c++ - Difference in two versions of QDBusPendingReply::argumentAt -

i use following helper functions call d-bus methods in implementations of d-bus interfaces.

template< typename ...out > bool iserror(const qdbuspendingreply< out... > & reply) {     q_assert(reply.isfinished());     if (!reply.iserror()) {         return false;     }     const auto error = reply.error();     qcritical().noquote()             << qdbusinterface::tr("asynchronous call finished error: %1 (%2)")                .arg(error.name(), error.message());     return true; }  template< typename ...out, typename r, int ...indices > void unpackreply(const r & reply, std::integer_sequence< int, indices... >, out &... results) {     ((results = reply.template argumentat< indices >()), ...); }  template< typename ...out > bool extractresults(qdbusmessage message, out &... results) {     qdbuspendingreply< out... > reply = message;     if (!reply.isfinished()) {         reply.waitforfinished();     }     if (iserror(reply)) {         return false;     }     q_assert(reply.isvalid()); #if 1     // 1     (reply.argumentat(0).template value< qdbusargument >() >> ... >> results); #elif 0     // 2     int = 0;     ((reply.argumentat(i++).template value< qdbusargument >() >> results), ...); #else     // 3     unpackreply< out... >(reply, std::make_integer_sequence< int, sizeof...(out) >{}, results...); #endif     return true; } 

in subclass of qdbusabstractinterface use helper dbuscall:

template< typename ...out > bool dbuscall(qstring method, qvariantlist arguments, out &... results) {     return extractresults(callwithargumentlist(qdbus::blockwithgui, method, qmove(arguments)), results...); } 

call of method looks following:

qdbusobjectpath networkmanagerinterface::addandactivateconnection(nmvariantmapmap connection,                                                   qdbusobjectpath device,                                                   qdbusobjectpath specificobject,                                                   qdbusobjectpath & activeconnection) {     qdbusobjectpath path;     dbuscall("addandactivateconnection", makevariantlist{connection, device, specificobject}, path, activeconnection);     return path; } 

why 3 variants, marked // 1, // 2, // 3, behaves equivalently?

// 3 works fine - expected, how // 1 can correct? it's equivalency // 2 means, qdbusargument returned argumentat ascending indices descending parts of argumentat(0).

why qdbuspendingreply::argumentat(int) return qvariant containing qdbusargument. 3 variants executed when user types registered qdbusregistermetatype.

seems, qdbuspendingreply::argumentat(int) return not de-marshalled qdbusargument (underlying corresponding call), implicitly-shared , modified successive applying of operator >>.

qtdbus looks outdated , abandoned part of qt. say, there no support new syntax in signal/slot connections d-bus signals. arity of qdbuspendingreply can not limited 8, variadic (not useful, generic). maybe there place clarification , refactoring of library structure.

additional:

the correct way use qdbus_cast unary (there two) overloading qvariant (it not documented, present sources):

template< typename ...out > bool extractresults(qdbusmessage message, out &... results) {     qdbuspendingreply< out... > reply = message;     if (!reply.isfinished()) {         reply.waitforfinished();     }     if (iserror(reply)) {         return false;     }     q_assert(reply.isvalid());     int = 0;     ((results = qdbus_cast< out >(reply.argumentat(i++))), ...);     return true; }