Struct template child

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## Struct template child

boost::proto::functional::child — A callable PolymorphicFunctionObject that is equivalent to the `proto::child()` function.

## Synopsis

    template<typename N = mpl::long_<0> >
    struct child :  proto::callable {
      template<typename This, typename Expr>
      struct result<This(Expr)> :  proto::result_of::child< Expr, N > {
      };

      // public member functions
      template<typename Expr>
        typename proto::result_of::child< Expr &, N >::type
        operator()(Expr &) const;
      template<typename Expr>
        typename proto::result_of::child< Expr const &, N >::type
        operator()(Expr const &) const;
    };

## Description

A callable PolymorphicFunctionObject that is equivalent to the `proto::child()` function. `N` is required to be an MPL Integral Constant.

### `child` public member functions

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

   Return the `N`th child of the given expression.

   Parameters:

   `expr` | The expression node.

   Requires: | `proto::is_expr<Expr>::value` is `true` `N::value < Expr::proto_arity::value`
   Returns: | `proto::child<N>(expr)`
   Throws: | Will not throw.

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

   This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.