QHttpServer: use function pointer as ViewHandler

Tarae · 30 Mar 2023, 12:34

Using Qt 6.4.3, playing with QHttpServer.

Documentation: ViewHandler can be a function pointer, non-mutable lambda...

But I don't know how to use it with a function pointer:

#include <QCoreApplication>
#include <QHttpServer>

const char * responseFunction()
{
  return "Hello world!";
}

int main(int argc, char *argv[])
{
  QCoreApplication a(argc, argv);

  QHttpServer server;

  //server.route("/", [](){  return "Hello world!";}); // this compiles
  server.route("/", &responseFunction); // this doesn't compile

  server.listen(QHostAddress::Any, 2222);

  return a.exec();
}

The program compiles and works when lambda is used. But I can't compile it with a function pointer. What am I doing wrong?

Christian Ehrlicher · T Tarae 31 Mar 2023, 13:47

And fixed

JonB · 30 Mar 2023, 12:41

@Tarae said in QHttpServer: use function pointer as ViewHandler:

server.route("/", &responseFunction); // this doesn't compile

Usually I thought it does not matter with C function pointers, but try removing that &. Plain responseFunction is a function pointer. Does that resolve?

Having said that, I don't think your function is acceptable anyway:

ViewHandler can be a function pointer, non-mutable lambda, or any other copiable callable with const call operator. The callable can take two optional special arguments: const QHttpServerRequest& and QHttpServerResponder&&. These special arguments must be the last in the parameter list, but in any order, and there can be none, one, or both of them present.

Oh, no, I see, the parameters are optional anyway.

jsulm · 30 Mar 2023, 12:44

@Tarae And it could also help if you would post the compiler error...

Tarae · J JonB 30 Mar 2023, 12:34

@JonB I've tried with &, without & , with * or && or **. Nothing works.

Tarae · 30 Mar 2023, 12:46

@jsulm there is compiler output: https://pastebin.com/NLrxHdTE

jsulm · 30 Mar 2023, 12:52

@Tarae I can't open this URL.
Please post the error here, it's simple text...

Tarae · 30 Mar 2023, 12:54

@jsulm Ok, here it is:

