Re: [Boost-users] [move] differences between results in C++03 and C++11 modes

Subject: Re: [Boost-users] [move] differences between results in C++03 and C++11 modes
From: Krzysztof Czainski (1czajnik_at_[hidden])
Date: 2014-02-11 09:14:51

2014-02-11 14:11 GMT+01:00 Ion Gaztañaga <igaztanaga_at_[hidden]>:

> El 11/02/2014 8:55, Adam Romanek escribió:
>
>  Hi!
>>
>> Some time ago I encountered a problem with assigning the return value of
>> boost::move() to a non-const reference in C++11 mode, but not in C++03
>> mode. See [1] for a StackOverflow question that I created for this
>> issue. It contains all the details so for brevity I won't repeat them
>> here. Could anyone explain this issue? Is this a limitation of C++03
>> emulation or a defect in the implementation?
>>
>
> Yes, it's a limitation of boost move. When called with the emulation code
> boost::move() returns a type rv<T> & that is convertible to T (and that is
> the key point to implement emulated move semantics). However in C++11 you
> return a T&& (unnamed rvalue reference) which is not assignable to T &.
>
> In your case, as R is std::ostream &, the emulation return
> rv<std::ostrean> (convertible to std::ostream &) and the C++11 version
> returns std::ostream &&, which is not convertible to std::ostream &.
>
> I guess we could add a new macro to boost move for return types. In
> emulated mode, it moves the return value, In C++11 mode it only return the
> value (If RVO can't be applied the compiler will do an implicit move).
> Something like:
>
> //return boost::move(r) in emulation
> //return r in C++11
> return BOOST_MOVE_RET(r);
>
+1
I've written and use such a macro myself.
Regards,
Kris