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## Struct template impl

boost::proto::if_::impl

## Synopsis

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

    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

### `impl` public member functions

1.     result_type operator()(typename impl::expr_param expr,
                              typename impl::state_param state,
                              typename impl::data_param data) const;

   Parameters:

   `data` | A data of arbitrary type
   `expr` | An expression
   `state` | The current state

   Returns: | `proto::when<proto::_, `Then-or-Else`>()(expr, state, data)`