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Struct template if_

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_base_expr;

  // 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_()
    >
  >
{};


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