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Subject: Re: [std-proposals] Flexible function parameters
From: Filippo Casarin (casarin.filippo17_at_[hidden])
Date: 2020-07-09 10:16:22


>
> I recall, some time ago, a proposal (or at least a discussion of a
> potential proposal) for bringing template parameters into the function
> parameter set, and with the same syntax. I cannot find it at the moment,
> but I believe it is on the old mailing list somewhere, I think around the
> 2017-2018 mark. I'll see if I can dig it up - there might be some merit to
> merging the two. IIRC I was against it at the time, but who knows - the two
> may complement each other.
>

I saw a talk on youtube on that topic.
I don't know if there is already a solution but this could be used for
template argument deduction.
Example:

> void foo(constexpr size_t n; std::array<int, n> arr);
> foo(; std::array{4, 5, 6}); // equivalent to foo(3; std::array<int, 3>{4,
> 5, 6});
>

Il giorno gio 9 lug 2020 alle ore 15:32 Jake Arkinstall via Std-Proposals <
std-proposals_at_[hidden]> ha scritto:

> The why is clear. It's a way of forming groupings of parameters, which
> currently isn't possible without bringing structs into the equation. This
> opens up some decent possibilities: Defaults can apply to the group (rather
> than having to put them at the end of the call), parameter packs can be
> logically separated in a way they currently cannot, and so we can pass
> multiple parameters to operator[] without interfering with the comma
> operator. All lovely things. I'm for it.
>
> It presents a very clean method of solving some long standing niggles with
> the language, and does so in a way that (as far as I can see) does not
> interfere with any existing valid syntax. In fact, it's clean enough that
> it is even being used as a scoping tool for if statements, so the syntax
> does carry some favour in related areas.
>
> As a language change, and a reasonably big one at that, I can't imagine it
> even being considered until c++26 at the very earliest. It's also going to
> need a very convincing writeup. None of those should ever, ever detract you
> from doing it, though.
>
> I recall, some time ago, a proposal (or at least a discussion of a
> potential proposal) for bringing template parameters into the function
> parameter set, and with the same syntax. I cannot find it at the moment,
> but I believe it is on the old mailing list somewhere, I think around the
> 2017-2018 mark. I'll see if I can dig it up - there might be some merit to
> merging the two. IIRC I was against it at the time, but who knows - the two
> may complement each other.
>
>
> On Thu, 9 Jul 2020, 10:56 Magnus Fromreide via Std-Proposals, <
> std-proposals_at_[hidden]> wrote:
>
>> On Thu, Jul 09, 2020 at 10:32:03AM +0200, Filippo Casarin via
>> Std-Proposals wrote:
>> > I would like to allow colons and semicolons to appear in function
>> > parameters
>>
>> Why?
>> What is your use case?
>> What does C++ as a language stand to gain from this change?
>>
>> The kneejerk reaction to any proposal with no rationale is generally
>> 'Over my dead body' so you really have to make an argument for why the
>> change should happen.
>>
>> There is also the time factor, even if your change is agreed upon by
>> everybody as the best thing ever you won't see it in compilers for
>> 4-6 years so if the change is because you need it NOW then this is the
>> wrong path.
>>
>> /MF
>>
>> > >
>> > Example:
>> >
>> > > void foo(int a, int b; int c);
>> > > foo(0, 4; 3);
>> >
>> >
>> > void foo(int a=0; int b);
>> > > foo(/* default vaue for a */ ; 43);
>> > >
>> >
>> >
>> > > template <class... Args1; class... Args2>
>> >
>> > void foo(Args1&&... args1; Args2&&... args2);
>> > >
>> >
>> > I would also like this to extend this to the operator[]
>> >
>> > Example:
>> >
>> > > template <class T>
>> > > struct matrix {
>> > > T &operator[](size_t i, size_t j);
>> > > };
>> > >
>> >
>> >
>> > > template <class T, size_t N>
>> > > struct array {
>> > > std::span<T> operator[](size_t i : size_t j);
>> > > };
>> > >
>>
>> > --
>> > Std-Proposals mailing list
>> > Std-Proposals_at_[hidden]
>> > https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>
>> --
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>>
> --
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