Function count

lesson6-Segmentation/json.hpp:20863–20867

    */
    template<typename KeyT>
    size_type count(KeyT&& key) const
    {
        // return 0 for all nonobject types
        return is_object() ? m_value.object->count(std::forward<KeyT>(key)) : 0;
    }

    /*!
    @brief check the existence of an element in a JSON object