STRUCT TEMP(3)

NAME
boost::proto::external_transforms - A map from grammars to transforms, used as a way to externally associate transforms.

SYNOPSIS

// In header: <boost/proto/transform/when.hpp>
template<typename... When>
struct external_transforms {
  // types
  typedef mpl::map< typename to_mpl_pair< When >::type... > map_type;  // For exposition only.
  // member classes/structs/unions
  template<typename Grammar>
  struct when :
     proto::otherwise< typename mpl::at< map_type, Grammar >::type >
  {
  };
};

DESCRIPTION

It is sometimes desirable to define a grammar that can be customized with different sets of transforms. To do that, where you would normally specify a transform within a grammar, you can instead put proto::external_transform; for example: proto::when< some_grammar, proto::external_transform >. Then, when invoking the grammar, you can pass an approriately-defined instance of proto::external_transforms as the Data parameter. When an expression matches some_grammar, Proto will look up the approprite transform in the Data parameter using some_grammar as a key.

    struct int_terminal
      : proto::terminal<int>
    {};
    struct char_terminal
      : proto::terminal<char>
    {};
    struct my_grammar
      : proto::or_<
            // The next two grammar rules are customization points.
            // The associated transforms are specified externally
            // using external_transforms below.
            proto::when< int_terminal, proto::external_transform >
          , proto::when< char_terminal, proto::external_transform >
          , proto::when<
                proto::plus< my_grammar, my_grammar >
              , proto::fold< proto::_, int(), my_grammar >
            >
        >
    {};
    // Here is where the transforms are associated with the
    // grammar rules above.
    struct my_transforms
      : proto::external_transforms<
            proto::when<int_terminal, print(proto::_value)>
          , proto::when<char_terminal, print(proto::_value)>
        >
    {};
    // ...
    proto::literal<int> i(1);
    proto::literal<char> c('a');
    my_transforms trx;
    // Evaluate "i+c" using my_grammar with the specified transforms:
    my_grammar()(i + c, 0, trx);