[Boost-users] Re: enable_if and constructors

From: David Abrahams (dave_at_[hidden])
Date: 2005-04-05 09:11:37

Angus Leeming <angus.leeming_at_[hidden]> writes:

> Is it possible to use enable_if with a constructor? 

In principle, yes.  You just add a dummy default parameter for the
enabler.

> For example, I'd like to define a constructor that allows only
> certain values of an enum:

No chance; that's a runtime test... or you'd have to specify the enum
as a template argument to the ctor explicitly... but you can't provide
explicit template arguments to ctors.

Well, you can do something like:

  template <class N>
  foo(
        N

      , typename boost::enable_if<
            typename contains_c<
                mpl::vector_c<state, state2, state3>
              , N
            >::type
        >::type*=0
  );
  
where 

  template <class S, class N>
  struct contains_c
    : mpl::not_<
          mpl::is_same<
              typename mpl::find_if<S,mpl::equal_to<N,_> >::type
            , typename mpl::end<S>::type
          >
      >
  {};

and then a foo is constructed something like this:

    foo x( mpl::int_<state2>() );

HTH,

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com