multiple definition of qt_plugin_query_metadata

dustije

5 years ago

I have a project with several plugins i want to compile into one library. I get the error:
@./debug\moc_stringoperationsplugin.o: In function qt_plugin_query_metadata': C:\...\ARGS-Plugins\ARGS-Plugins-build-Desktop_Qt_5_0_1_MinGW_32bit-Debug/debug/moc_stringoperationsplugin.cpp:153: multiple definition ofqt_plugin_query_metadata'
./debug\moc_tplugin.o:C:...\ARGS-Plugins\ARGS-Plugins-build-Desktop_Qt_5_0_1_MinGW_32bit-Debug/debug/moc_tplugin.cpp:153: first defined here
./debug\moc_stringoperationsplugin.o: In function qt_plugin_instance': C:\...\ARGS-Plugins\ARGS-Plugins-build-Desktop_Qt_5_0_1_MinGW_32bit-Debug/debug/moc_stringoperationsplugin.cpp:153: multiple definition ofqt_plugin_instance'
./debug\moc_tplugin.o:C:...\ARGS-Plugins\ARGS-Plugins-build-Desktop_Qt_5_0_1_MinGW_32bit-Debug/debug/moc_tplugin.cpp:153: first defined here@
Do i have to have a project for every single plugin or is it possible to have several plugins in one library? If it's possible to have several plugins in one library, what else could cause this error? I could not find a single helpful post online so i think this problem is very rare.
I'll post code if someone thinks that will be necessary but i hope to get a general answer to what could cause this error message.
Thanks in advance!

Edit: Quick question: does the IID in the "Q_PLUGIN_METADATA(IID "reversed.url")" need to be unique, e.g. does every plugin needs its own IID or should it be the same for every plugin? I couldn't find information regarding this.

 

 

lgeyer

5 years ago

[quote] Q_PLUGIN_METADATA(...)
This macro is being used to declare meta data that is part of a plugin that instantiates this object.
The macro needs to declare the IID of the interface implemented through the object, and reference a file containing the meta data for the plugin.
There should be exactly one occurrence of this macro in the source code for a Qt plugin.[/quote]

You can't have multiple Q_PLUGIN_METADATA definitions per plugin (for technical reasons, the plugin metadata is placed in a special region of the binary so the metadata can be fetched without dlopen()'ening the library and for logical reasons, as there has to be a single root object which is instantiated on load), but you can have multiple classes implementing various interfaces.

@
class PluginA : public QObject, public InterfaceA
{
Q_OBJECT
Q_INTERFACES(InterfaceA)
}

class PluginB : public QObject, public InterfaceB
{
Q_OBJECT
Q_INTERFACES(InterfaceB)
}

class PluginCollection : public QObject, public PluginCollectionInterface
{
Q_OBJECT
Q_PLUGIN_METADATA(IID "PluginCollectionInterface")

public:
QList<QObject*> plugins() { return QList<QObject*>{new PluginA, new PluginB}; }

// or
QList<QString> plugins() { ... }
QObject *create(const QString &plugin) { ... };

}
@
Brain to terminal.