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## Struct template nullary_expr

boost::proto::nullary_expr — A metafunction for generating nullary expression types, a grammar element for matching nullary expressions, and a PrimitiveTransform that returns the current expression unchanged.

## Synopsis

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

    template<typename Tag, typename T>
    struct nullary_expr :  proto::transform< nullary_expr<Tag, T> > {
      // types
      typedef proto::expr< Tag, proto::term< T > > type;
      typedef type                                 proto_base_expr;

      // member classes/structs/unions
      template<typename Expr, typename State, typename Data>
      struct impl :  proto::transform_impl< Expr, State, Data > {
        // types
        typedef Expr result_type;

        // public member functions
        Expr operator()(typename impl::expr_param, typename impl::state_param,
                        typename impl::data_param) const;
      };
    };

## Description

Use `proto::nullary_expr<proto::_, proto::_>` as a grammar element to match any nullary expression.