Re: [Boost-users] Boost.Move template-function parameter

Subject: Re: [Boost-users] Boost.Move template-function parameter
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2012-01-12 18:20:27

On Thu, Jan 12, 2012 at 2:27 AM, Luke J. West <ljw_at_[hidden]> wrote:

> Hi. I'm trying to implement a template function that 'moves' it's
> parameter. Here's a simple moveable class...
>
>
>  struct my_class {
>   my_class() {}
>   my_class(BOOST_RV_REF(my_**class)) {}
>
>   my_class& operator=(BOOST_COPY_ASSIGN_**REF(my_class) c) {return *this;}
>
>   BOOST_COPYABLE_AND_MOVABLE(my_**class);
>  };
>
>
> And here is the function...
>
>
>  template<typename T > void steal(BOOST_COPY_ASSIGN_REF(T) c) {}
>

Unrelated: Why use BOOST_COPY_ASSIGN_REF( T ) as the function parameter?
Pretty sure this macro is only to be used within a operator=
declaration/definition. Should probably use BOOST_RV_REF( T ).

> so it works fine when I do this...
>
>  int main() {
>    my_class c;
>    steal<my_class >(c);
>  }
>
> but not when I do this...
>
>  int main() {
>    int c;
>    steal<int >(c);
>  }
>
>
> It's as if 'int' is not a movable type which of course it trivially is. I
> read that Boost.Container classes are 'move aware' and I assume
> boost::vector<int > works, so what am I doing wrong - how to write my
> 'steal' function capable of moving it's argument when it's moveable class,
> but not breaking for primitives?
>
> Thanks,
>
> Luke (hopeful newbie!)
>

Hmmm...right...I think the compiler doesn't like instantiating
boost::rv<int>. AFAIK a trac ticket had just been recently filed to this
effect.

Note that in C++11, lvalues are not implicitly convertible to rvalue
references. So if you actually wanted steal to take an (emulated) rvalue
reference, you should still call it with an enclosed boost::move:

steal(boost::move(c))

and if you know that steal will always be accepting a moveable object, you
can declare it as

template< class T > void steal(BOOST_RV_REF( T ));

Can you give a better idea of what you're trying to achieve?

- Jeff