STRUCT TEMPLATE IF_(3)

NAME
boost::proto::if_ - Used to select one grammar or another based on the result of a compile-time Boolean. When used as a transform, proto::if_<> selects between two transforms based on a compile-time Boolean.

SYNOPSIS

// In header: <boost/proto/matches.hpp>
template<typename If, typename Then = proto::_,
         typename Else = proto::not_<proto::_> >
struct if_ :  proto::transform<if_<If, Then, Else> > {
  // types
  typedef if_ proto_grammar;
  // member classes/structs/unions
  template<typename Expr, typename State, typename Data>
  struct impl :  proto::transform_impl< Expr, State, Data > {
    // types
    typedef typename mpl::if_<
      typename boost::result_of<proto::when<proto::_, If>(Expr, State, Data)>::type,
      typename boost::result_of<proto::when<proto::_, Then>(Expr, State, Data)>::type,
      typename boost::result_of<proto::when<proto::_, Else>(Expr, State, Data)>::type
    >::type result_type;
    // public member functions
    result_type operator()(typename impl::expr_param,
                           typename impl::state_param,
                           typename impl::data_param) const;
  };
};

DESCRIPTION

When proto::if_<If, Then, Else> is used as a grammar, If must be a Proto transform and Then and Else must be grammars. An expression type E matches proto::if_<If, Then, Else> if boost::result_of<proto::when<proto::_, If>(E)>::type::value is true and E matches Then; or, if boost::result_of<proto::when<proto::_, If>(E)>::type::value is false and E matches Else.

The template parameter Then defaults to proto::_ and Else defaults to proto::not_<proto::_>, so an expression type E will match proto::if_<If> if and only if boost::result_of<proto::when<proto::_, If>(E)>::type::value is true.

    // A grammar that only matches integral terminals,
    // using is_integral<> from Boost.Type_traits.
    struct IsIntegral :
      proto::and_<
        proto::terminal<proto::_>,
        proto::if_< boost::is_integral<proto::_value>()>
      >
    {};

When proto::if_<If, Then, Else> is used as a transform, If, Then and Else must be Proto transforms. When applying the transform to an expression E, state S and data V, if boost::result_of<proto::when<proto::_, If>(E,S,V)>::type::value is true then the Then transform is applied; otherwise the Else transform is applied.

    // Match a terminal. If the terminal is integral, return
    // mpl::true_; otherwise, return mpl::false_.
    struct IsIntegral2 :
      proto::when<
        proto::terminal<_>,
        proto::if_<
          boost::is_integral<proto::_value>(),
          mpl::true_(),
          mpl::false_()
        >
      >
    {};