On 10/13/14 21:59, Chen Gang wrote:
>
> On 10/13/14 21:31, Jakub Jelinek wrote:
>> On Mon, Oct 13, 2014 at 09:10:31PM +0800, Chen Gang wrote:
>>> Oh, yes. Originally I got this warning by compiling Qemu. And sorry for
>>> my sample (test.cc) may be not quite precise.
>>>
>>> For me, I guess:
>>>
>>> - If the constant number is defined in the header file, and never be
>>> used, our g++ need not report warning about [-Wunused-variable].
>>
>> That is nonsense, even if you define such a "constant" in a header file,
>> it still means runtime overhead (the variable needs to be constructed at
>> runtime, const is not the same thing as constexpr).
>> So, IMHO the warning is desirable even if it is in headers, it is something
>> you should reconsider. Making the function constexpr makes the warning of
>> course go away, then there is no runtime overhead associated with it; but
>> you'll need C++11 for that.
>>
>
> OK, thank you for your explanation. Excuse me, I am not quite familiar
> with C++. I guess your meaning is:
>
> - "char f(){return 1}; const char a = f();", if 'a' is not used, need
> report warning (no matter whether it is in header or source file).
>
> - "const float a = 3.1 + 7.0/2;", if 'a' is not used, need nor report
Sorry, remove "nor". "If 'a' is not used, need report warning".
> warning (no matter whether it is in header or source file).
>
> - this warning is based on C++11, so old gcc version may not support
> this warning, but new gcc (e.g. 5.0), need support it.
>
> Is it correct?
>
>
> Thanks.
>
--
Chen Gang
Open, share, and attitude like air, water, and life which God blessed
