Re: [Boost-users] Problem with type traits

From: David Abrahams (dave_at_[hidden])
Date: 2008-07-21 00:24:29

on Sun Jul 20 2008, "Robert Dailey" <rcdailey-AT-gmail.com> wrote:

> Hi,
>
> I'm trying to create my own struct that is compatible with boost::mpl::or_ as well as
> boost::enable_if.
>
> I find that using boost::mpl::or_ requires that my structure have a
> 'type' member, and enable_if looks for 'value'.

Both metafunctions are defined so they will work with an MPL integral constant
http://www.boost.org/doc/libs/1_35_0/libs/mpl/doc/refmanual/integral-constant.html

> Must I define both?

Usually the best way to do this is to derive your struct from mpl::true_
or mpl::false_ or some other bool-valued nullary MPL metafunction.

> What is the proper way to handle this? Below is my structure:
>
> template< typename t_type >
> struct is_sink
> {
>     typedef typename boost::mpl::or_<
>         vfx::io::is_device<t_type>,
>         boost::is_same<t_type, vfx::io::Sink>
>     > type;
       ^
missing "::type" here? Otherwise, the "typename" above is probably illegal

> };

provided of course that vfx::io::is_device<t_type> is a integral-valued
nullary metafunction, 

  template <class t_type>
  struct is_sink
    : mpl::or_<
          vfx::io::is_device<t_type>,
          boost::is_same<t_type, vfx::io::Sink>
      >
  {};

> The above structure works perfectly fine with boost::mpl::or_, but
> does not work for boost::enable_if because it is looking for a 'value'
> member. I feel what it should be looking for is
> is_sink::type::value. An example of how I use the above structure is
> below:
>
> template< typename t_source, typename t_data >
> typename boost::enable_if<
>     boost::mpl::and_<is_source<t_source>, boost::is_pod<t_data> >,
>     t_source
>>::type& operator>> ( t_source& source, t_data& data )
> {
>     source.Read( reinterpret_cast<boost::uint8_t*>( &data ), sizeof( t_data ) );
>     return source;
> }

I don't see is_sink in there.

This might help you:

<plug>
  http://boostpro.com/tmpbook
</plug>

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