Re: [boost] C++03 unique_ptr emulation

Subject: Re: [boost] C++03 unique_ptr emulation
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2009-01-09 10:43:15

David Abrahams wrote:
> on Thu Jan 08 2009, Ion Gaztañaga <igaztanaga-AT-gmail.com> wrote:
> 
>> The same happens when implementing forwarding functions, so I'm starting to think that
>> putting T() in functions taking movable-only types by value is not a bad idea ;-)
> 
> Sorry, but what do you mean by "putting T() in functions?"
> 
> putting-milk-in-his-T'ly y'rs,

void function (movable m);

int main()
{
    movable m;
    function(movable(boost::move(m)));
    function(movable());
    return 0;
}

I mean that to pass movable-only objects per-value you need to put 
movable() in the argument. This is needed if boost::move returns 
boost::detail::rv<T> instead of T. Whether we return T or rv<T>, 
returning a movable type must types must construct a movable object in 
the return statement (see the attached test case and change the #define 
BOOST_MOVE_RETURN_OPTION define to play with both approaches):

movable move_return_function ()
{
    if(cond){
       return movable();
    }
    else(cond){
       movable m;
       return movable(boost::move(m));
    }
}

This won't work in both cases:

movable move_return_function ()
{
     movable m;
     return movable;
}

Returning T from move() avoids the need of explicitly specify movable() 
when passing arguments by value but makes forwarding really hard. On the 
other hand returning rv<T> makes forwarding easy but passing by value 
ugly. Of course, the question is if we can return something from move() 
that can be different from T so that forwarding is easy but has no need 
to specify movable() when passing by value.

Regards,

Ion