Custom Terminals are used in Phoenix to handle special values transparently. For example, as Phoenix captures everything by value, we needed to use boost::reference_wrapper to bring reference semantics into Phoenix.

Custom terminals could be any wrapper class:

template <typename T>
struct is_custom_terminal;

needs to be specialized in order for Phoenix to recognize this wrapper type. default_action calls custom_terminal<T>.

Example:

// Call out boost::reference_wrapper for special handling
template<typename T>
struct is_custom_terminal<boost::reference_wrapper<T> >
  : mpl::true_
{};

// Special handling for boost::reference_wrapper
template<typename T>
struct custom_terminal<boost::reference_wrapper<T> >
{
    typedef T &result_type;

    template <typename Context>
    T &operator()(boost::reference_wrapper<T> r, Context &) const
    {
        return r;
    }
};