Re: [boost] move library / trait needed

Subject: Re: [boost] move library / trait needed
From: David Abrahams (dave_at_[hidden])
Date: 2009-04-30 14:10:26

on Thu Apr 30 2009, Sebastian Redl <sebastian.redl-AT-getdesigned.at> wrote:

> David Abrahams wrote:
>> Hi Ion,
>>
>> are you familiar with
>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2855.html
>> ?
>>
>> Until the committee decides exactly how to handle it and compilers catch
>> up with the feature they add, we need a workaround for that problem,
>> both for C++03 and C++0x.  The simplest approach requires a trait called
>> has_nothrow_move<T> which can be used to SFINAE-out pair's move
>> constructor unless its members all have a nonthrowing move constructor.
>> It would be reasonable to say
>>
>>    template <class T> 
>>    struct has_nothrow_move : has_move_constructor {};
>>
>> as a default, since move constructors shouldn't throw.  But still, we
>> need an implementation of has_move_constructor.  Obviously, that will
>> require compiler support to be optimal, but in the meantime it can be
>> specialized.
>>   
> What if we say
>
> template <class T>
> struct move_is_nothrow : mpl::true_type {};
>
> and have it defined whether there actually is a move constructor or not?
> Would that work? Or would it interact badly with copy-only types?

The latter.  We can probably count on users not to write throwing move
ctors, but we can't count on nonthrowing copy ctors.  It needs to be
conservative, i.e. something like:

    template <class T> 
    struct has_nothrow_move 
      : mpl::or_<
            has_nothrow_copy<T>     // handles all builtins
          , has_move_constructor<T> // handle UDTs
        >
     {};
 
    template <class T>
    struct has_move_constructor     // Must assume UDTs don't have a move ctor
      : mpl::false_ {};

    template <class T>
    struct has_nothrow_copy         // Must assume UDTs have a copy ctor
      : has_trivial_copy<T> {};

otherwise, 

   pair<movable_type, type_with_throwing_copy_and_no_move_ctor>

has a throwing move constructor.  

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com