Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Boost
C++ Libraries
...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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));