Failure to build pcl against Qt 6

[fxcoudert]

Describe the bug

Building pcl 1.13.1 against Qt 6.5.1 as part of Homebrew (where we previously used Qt 5.15.10) leads to a build failure:

  In file included from /tmp/pcl-20230731-43792-ujwqr0/pcl-pcl-1.13.1/apps/cloud_composer/src/signal_multiplexer.cpp:1:
  /tmp/pcl-20230731-43792-ujwqr0/pcl-pcl-1.13.1/apps/cloud_composer/include/pcl/apps/cloud_composer/signal_multiplexer.h:51:38: error: base class has incomplete type
      class SignalMultiplexer : public QObject
                                ~~~~~~~^~~~~~~
  /opt/homebrew/include/QtCore/qtclasshelpermacros.h:81:7: note: forward declaration of 'QObject'
  class QObject;
        ^
  In file included from /tmp/pcl-20230731-43792-ujwqr0/pcl-pcl-1.13.1/apps/cloud_composer/src/signal_multiplexer.cpp:1:
  /tmp/pcl-20230731-43792-ujwqr0/pcl-pcl-1.13.1/apps/cloud_composer/include/pcl/apps/cloud_composer/signal_multiplexer.h:146:27: error: implicit instantiation of undefined template 'QList<pcl::cloud_composer::SignalMultiplexer::Connection>'
          QList<Connection> connections;
                            ^
  /opt/homebrew/include/QtCore/qcontainerfwd.h:29:29: note: template is declared here
  template <typename T> class QList;
                              ^
  /tmp/pcl-20230731-43792-ujwqr0/pcl-pcl-1.13.1/apps/cloud_composer/src/signal_multiplexer.cpp:5:5: error: type 'QObject' is not a direct or virtual base of 'pcl::cloud_composer::SignalMultiplexer'
    : QObject (parent)
      ^~~~~~~
  3 errors generated.

Context

Trying to move, in Homebrew (a macOS open source distro), as many packages to Qt 6 as possible given the near EOL of Qt 5: Homebrew/homebrew-core#138034

Expected behavior

Build with Qt 5 is successful. I hoped it would be the same with Qt 6 :)

Current Behavior

See error above.

To Reproduce

We build pcl from source with:

cmake .. -DCMAKE_INSTALL_PREFIX=/opt/homebrew/Cellar/pcl/1.13.1_2 -DCMAKE_INSTALL_LIBDIR=lib -DCMAKE_BUILD_TYPE=Release -DCMAKE_FIND_FRAMEWORK=LAST -DCMAKE_VERBOSE_MAKEFILE=ON -Wno-dev -DBUILD_TESTING=OFF -DCMAKE_OSX_SYSROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX11.sdk -DBUILD_SHARED_LIBS:BOOL=ON -DBUILD_apps=AUTO_OFF -DBUILD_apps_3d_rec_framework=AUTO_OFF -DBUILD_apps_cloud_composer=AUTO_OFF -DBUILD_apps_in_hand_scanner=AUTO_OFF -DBUILD_apps_point_cloud_editor=AUTO_OFF -DBUILD_examples:BOOL=OFF -DBUILD_global_tests:BOOL=OFF -DBUILD_outofcore:BOOL=AUTO_OFF -DBUILD_people:BOOL=AUTO_OFF -DBUILD_simulation:BOOL=ON -DWITH_CUDA:BOOL=OFF -DWITH_DOCS:BOOL=OFF -DWITH_TUTORIALS:BOOL=OFF -DBoost_USE_DEBUG_RUNTIME:BOOL=OFF -DBUILD_apps_modeler:BOOL=OFF -DPCL_ENABLE_MARCHNATIVE:BOOL=OFF && make install

Your Environment (please complete the following information):

• OS: macOS 11
• Compiler: AppleClang 13.0.0.13000029
• PCL Version: 1.13.1

[mvieth]

@fxcoudert Can you try the following, please:

diff --git a/apps/cloud_composer/include/pcl/apps/cloud_composer/signal_multiplexer.h b/apps/cloud_composer/include/pcl/apps/cloud_composer/signal_multiplexer.h
index 86c72299a..c8a5224f5 100644
--- a/apps/cloud_composer/include/pcl/apps/cloud_composer/signal_multiplexer.h
+++ b/apps/cloud_composer/include/pcl/apps/cloud_composer/signal_multiplexer.h
@@ -42,6 +42,8 @@
 
 #pragma once
 
+#include <QList>
+#include <QObject>
 #include <QPointer>
 
 namespace pcl

[fxcoudert]

Yes, that diff fixes the build issue. Thanks!