14:38:56: Running steps for project httptest...
14:38:56: Configuration unchanged, skipping qmake step.
14:38:56: Starting: "/usr/bin/make" -j8
/home/user/Qt/6.4.3/gcc_64/bin/qmake -o Makefile ../httptest/httptest.pro -spec linux-g++ CONFIG+=debug CONFIG+=qml_debug
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -fPIC -D_REENTRANT -DQT_QML_DEBUG -DQT_HTTPSERVER_LIB -DQT_WEBSOCKETS_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -I../httptest -I. -I/home/user/Qt/6.4.3/gcc_64/include -I/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer -I/home/user/Qt/6.4.3/gcc_64/include/QtWebSockets -I/home/user/Qt/6.4.3/gcc_64/include/QtNetwork -I/home/user/Qt/6.4.3/gcc_64/include/QtCore -I. -I/home/user/Qt/6.4.3/gcc_64/mkspecs/linux-g++ -o main.o ../httptest/main.cpp
In file included from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouterviewtraits.h:8,
                 from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouter.h:8,
                 from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:9,
                 from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/QHttpServer:1,
                 from ../httptest/main.cpp:2:
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverviewtraits_impl.h: In instantiation of ‘struct QtPrivate::FunctionTraits<const char* (* const&)()>’:
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverviewtraits_impl.h:65:11:   required from ‘struct QtPrivate::ViewTraits<const char* (* const&)(), false>’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouterviewtraits.h:18:8:   required from ‘struct QtPrivate::RouterViewTraitsHelper<const char* (* const&)(), false>’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouterviewtraits.h:122:11:   required from ‘struct QHttpServerRouterViewTraits<const char* (* const&)(), false>’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouter.h:57:39:   required by substitution of ‘template<class ViewHandler, class ViewTraits> typename ViewTraits::BindableType QHttpServerRouter::bindCaptured(ViewHandler&&, const QRegularExpressionMatch&) const [with ViewHandler = const char* (* const&)(); ViewTraits = QHttpServerRouterViewTraits<const char* (* const&)(), false>]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:135:59:   required from ‘bool QHttpServer::routeImpl(Args&& ..., ViewHandler&&) [with Rule = QHttpServerRouterRule; ViewHandler = const char* (*)(); ViewTraits = QHttpServerRouterViewTraits<const char* (*)(), false>; Args = {const char (&)[2]}]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:125:71:   required from ‘bool QHttpServer::routeHelper(QtPrivate::IndexesList<I ...>, Args&& ...) [with Rule = QHttpServerRouterRule; ViewHandler = const char* (*)(); ViewTraits = QHttpServerRouterViewTraits<const char* (*)(), false>; int ...I = {0}; Args = {const char (&)[2], const char* (*)()}]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:69:58:   required from ‘bool QHttpServer::route(Args&& ...) [with Rule = QHttpServerRouterRule; Args = {const char (&)[2], const char* (*)()}]’
../httptest/main.cpp:16:15:   required from here
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverviewtraits_impl.h:42:56: error: ‘operator()’ is not a member of ‘std::remove_cv<const char* (* const)()>::type’ {aka ‘const char* (*)()’}
   42 | struct FunctionTraits : public FunctionTraits<decltype(&q20::remove_cvref_t<T>::operator())>{};
      |                                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverviewtraits_impl.h: In instantiation of ‘struct QtPrivate::ViewTraits<const char* (* const&)(), false>’:
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouterviewtraits.h:18:8:   required from ‘struct QtPrivate::RouterViewTraitsHelper<const char* (* const&)(), false>’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouterviewtraits.h:122:11:   required from ‘struct QHttpServerRouterViewTraits<const char* (* const&)(), false>’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouter.h:57:39:   required by substitution of ‘template<class ViewHandler, class ViewTraits> typename ViewTraits::BindableType QHttpServerRouter::bindCaptured(ViewHandler&&, const QRegularExpressionMatch&) const [with ViewHandler = const char* (* const&)(); ViewTraits = QHttpServerRouterViewTraits<const char* (* const&)(), false>]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:135:59:   required from ‘bool QHttpServer::routeImpl(Args&& ..., ViewHandler&&) [with Rule = QHttpServerRouterRule; ViewHandler = const char* (*)(); ViewTraits = QHttpServerRouterViewTraits<const char* (*)(), false>; Args = {const char (&)[2]}]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:125:71:   required from ‘bool QHttpServer::routeHelper(QtPrivate::IndexesList<I ...>, Args&& ...) [with Rule = QHttpServerRouterRule; ViewHandler = const char* (*)(); ViewTraits = QHttpServerRouterViewTraits<const char* (*)(), false>; int ...I = {0}; Args = {const char (&)[2], const char* (*)()}]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:69:58:   required from ‘bool QHttpServer::route(Args&& ...) [with Rule = QHttpServerRouterRule; Args = {const char (&)[2], const char* (*)()}]’
../httptest/main.cpp:16:15:   required from here
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverviewtraits_impl.h:65:11: error: ‘ArgumentCount’ is not a member of ‘QtPrivate::FunctionTraits<const char* (* const&)()>’
   65 |     using ArgumentIndexes = typename Indexes<FTraits::ArgumentCount>::Value;
      |           ^~~~~~~~~~~~~~~
In file included from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouter.h:8,
                 from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:9,
                 from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/QHttpServer:1,
                 from ../httptest/main.cpp:2:
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouterviewtraits.h: In instantiation of ‘struct QHttpServerRouterViewTraits<const char* (* const&)(), false>’:
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouter.h:57:39:   required by substitution of ‘template<class ViewHandler, class ViewTraits> typename ViewTraits::BindableType QHttpServerRouter::bindCaptured(ViewHandler&&, const QRegularExpressionMatch&) const [with ViewHandler = const char* (* const&)(); ViewTraits = QHttpServerRouterViewTraits<const char* (* const&)(), false>]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:135:59:   required from ‘bool QHttpServer::routeImpl(Args&& ..., ViewHandler&&) [with Rule = QHttpServerRouterRule; ViewHandler = const char* (*)(); ViewTraits = QHttpServerRouterViewTraits<const char* (*)(), false>; Args = {const char (&)[2]}]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:125:71:   required from ‘bool QHttpServer::routeHelper(QtPrivate::IndexesList<I ...>, Args&& ...) [with Rule = QHttpServerRouterRule; ViewHandler = const char* (*)(); ViewTraits = QHttpServerRouterViewTraits<const char* (*)(), false>; int ...I = {0}; Args = {const char (&)[2], const char* (*)()}]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:69:58:   required from ‘bool QHttpServer::route(Args&& ...) [with Rule = QHttpServerRouterRule; Args = {const char (&)[2], const char* (*)()}]’
../httptest/main.cpp:16:15:   required from here
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouterviewtraits.h:122:11: error: no type named ‘ReturnType’ in ‘using FunctionTraits = using FTraits = struct QtPrivate::FunctionTraits<const char* (* const&)()>’ {aka ‘struct QtPrivate::FunctionTraits<const char* (* const&)()>’}
  122 |     using ReturnType = typename Helpers::FunctionTraits::ReturnType;
      |           ^~~~~~~~~~
In file included from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/QHttpServer:1,
                 from ../httptest/main.cpp:2:
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h: In instantiation of ‘bool QHttpServer::routeImpl(Args&& ..., ViewHandler&&) [with Rule = QHttpServerRouterRule; ViewHandler = const char* (*)(); ViewTraits = QHttpServerRouterViewTraits<const char* (*)(), false>; Args = {const char (&)[2]}]’:
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:125:71:   required from ‘bool QHttpServer::routeHelper(QtPrivate::IndexesList<I ...>, Args&& ...) [with Rule = QHttpServerRouterRule; ViewHandler = const char* (*)(); ViewTraits = QHttpServerRouterViewTraits<const char* (*)(), false>; int ...I = {0}; Args = {const char (&)[2], const char* (*)()}]’
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:69:58:   required from ‘bool QHttpServer::route(Args&& ...) [with Rule = QHttpServerRouterRule; Args = {const char (&)[2], const char* (*)()}]’
../httptest/main.cpp:16:15:   required from here
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:135:59: error: no matching function for call to ‘QHttpServerRouter::bindCaptured(const char* (* const&)(), const QRegularExpressionMatch&)’
  135 |             auto boundViewHandler = router()->bindCaptured(viewHandler, match);
      |                                     ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
In file included from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserver.h:9,
                 from /home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/QHttpServer:1,
                 from ../httptest/main.cpp:2:
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouter.h:57:39: note: candidate: ‘template<class ViewHandler, class ViewTraits> typename ViewTraits::BindableType QHttpServerRouter::bindCaptured(ViewHandler&&, const QRegularExpressionMatch&) const’
   57 |     typename ViewTraits::BindableType bindCaptured(ViewHandler &&handler,
      |                                       ^~~~~~~~~~~~
/home/user/Qt/6.4.3/gcc_64/include/QtHttpServer/qhttpserverrouter.h:57:39: note:   substitution of deduced template arguments resulted in errors seen above
make: *** [Makefile:834: main.o] Error 1
14:38:56: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project httptest (kit: Desktop Qt 6.4.3 GCC 64bit)
When executing step "Make"
14:38:56: Elapsed time: 00:01.

