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## Function child

boost::proto::child — Return the `N`th child of the specified Proto expression.

## Synopsis

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

    template<typename N, typename Expr>
      typename proto::result_of::child< Expr &, N >::type child(Expr & expr);
    template<typename N, typename Expr>
      typename proto::result_of::child< Expr const &, N >::type
      child(Expr const & expr);
    template<typename Expr>
      typename proto::result_of::child< Expr & >::type child(Expr & expr);
    template<typename Expr>
      typename proto::result_of::child< Expr const & >::type
      child(Expr const & expr);

## Description

Return the `N`th child of the specified Proto expression. If `N` is not specified, as in `proto::child(expr)`, then `N` is assumed to be `mpl::long_<0>`. The child is returned by reference.

Parameters: |
`expr` | The Proto expression.
Requires: | `proto::is_expr<Expr>::value` is `true`. `N` is an MPL Integral Constant. `N::value < Expr::proto_arity::value`
Returns: | A reference to the `N`th child of `expr`.
Throws: | Will not throw.