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## Struct template as_child

boost::proto::result_of::as_child — A metafunction that computes the return type of the `proto::as_child()` function.

## Synopsis

    template<typename T, typename Domain = proto::default_domain>
    struct as_child {
      // types
      typedef see-below type;
    };

## Description

The `proto::result_of::as_child<>` metafunction turns types into Proto types if they are not already by making them Proto terminals held by reference. Types which are already Proto types are left alone.

When `T` is not yet a Proto type, the result type `proto::result_of::as_child<T, Domain>::type` is `boost::result_of<Domain(proto::expr< proto::tag::terminal, proto::term<T &> >)>::type`.

When `T` is already a Proto type The result type `proto::result_of::as_child<T, Domain>::type` is `T &`.