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Subject: Re: [std-proposals] Template qualifiers
From: Brian Bi (bbi5291_at_[hidden])
Date: 2019-10-04 09:47:21


On Fri, Oct 4, 2019 at 3:27 AM Andrey Semashev via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On 2019-10-04 08:15, Phil Bouchard via Std-Proposals wrote:
> >
> > 2. a) Like I was saying before, the need for the "const" overloads on
> > the "this" parameter forces us to create redundant code and disregards
> > the "volatile" qualifier:
> >
> > https://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a06712.html
> >
> >
> > 2. b) The only solution I can foresee is to add a new "qualifier"
> > template token type:
> >
> > template <qualifier Q>
> >
> > iterator end () Q noexcept;
>
> This isn't right. The return type must be iterator or const_iterator,
> depending on the method qualification.
>
> There isn't such thing as "qualifier" in C++ in the sense distinct from
> a type. IOW, qualifier is always part of a type. If you propose to
> introduce qualifiers as a distinct entity, you will have to define how
> the shall interact with other parts of the language. Especially, what
> parts it will break.
>
> Honestly, I don't find your arguments compelling enough for introducing
> qualifiers as a distinct entity.
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>

As an alternative to introducing a "qualifier" entity, could we not have
const(bool) the way we have noexcept(bool) and in C++20, explicit(bool)?
But the bool would need to be deducible. We would also want to make the
same change for noexcept.

template <bool c>
auto end() const(c) noexcept -> std::conditional_t<c, const_iterator,
iterator>;

--
Here is another example: currently, to implement something like the
std::is_function type trait, you need 48 partial specializations
<https://en.cppreference.com/w/cpp/types/is_function#Possible_implementation>
but
we could cut it down to 6:
template <class F> struct is_function : false_type {};
template <class R, class... Args, bool c, bool v, bool n>
struct is_function<R(Args...) const(c) volatile(v) noexcept(n)> : true_type
{};
template <class R, class... Args, bool c, bool v, bool n>
struct is_function<R(Args...) const(c) volatile(v) & noexcept(n)> :
true_type {};
template <class R, class... Args, bool c, bool v, bool n>
struct is_function<R(Args...) const(c) volatile(v) && noexcept(n)> :
true_type {};
template <class R, class... Args, bool c, bool v, bool n>
struct is_function<R(Args..., ...) const(c) volatile(v) noexcept(n)> :
true_type {};
template <class R, class... Args, bool c, bool v, bool n>
struct is_function<R(Args..., ...) const(c) volatile(v) & noexcept(n)> :
true_type {};
template <class R, class... Args, bool c, bool v, bool n>
struct is_function<R(Args..., ...) const(c) volatile(v) && noexcept(n)> :
true_type {};
--
Obviously this would still require some fairly nontrivial changes to the
standard, but I think it's less complicated than introducing qualifiers as
a new entity, and more usable since it lets you get all the qualifiers you
care about separately rather than deducing a "Q" that has all of them (and
needs to be somehow decomposed). We might be able to handle the
ref-qualifiers in a similar way (syntax to be bikeshedded later).
I'm not sure if there's a point to writing a proposal for this now since
P0847 will solve most of the problems this solves plus various others. But
in my ideal world, we would have both.
-- 
*Brian Bi*


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