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Struct template external_transforms

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


// 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 >


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);

// If you would also like to pass arbitrary data along with the
// transforms, you can use a transform environment, as so:
my_grammar()(i + c, 0, (proto::data = 42, proto::transforms = trx));