Shared Library: undefined reference to `Plugin_API::getName() const'

[fem_dev]

I'm developing a Qt C++ shared library and unintentionally I changed my source-code in a way that not only the plugin interface definition (*.h) must be in the main app side, but the implementation too.

I know that is wrong, but I can't found how to fix it.
If I remove the implementation on the main app side, I got this compilation time error:

undefined reference to `Plugin_API::getName() const'

The code is very long, but I will try to resume here:

My Plugin_API class have:
a) Some pure virtual methods (that will be called by the Main App - API)
b) Another pure virtual methods (Only to force the MyLib class to implement them. They are not called direct by the Main App)
c) Private, protected and public methods
d) Some signals (that will be used for all developed plugins)

#ifndef PLUGIN_QT_API_H
#define PLUGIN_QT_API_H

#include <QtPlugin>
#include <QString>

class Plugin_API : public QObject
{
    Q_OBJECT

private:
    ...

protected:
    ...

public:
    Plugin_API();
    virtual ~Plugin_API() = default;
 ...
    const QString getName(void) const;

public slots:
    ...

signals:
    ...
};

Q_DECLARE_INTERFACE(Plugin_API, "com.rotortest.plugin")

#endif // PLUGIN_QT_API_H

This header file was copied and pasted inside the Main App, and include it in the project. All ok!

I have the plugin_api.cpp that implements the b) and c) cases above. Example:

const QString Plugin_API::getName(void) const {
    return _pluginName;
}

Finally, in the Main App project I included #include "plugin_api.h".
I loaded it using QPluginLoader. It works!
So, I'm trying to use the loaded plugin in this way:

void App::SomeMethod()
{
    const QString pluginName = plugin->getName();
}

But I got this compile time error:

undefined reference to `Plugin_API::getName() const'

The only way that I found to solve this is copying and pasting the plugin_api.cpp inside the Main App project.

Could you help me?

UPDATE 1

Here is how I loaded all plugins inside the Main App:

void App::loadPlugins(const QStringList pluginsList)
{
    foreach(QString file, pluginsList) {

        QPluginLoader loader(file);

        // Error checking:
        if (!loader.load()) {
            sendErrorMsg(tr("%1: ").arg(loader.errorString()));
            continue;
        }

        // Get and display the plugin file name:
        const QStringList pathSplited = loader.fileName().split("/");
        const QString fileName = pathSplited.last();
        sendStatusMsg(tr("Loading plugin: %1").arg(fileName));

        // Cast plugin interface:
        Plugin_API* plugin = qobject_cast<Plugin_API*>(loader.instance());

        // Error chekcing: nullptr
        if (!plugin) {
            sendErrorMsg(tr("Wrong Plugin API! Could not cast: %1").arg(loader.fileName()));
            continue;
        }

        // ===== CONNECT ALL SIGNALS /  SLOTS ===== //
        bool conn1 = connect(plugin, &Plugin_API::sendMsg, this, &App::receivedMsg);
        bool conn2 = connect(plugin, &Plugin_API::savePluginSettings, this, &App::receivedPluginSettings);

        bool allConnections = conn1 & conn2;

        if (!allConnections) {
            sendErrorMsg(tr("Unable to connect: %1").arg(plugin->getName()));
            continue;
        }

        plugin->setProject(&_project);

        // Store the plugin in a map:
        _pluginsMapMap[plugin->getType()][plugin->getName()] = plugin;
    }
}

In the app.h I declared the _pluginsMapMap as private member of the App class:

std::map<QString, std::map<QString, Plugin_API*>> _pluginsMapMap;

[SGaist]

Then you should start with the habit of properly exporting the symbols you want to use otherwise you'll have surprises on Windows.

Your plugin interface is not a pure virtual interface hence your app has to know its implementation.

In this case the proper design is:

• shared library
• plugin
• application

The shared library provides the core class that will be used by both the plugin and the application.

[SGaist]

Hi,

As already asked on your other thread, are you developing on Windows ?

[fem_dev]

@SGaist No...I'm developing both plugins and main app in Linux Ubuntu 20.04.

But, in the future, I will compile both in 3 OS: Windows, Linux and Mac OSX.

My system:

• Qt Creator IDE 4.12.4
• Qt 5.15.0
• Ubuntu 20.04 x64

[SGaist]

Then you should start with the habit of properly exporting the symbols you want to use otherwise you'll have surprises on Windows.

Your plugin interface is not a pure virtual interface hence your app has to know its implementation.

In this case the proper design is:

• shared library
• plugin
• application

The shared library provides the core class that will be used by both the plugin and the application.

[fem_dev]

@SGaist said in Shared Library: undefined reference to `Plugin_API::getName() const':

Then you should start with the habit of properly exporting the symbols you want to use otherwise you'll have surprises on Windows.

Perfect explanation! Thank you so much!