Qt Plugin Loading fails (specified module could not be found) - qt

Question

For some reason I am unable to load my plugins anymore, although it has worked previously. I have a plugin loader code in my MainWindow, which is supposed to load every .dll found in a specific folder. The MainWindow Code contains the following:
Interface
#ifndef PLUGININTERFACE_H
#define PLUGININTERFACE_H
#include <QtPlugin>
// forward declarations
class MainWindow;
struct P3DData;
class PluginInterface
{
public:
virtual bool createPublisher(MainWindow*, P3DData*) = 0;
};
#define PLUGIN_INTERFACE_iid "PluginInterface"
Q_DECLARE_INTERFACE(PluginInterface, PLUGIN_INTERFACE_iid)
#endif // PLUGININTERFACE_H
loadPlugins()
bool MainWindow::loadPlugins()
{
QDir pluginsDir(qApp->applicationDirPath());
pluginsDir.cd("plugins");
const auto entryList = pluginsDir.entryList(QDir::Files);
for(const QString &fileName : entryList)
{
QString dllPath = pluginsDir.absoluteFilePath(fileName);
QPluginLoader* loader = new QPluginLoader(dllPath);
loaderList.push_back(loader);
QObject *plugin = loader->instance();
if (plugin)
{
pluginList.push_back(qobject_cast<PluginInterface *>(plugin));
pluginList.last()->createPublisher(this, simDataPtr);
pluginCount++;
continue;
}
else
{
qDebug() << "[PluginLoader] '" + fileName + "': " + loader->errorString();
delete loaderList.takeLast();
}
}
}
Besides my MainWindow, there is another subdir in my project, which is the plugin "Position". The plugin is deployed into the correct "plugins" folder, which the loadPlugin() method from the MainWindow iterates through. The plugin uses following code to implement the interface:
#include <QObject>
#include "MainWindow.h"
class PositionPublisher : public QObject, PluginInterface
{
Q_OBJECT
Q_PLUGIN_METADATA(IID "PluginInterface")
Q_INTERFACES(PluginInterface)
public:
PositionPublisher();
~PositionPublisher();
bool createPublisher(MainWindow* _window, P3DData* _simDataPtr) override;
//...
};
When trying to run loadPlugins() now, it checks the correct Position.dll file but the "if(plugin)" part returns false and loader->errorString() is executed giving the following error:
[PluginLoader] 'Position.dll': Cannot load library
F:\DEV\build\simNET\bin\plugins\Position.dll: Cannot find the
specified module.
I have already checked and tried the following:
the Plugins folder actually contains the Position.dll file
both projects (MainWindow and Plugin) are built in release mode
the dependencies of the plugin (two libs) exist and the specified path in the pro file is correct
.pro file of plugin:
QT += widgets
TEMPLATE = lib
CONFIG += c++11
CONFIG += plugin
CONFIG += release
SOURCES += \
Position.cpp \
PositionPubSubTypes.cpp \
PositionPublisher.cpp
HEADERS += \
Position.h \
PositionPubSubTypes.h \
PositionPublisher.h
DISTFILES += \
Position.idl
INCLUDEPATH += ../../application
TARGET = $$qtLibraryTarget(Position)
DESTDIR = ../../bin/plugins
INCLUDEPATH += "F:/DEV/prog/FastRTPSv1.5/include"
DEPENDPATH += "F:/DEV/prog/FastRTPSv1.5/include"
LIBS += -L"F:/DEV/prog/FastRTPSv1.5/lib/x64Win64VS2015" -lfastrtps-1.5
PRE_TARGETDEPS += F:/DEV/prog/FastRTPSv1.5/lib/x64Win64VS2015/fastrtps-1.5.lib
INCLUDEPATH += 'F:/Programme/Prepar3D v4/SDK/inc/SimConnect'
DEPENDPATH += 'F:/Programme/Prepar3D v4/SDK/inc/SimConnect'
LIBS += -L'F:/Programme/Prepar3D v4/SDK/lib/SimConnect' -lSimConnect
PRE_TARGETDEPS += 'F:/Programme/Prepar3D v4/SDK/lib/SimConnect/SimConnect.lib'
Does anyone have an idea why it does not load the plugin??

Answer 1

Qt plugin loader is trying to tell you that a Qt module used in your plugin doesn't exist so first you need to check which Qt modules your plugin depends on then,
If you're trying to run the application from Qt Creator itself, and it gives you this error, then maybe the DLL file got deleted for some reason or you're trying to use the plugin with a Qt version that doesn't have its needed modules. (for example, Qt6 doesn't contain the QtSerialPort module but Qt5 did (at the time of writing this)).
If you're trying to run the application directly outside Qt Creator then you need to copy the required module's dll file into your applications root directory.