rvalue delegation, or the rvalue equivalent of a const member function

Question

struct that
{
    that &frob()
    {
        return *this;
    }
    that frob() const
    {
        return that(*this);
    }

    //that &&frob() && //<-called only if *this is an rvalue
    //{
    //    return move(*this);
    //}

    that()
    {
        // make things
    }
    that(const that &other)
    {
        // copy things
    }
    that(that &&other)
    {
        // move things
    }
};

Obviously the function in comments above is not legal C++, but I need to know if there is a way to accomplish this:

that().frob().frob().frob();

and so on, while each call to frob() would effectively call the "move" version thereof. As this is something that can be determined at compile time, I can't think of any reason for it to not exist in some form.

I can write something like this:

that &&frob(that &&t)
{
    return t;
}

Which would result in this:

frob(frob(frob(that())));

Which is somewhat annoying to read and doesn't accomplish my goal of "spelling things out" with delegation.

[related question](http://stackoverflow.com/questions/3028632/) — fredoverflow, Jul 26 '12 at 12:28
Just to be pedantic, note that `*this` is *always* an lvalue. It's very tempting to confuse `*this` with "the implicit instance parameter". — Kerrek SB, Jul 26 '12 at 13:51

score 2 · Accepted Answer · answered Jul 26 '12 at 11:56

2

If you want the && annotated function to play well with others, you should use the & annotation on the others.

that &frob() &
{
    return *this;
}
that frob() const &
{
    return that(*this);
}

answered Jul 26 '12 at 11:56

Potatoswatter

134,909
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That is a compilation error using Intel C++ v12: `error: expected a ';'`. – defube Jul 26 '12 at 11:58
@i_photon Hmm, that missing feature kills a lot of the usefulness of rvalue `this`. Sorry. – Potatoswatter Jul 26 '12 at 11:59
It seems to be the same for MSC as well :c – defube Jul 26 '12 at 12:00
@i_photon ICC is one of the better ones… well it gets a similar error in GCC 4.7 but Clang 3.1 works fine, and does what you describe for `that().frob().frob().frob();`. – Potatoswatter Jul 26 '12 at 12:17
3

Actually, Clang is currently the only compiler to support rvalue for `*this`. – Morwenn Jul 26 '12 at 12:27
Looks like this weekend will involve poking around with Clang. Also, does anyone know if it supports `__declspec(property(...))`? I read something in a mailing list somewhere, but I'm not sure. – defube Jul 26 '12 at 12:43
@i_photon that's not standard, that's an MSVC extension. I doubt clang supports it. – R. Martinho Fernandes Jul 26 '12 at 13:33

yuri kilochek · Answer 2 · 2012-07-26T12:34:04.337

0

No, there is no way to determine if an object is an rvalue from within the object. The best you can do is copy it if it is const and move it if its not:

that frob() const
{
    return *this;
}

that frob()
{
    return std::move(*this);
}

EDIT

Further reaserch shows that you actually can, as @Potatoswatter pointed out, but its called ref-qualification and not annotation.

edited Jul 26 '12 at 12:34

answered Jul 26 '12 at 12:12

yuri kilochek

12,709
2
32
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rvalue delegation, or the rvalue equivalent of a const member function

2 Answers2