JonB · 30 Mar 2023, 12:56

@Tarae
Yep, that's pretty complex!

Wait for @jsulm's better reply, but maybe you are required to put in the two optional parameters for this free/C function to be acceptable? Worth a try?

Tarae · 30 Mar 2023, 12:58

@JonB As you suggested, I've tried this:

const char * responseFunction(const QHttpServerRequest & request, QHttpServerResponder && responder)
{
  return "Hello world!";
}

Still doesn't compile, same error.

jsulm · 30 Mar 2023, 13:01

@Tarae said in QHttpServer: use function pointer as ViewHandler:

const char * responseFunction

Try without const:

char * responseFunction(...

Tarae · J jsulm 30 Mar 2023, 12:58

@jsulm No, const char *, char *, QString, const QString, QByteArray, const QByteArray. Still same error.

JonB · 30 Mar 2023, 13:23

@Tarae
I'm only guessing, but since those parameters are optional try specifying defaults for them. Assuming you can do that in a free/C function, I don't know.

The const char * for the return value is correct.

BTW: if all else fails, you can always make it a lambda like the examples show and have that lambda simply call your free function....

Tarae · 31 Mar 2023, 08:04

@JonB specifying defaults for const QHttpServerRequest & and QHttpServerResponder && is not that easy. But I don't feel like it will solve my problem.
I know I can use a lambda to call my function, but the documentation says function pointer, so I want to figure it out.

Christian Ehrlicher · 31 Mar 2023, 13:47

From my pov a plain function is currently not possible so either the documentation is wrong or the implementation. Since there is an easy workaround for it I would guess the solution is to adjust the documentation. Please create a bug report for it and post the bug number here.

JonB · 31 Mar 2023, 08:15

If this is what @Christian-Ehrlicher says, no wonder we could not figure it yesterday! :)

Christian Ehrlicher · 31 Mar 2023, 08:39

A plain function does not work but a std::function<> since the template magic expects a callable:

std::function<const char* ()>  resp = &responseFunction;
server.route("/", resp);

JonB · wrote on 31 Mar 2023, 08:39

@Christian-Ehrlicher
I am whistling in the dark here, but I'm a student of C++ so I am interested. I am not as expert as you. The issue is "a callable". I do not have Qt6 so cannot test QHttpServer::route() If you don't want to use a lambda or std::function here, could you use something like:

struct callable
{
  const char *operator()() const
  {
    std::cout << "Hello World from a callable!\n";
    return "Hello World!";
  }
};

const callable responseFunction;
server.route("/", &responseFunction);

Maybe this is the sort of thing a lambda or std::function() does which is different from a plain function?

Tarae · 25 Apr 2023, 18:34

@Christian-Ehrlicher I've created a bug report: https://bugreports.qt.io/browse/QTBUG-112484

Christian Ehrlicher · T Tarae 31 Mar 2023, 13:47

And fixed