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boost::proto::domain::as_child — A callable unary MonomorphicFunctionObject that specifies how objects are turned into Proto expressions in this domain, for use in scenarios where the resulting expression is intended to be made a child of another expression.
// In header: <boost/proto/domain.hpp>
// A callable unary MonomorphicFunctionObject that specifies how objects are
// turned into Proto expressions in this domain, for use in scenarios where
// the resulting expression is intended to be made a child of another
// expression.
template<typename T> 
struct as_child :  proto::callable {
  // types
  typedef see-below result_type;
  // public member functions
  result_type operator()(T &) const;
};
              A unary MonomorphicFunctionObject that specifies how objects are turned into Proto
              expressions in this domain. The resulting expression object is suitable for storage
              as a child of another expression. In that scenario, it is usually
              preferable to store child expressions by reference; or, in the case of objects that
              are not yet Proto expressions, to wrap them by reference in a new Proto terminal
              expression. (Contrast this description with the description for
              proto::domain::as_expr.)
            
              The as_child function object turns objects into
              Proto expressions, if  they are not already, by making them Proto terminals held by
              reference. Objects that are already Proto expressions are simply returned by
              reference. If
              wants_basic_expr<Generator>::value is true,
              then let E be
              proto::basic_expr;
              otherwise, let E be
              proto::expr.
              Given an lvalue t of type
              T:
              
T is not a Proto expression type, the resulting
                  terminal is
                  Generator()(E<tag::terminal, 
                  term< T & > >::make(t)).
                t.