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Subject: Re: [std-proposals] Operator for reference casting
From: Arthur O'Dwyer (arthur.j.odwyer_at_[hidden])
Date: 2021-01-15 10:00:18


On Fri, Jan 15, 2021 at 10:44 AM Bengt Gustafsson via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
> I think it should be a prefix operator, [...]

Yes, >>x has been proposed before, but shot down.

The OP's specific idea of postfix `&` or `&&` doesn't work because
    return x&&-y;
    return x&-y;
 already parse as meaningful expressions in C++.

Anyway, with OP's specific idea for syntax gone, all that remains is OP's
specific idea for semantics. OP should think about what this hypothetical
facility would do with a variable `x` whose decltype is a non-reference
type, e.g.
    auto foo(auto x) { return bar(HYPOTHETICAL_FWD_SYNTAX(x)); } // Moves
(like std::forward/static_cast<decltype(x)>)? Copies? Ill-formed?

Bengt wrote:

> Preferably the operator should work both as move and as forward, it is a
> struggle understanding which one to use

Move and forward are different operations, though.

    auto move_from(auto&& x) { return std::move(x); }
    auto forward_from(auto&& x) { return std::forward<decltype(x)>(x); }
    int main() {
        std::shared_ptr<int> o = std::make_shared<int>(42);
        auto x = forward_from(o); // copies
        auto y = move_from(o); // moves
        assert(o == nullptr && x != nullptr && y != nullptr && x == y);
    }

But my understanding is that you can always use forward<T>(x) and
> move(x) is just a shortcut which is allowed when you know that x is a
> parameter passed by rvalue reference.
>

No! `move` unconditionally moves; `forward<T>` is conditional, either
moving or not-moving, based on the value of T (which is why it has to take
a template parameter like that).
Unfortunately I haven't written a blog post on this topic yet. But now it's
on my list.

HTH,
–Arthur



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