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

[David Blaikie]

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?

> 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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>>
>>
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>