在另一个类的方法中创建类的实例

Question

this is probably a really basic C++ question but I simply cannot find out a way to properly do this:

I need to create some instances of class "QPluginLoader" within a method of my GUI class and want to store these instances within a QList. The QPluginLoader instances have to be available throughout the whole GUI class. But whenever I try to access a QPluginLoader through my QList the program crashes. I assume that is because once out of the scope where I instantiated the QPluginLoader class, it is destroyed, so it's not available outside of the method where I created the QPluginLoader and stored it into the QList.

But how do I make the instance available in the whole class?

Here is what I tried:

mainwindow.h

class MainWindow : public QMainWindow
{
    Q_OBJECT

public slots:
    void onThreadTerminated();

private:
    bool loadPlugins();
    QList<QPluginLoader*> loaderList;
};

mainwindow.cpp

bool MainWindow::loadPlugins()
{
    QDir pluginsDir(qApp->applicationDirPath());
    pluginsDir.cd("plugins");

    foreach (QString fileName, pluginsDir.entryList(QDir::Files))
    {
        QPluginLoader pluginLoader(pluginsDir.absoluteFilePath(fileName));
        QObject* plugin = pluginLoader.instance();

        if (plugin)
        {
            loaderList.push_back(&pluginLoader);
            pluginCount++;
        }
    }
    return false;
}

last but not least, the slot which crashes my program

// SLOT
void MainWindow::onThreadTerminated()
{
    while (!loaderList.isEmpty()) {
       if(loaderList.takeFirst()->isLoaded()) loaderList.takeFirst()->unload();
    }
    loaderList.clear();
}

Answer 1

foreach (QString fileName, pluginsDir.entryList(QDir::Files))
{
    QPluginLoader pluginLoader(pluginsDir.absoluteFilePath(fileName));

pluginLoader will be destroyed at the next } and you keep a reference of this destroyed object in loaderList .

Allocate memory for pluginLoader dynamically using new and keep pointers to QPluginLoader in loaderList

Ofcourse you will have to remember to free the allocated memory for QPluginLoader s

Answer 2

The problem is these two lines in the loadPlugins function

QPluginLoader pluginLoader(pluginsDir.absoluteFilePath(fileName));
...
loaderList.push_back(&pluginLoader);

The first define the local object pluginLoader . The second pushes a pointer to that local object into the container.

The problem is that when the loop iterates (or ends), the life-time of pluginLoader ends and it is destructed, the pointer to it becomes invalid.

It seems to me that you should not have a container of plugin loaders, but of the actual plugins themselves.

If you really want the plugin loader in the container, you need to create those dynamically with new , or at least use smart pointers for it. Or, as mentioned in a comment to your question, use a container of QPluginLoader objects .