[boost] Re: Perfect moving (almost)

From: David Abrahams (dave_at_[hidden])
Date: 2004-02-09 09:44:48

Daniel Wallin <dalwan01_at_[hidden]> writes:

> Daniel Frey wrote:
>> David Abrahams wrote:
>>> Oh, really?  Too bad; I guess this isn't really much of an innovation
>>> then.
>> Maybe I misunderstood what you have done, but to me this was the
>> most important part of your code! No strange changes for the user
>> like returning special values from functions 
>
> I believe this is still needed, for the same reason as in Mojo.
>
>> (which is BTW breaking RVO/NRVO and which is what I never liked
>> about Mojo...) and no need for special function signatures as in
>> Mojo, where you had to declare function in a certain unnatural
>> style. IIRC, something like:
>> void f( X& );
>> void f( mojo::temporary< X >& );
>> void f( mojo::constant< X >& );
>
> You only need to do that if you want an rvalue reference as a parameter.
> Normally you would just take the parameter by value and trust the move
> constructor. The same thing needs to be done with Dave's version and
> also with some future addition of rvalue-references to C++.
>
>    void f(X); // can move automatically with move constructor
>               // in both Andrei's and Dave's code
>    void f(mojo::temporary<X> x); // rvalue ref in Andrei's code
>    void f(X::ref x); // rvalue ref in Dave's code

Not exactly.  

    void f(X const&)

Currently accepts rvalues of ordinary copyable classes, and that
wouldn't change if the rvalue reference (&&) notation were introduced.

The problem is that in order to use my technique (or Andrei's),
movable classes can't be "ordinary copyable" classes; they must have
a copy ctor taking a non-const rhs.  At least with a language
extension, an "ordinary copyable" class can also be movable.

One thing I think we overlooked in the move proposal, though, is the
fact that "only movable and not copyable" class rvalues *still* can't
legally be passed where a const reference is expected, unless the rule
in question is lifted.  That seems like a problem to me.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com