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Subject: Re: [std-proposals] Language Feature for Reducing Duplicated Code
From: Gašper Ažman (gasper.azman_at_[hidden])
Date: 2020-07-18 02:42:55


Already in the pipeline - deducing this.
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0847r4.html

It's well on track for acceptance.

On Sat, Jul 18, 2020, 06:29 David Ledger via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Hello Everyone,
>
> Duplicate function bodies seem to exist everywhere in C++ codebases.
> This attempts to reduce duplicate code by allowing deduction of const for
> a function. Allowing a const and non-const function to have the same
> function body but call the appropriate const or non-const functions.
>
> What I'm talking about it that everyone writes:
>
> iterator begin();
> iterator begin() const;
>
> T & operator[](size_t i);
> T const & operator[](size_t i) const;
>
> Same for operator[] and function at, and begin, end, front, back etc...
>
> For the const and non-const versions of the function, often the body of
> these functions is identical, all that changes is the return type and the
> selected overloads in the function body. I don't really see the benefit for
> this and want to improve this.
>
> So I want to propose the following:
>
> const(auto),
> const(boolean expr)
>
> noexcept(auto), we already have noexcept(boolean expr)
>
> This would let me write:
>
> iterator begin() const(auto);
>
> The problem this introduces is how is the return type determined here,
> well to do that we would need the bool available for the user:
>
> abbreviated syntax:
> auto begin() const(is_const) -> iterator<is_const>;
>
> or,
>
> template syntax:
> template <bool is_const>
> auto begin() const(is_const) -> iterator<is_const>;
>
> or,
>
> template syntax with return using conditional_t
> auto begin() const(is_const) -> conditional_t<is_const, citerator,
> iterator>;
>
> There are additional benefits here:
> - Keep function logic in one place, not many.
> - Use template parameters of a class to fully const or not-const all
> functions in a class.
> - Reduce the maintenance cost of std library code by halving (in some
> cases) the number of overloads.
>
> As I see it, what needs to be solved:
> - Member function pointers, how to get an exact one?
>
> I'm happy to write up a proposal for this to submit.
> Anyone have and feedback before I write it up?
>
> Regards,
> David Ledger
> --
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> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>



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