Method on_document_begin

example/validate.cpp:39–39

        constexpr static std::size_t max_string_size = std::size_t(-1);

        bool on_document_begin( boost::system::error_code& ) { return true; }
        bool on_document_end( boost::system::error_code& ) { return true; }
        bool on_object_begin( boost::system::error_code& ) { return true; }
        bool on_object_end( std::size_t, boost::system::error_code& ) { return true; }