be able to
>> >>> warning-less include
>> >>> (with -Weverything, more or less) the LLVM headers that I use in my
>> >>> project.
>> >>> I think (and hope) more will follow.
>> >>>
>> >>> Thanks,
>> >>> Nicola
>> >>> _______________________________________________
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>> >>> cfe-dev at cs.uiuc.edu
>> >>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> >>
>> >>
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>> >>
>> >