Re: [Boost-users] Problem with type traits

On Sun, Jul 20, 2008 at 11:24 PM, David Abrahams wrote:

on Sun Jul 20 2008, "Robert Dailey" wrote:

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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-constan...

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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.

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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, boost::is_same > type; ^ missing "::type" here? Otherwise, the "typename" above is probably illegal

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};

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

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

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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_, boost::is_pod >, t_source

...

::type& operator>> ( t_source& source, t_data& data ) { source.Read( reinterpret_castboost::uint8_t*( &data ), sizeof( t_data ) ); return source; }

I don't see is_sink in there.

Sorry, I posted the wrong code example. The is_source in there should be is_sink. I didn't find it very easy to find this answer myself via the boost documentation, though I wish it had been. Thanks for everyone's help. I'll try inheritance and see what I get.