Multiple definition of `qt_plugin_instance', several plugins in one library

  • 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.

  • As you discovered; you should only use the Qt macros wrt plugins once in a library.

    Not knowing the Qt specifics for plugins, maybe you can register more than one plugin in the init code of the library?

  • Yes, it seems to work as long as i just have the macro "Q_PLUGIN_METADATA" once in the project. I thought i could implement several plugins in one project, but apparently i have to have a project for each plugin. Is there absolutely no way to implement more than one plugin in one project? It's alot of work to have a project per plugin.

  • [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

    class PluginB : public QObject, public InterfaceB

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

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

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

    Brain to terminal.

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