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Subject: Re: [SG10] [isocpp-lib] [isocpp-core] Missing feature-test macros
From: Barry Revzin (barry.revzin_at_[hidden])
Date: 2019-10-04 14:57:31


On Fri, Oct 4, 2019, 1:51 PM Richard Smith <richardsmith_at_[hidden]> wrote:

> On Fri, Oct 4, 2019 at 6:56 AM David Vandevoorde via Lib <
> lib_at_[hidden]> wrote:
>
>> Thanks for that work, Barry!
>>
>
> +1, this is great.
>
>
>> I don’t disagree with any of your recommendations, and wholeheartedly
>> agree with your constexpr changes.
>>
>> While going through it I found myself really not liking the name
>> “__cpp_familiar_template_lambda”: I find it quite opaque. Is there is an
>> opportunity to rename it to “__cpp_expl_lambda_template_parameters” or
>> something like that?
>>
>
> I'd prefer that we just bump the value of __cpp_generic_lambdas for this
> rather than adding a new macro.
>

I'm okay with either of these.

> __cpp_impl_constexpr_members_defined : not a huge fan of the name, but if
> it's likely only going to be used in the stdlib implementation, maybe
> that's fine.
>
> __cpp_lib_remove_cvref : does this need a macro? It seems like code
> wishing to support old compilers could define it themselves or
> unconditionally use remove_reference + remove_cv (I think this fails the
> "you lose nothing by always behaving like the feature is not present" test,
> unless I've overlooked a use case).
>

I wasn't sure about the traits. This one seems easily implantable (and the
next two), so maybe not.

I'm wondering the same thing for __cpp_lib_starts_ends_with. I'd imagine a
> typical usage of that macro would be something like this (possibly factored
> out into a helper function):
>
> #if __cpp_lib_starts_ends_with
> if (s.starts_with("foo"))
> #else
> if (s.size() >= 3 && s.compare(0, 3, "foo"))
> #endif
>
> I don't think that's worthwhile when you could instead write:
>
> // TODO(c++20): replace with s.starts_with("foo")
> if (s.size() >= 3 && s.compare(0, 3, "foo"))
>
> Same thought for: __cpp_lib_type_identity, __cpp_lib_unwrap_ref (small
> convenience functionality that you can write yourself with no loss of
> functionality if you need it and can't depend on it being in the library).
>

starts_with/ends_with seems similar to consistent erasure to me, which does
have a macro. It's implementable but obnoxious.

> __cpp_lib_constexpr_complex, __cpp_lib_constexpr_array_comparisons, __cpp_lib_constexpr_pointer_traits
> : I seem to recall a discussion in L(E?)WG about moving away from
> fine-grained constexpr feature test macros for the stdlib towards
> periodically bumping __cpp_lib_constexpr. How do these fit into that?
>

Do you have a link? The only paper I currently have associated with
__cpp_lib_constexpr on SD-6 is P1032R1.

> Regarding the new set of constexpr macros (__cpp_constexpr_virtual,
> try_catch, ...): do those benefit any implementation? (Clang at least has
> already released some of these features without the additional macros, so
> they're no help there for the previous release, and now implements the full
> set of these, so the fine-grained macros are no help for the next release
> either.) I think we need to be careful about adding too many macros -- each
> feature test macro we add introduces a small amount of overhead to every
> compilation, and that adds up. (Incidentally this is why the original
> proposal was for a single __has_feature(x) mechanism: that avoids the need
> to pre-populate all the feature test macros in the macro table. But that
> ship has sailed.) I think it's preferable for users to just have a value
> that means "you get to use the C++20 constexpr rules" if no-one is going to
> ship a compiler that has only part of the rules (apart from dynamic
> allocation, which has its own macro).
>

Maybe we could just remove the restriction that constexpr functions have to
be constant expressions for at least one set of arguments (otherwise
ILNDR). That way, we could just mark things constexpr even if some
compiler/library doesn't provide it yet?

My motivation with these finer grained macros was to let you do conditional
constexpr where possible. If implementations just batch these anyway, then
maybe the fine grain wouldn't provide any value, and you just check for the
one value you want?

> For __cpp_nodiscard_reason: I'm opposed to adding this macro in isolation.
> Either we need to accept that version numbers for feature test macros don't
> work (the magic numbers are too magical) and completely rethink our
> approach, or we do not have adequate rationale for this.
>

Understood.

Barry

>



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