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## Struct _expr

boost::proto::_expr — A PrimitiveTransform that returns the current expression unmodified.

## Synopsis

    // In header: <boost/proto/transform/arg.hpp>

    struct _expr :  proto::transform< _expr > {
      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

Example:

    proto::terminal<int>::type i = {42};
    proto::terminal<int>::type & j = proto::_expr()(i);
    assert( boost::addressof(i) == boost::addressof(j) );