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Subject: Re: [std-proposals] Flexible function parameters
From: Filippo Casarin (casarin.filippo17_at_[hidden])
Date: 2020-07-09 10:06:09


I haven't explained what it does in the first mail so i will explain now.

This proposal introduces more ways to overload a function.

What is `void f(int x, int y) {}` supposed to do differently from `void
> f(int x; int y) {}` or `void f(int x: int y) {}`?

In this case for the machine there is no difference other than the name
mangling.

Why is it useful?

1.
Given the code

> void foo(int a=1000696967, int b=1000696969);
>
In c++20 there is no way to call `foo` specifying a value for `b` but not
for `a`

With this proposal you will be allowed to do it in the following way

> void foo(int a=1000696967; int b=1000696969);
> foo(; 7); // calling foo with default valute for `a` and `b` = 7
>

2.
In c++20 you can't have multiple parameters packs in a function, the only
solution I know is from the std::pair constructor but it's too long to call

> std::pair<std::complex<double>, std::string> p6( // from
> https://en.cppreference.com/w/cpp/utility/pair/pair
> std::piecewise_construct,
> std::forward_as_tuple(0.123, 7.7),
> std::forward_as_tuple(10, 'a'));
>

 The new syntax would be

> std::pair<std::complex<double>, std::string> p6(0.123, 7.7; 10, 'a');
>

3.
The colon has been included just for the slice because other possibilities
didn't look as good
>
> arr[i:j] // slicing an array
>
arr[i, j] // indexing a matrix
>
arr[i; j] // does not mean anything to me
>

Il giorno gio 9 lug 2020 alle ore 16:02 Arthur O'Dwyer via Std-Proposals <
std-proposals_at_[hidden]> ha scritto:

> On Thu, Jul 9, 2020 at 9:32 AM Jake Arkinstall via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> The why is clear.
>>
>
> FWIW, no it's not. The OP didn't give any reason for the new syntax at
> all. In fact, the OP didn't even say what the new syntax is supposed to
> *do!*
> What is `void f(int x, int y) {}` supposed to do differently from `void
> f(int x; int y) {}` or `void f(int x: int y) {}`? Explaining *what the
> change does* is an important part of any change request.
>
>
>> 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.
>>
>
> FYI, there's already a deploy plan in motion (to the admittedly limited
> extent that WG21 can be said to make "deploy plans") to get multi-parameter
> `operator[]` overloads. The first step in the plan was committed in C++20:
> http://eel.is/c++draft/depr.comma.subscript
> The second step, targeted for C++23, is to support overloaded
> `operator[](int x, int y)` (or whatever parameter types) with the natural
> meaning and call syntax.
> The goal and justification, targeted also for C++23 as far as I know, is
> to finally ship `mdspan`. (Which I personally think is a silly end goal,
> but there seem to be a lot of people looking forward to it just as a lot of
> people look forward to C++20's `span`.)
>
> –Arthur
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>



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