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Re: -Wenum-compare and template metaprogramming
On 4 May 2017 at 07:32, Xi Ruoyao wrote:
> On 2017-05-03 11:26 +0100, Jonathan Wakely wrote:
>> On 3 May 2017 at 05:41, Xi Ruoyao wrote:
>> > In template metaprogramming, we may write something like
>> >
>> > ~~~~~
>> > template <typename A, typename B>
>> > struct BetterType
>> > {
>> > typedef If<(Priority<A>::value > Priority<B>::value), A, B>::type type;
>> > };
>> > ~~~~~
>> >
>> > However, GCC -Wenum-compare (enabled by default) would complain
>> > at each POT of BetterType. It seems annoying.
>> >
>> > Of course we can cast the operands of compare to int. But should we
>> > make GCC more clever and not to complain this causal idiom?
>>
>> You're comparing unrelated enumeration types, which is what the
>> warning is designed to warn about. Why should it not warn just because
>> it's in a template?
>
> No. My point is, in this "enum hack" case we just use the enum values as
> integer constants, not because they are in templates.
>
> Maybe we could silence the warning for the enums has only one
> enumerator with assigned value.
I suppose it could, but that would mean we fail to warn in cases where
we should be warning e.g.
enum Kilograms { kg_max = 0xffff };
enum Metres { m_max = 0xff };
constexpr Kilograms 10kg = Kilograms{10};
constexpr Metres 10m = Metres{10};
if (10kg > 10m) ...
The comparison is exactly the kind of thing the warning is designed for.
> I got thousands of warnings yesterday compiling a package with many
> enum hacks...
Because the code has hacks :-) It should be using constexpr variables
not enumeration types with a single value.