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## Struct template switch_

boost::proto::switch_ — For matching one of a set of alternate grammars, which are looked up based on an expression's tag type. When used as a transform, `proto::switch_<>` applies the transform associated with the sub-grammar that matches the expression.

## Synopsis

    // In header: <boost/proto/matches.hpp>

    template<typename Cases>
    struct switch_ :  proto::transform<switch_<Cases> > {
      // types
      typedef switch_ proto_base_expr;
      template<typename Expr, typename State, typename Data>
      struct impl :
        Cases::template case_<typename Expr::tag_type>::template impl<Expr, State, Data>
      {
      };
    };

## Description

An expression type `E` matches `proto::switch_<C>` if `E` matches `C::case_<E::proto_tag>`.

When applying `proto::switch_<C>` as a transform with an expression `e` of type `E`, state `s` and data `v`, it is equivalent to `C::case_<E::proto_tag>()(e, s, v)`.