How to iteratively qobject_cast items from a QList - c++

Question

I was trying out the "echo plugin" from the qt examples and i changed somethings to try out and stumbled upon an error that i cant fix.
here is the original Echo Window class
#include <QWidget>
#include "echointerface.h"
QT_BEGIN_NAMESPACE
class QString;
class QLineEdit;
class QLabel;
class QPushButton;
class QGridLayout;
QT_END_NAMESPACE
//! [0]
class EchoWindow : public QWidget
{
Q_OBJECT
public:
EchoWindow();
private slots:
void sendEcho();
private:
void createGUI();
bool loadPlugin();
EchoInterface *echoInterface; ***********
QLineEdit *lineEdit;
QLabel *label;
QPushButton *button;
QGridLayout *layout;
};
what i changed is the the line i highlighted with the stars. i changed it into A Qlist like this
QList<EchoInterface *>echoInterfaces
also changed the implementation of the function LoadPlugin() From this:-
bool EchoWindow::loadPlugin()
{
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) {
echoInterface = qobject_cast<EchoInterface *>(plugin);
if (echoInterface)
return true;
}
}
return false;
}
To this :-
bool ModifyWindow::loadPlugin()
{
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) {
foreach ( EchoInterface *a, echoInterfaces)
{
a = qobject_cast <EchoInterface *>(plugin);
}
if(!echoInterfaces.contains(NULL))
return true;
}
}
return false;
}
and thanks in advance

Answer 1

If my guess is correct, you'd like to store a collection of interfaces and your question is about how to read it properly.
I do understand that
assumption is the mother of all screw-ups
:) But let's not focus on the reason for doing this and just guess (again) that the EchoInterface is a base class for a variety of some available interfaces.
What's wrong in your code:
bool ModifyWindow::loadPlugin()
{
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) {
foreach ( EchoInterface *a, echoInterfaces)
{
// this is pointless - only a tmp variable 'a' is modified for a while
// just to be reset on the next iteration
a = qobject_cast <EchoInterface *>(plugin);
}
if(!echoInterfaces.contains(NULL)) // use nullptr instead of NULL
return true; // echoInterfaces is always empty, so it doesnt contain null
}
}
return false;
}
The changes you're loking for:
bool EchoWindow::loadPlugin()
{
QDir pluginsDir(QCoreApplication::applicationDirPath());
pluginsDir.cd("plugins");
const QStringList entries = pluginsDir.entryList(QDir::Files);
for (const QString &fileName : entries) {
QPluginLoader pluginLoader(pluginsDir.absoluteFilePath(fileName));
QObject *plugin = pluginLoader.instance();
// it's unnecessary to check if plugin == nullptr,
// because in this case qobject_cast returns nullptr
// and we go directly to pluginLoader.unload();
if (EchoInterface * echoIface = qobject_cast<EchoInterface *>(plugin)) {
echoInterfaces.append(echoIface); // populate the collection with a valid pointer
continue; // to avoid the pluginLoader.unload() call which would destroy the echoIface
}
pluginLoader.unload();
}
// Since we ensured that the echoInterfaces can contain only valid pointers,
// no additional checks are necessary — the fact that something is stored in the collection is enough
return !echoInterfaces.isEmpty();
// or
// return echoInterfaces.size() > 0;
// or even just
// return echoInterfaces.size();
}
In case my assumptions were wrong, then we definitely need a more detailed question.