Mess with inherited constructors

[jpn]

Could we see the implementation of the problematic constructor?

[Pepe]

MyAction::MyAction(QObject * parent, const QString & name) : QAction(parent)
{
	setObjectName(name);
}

MyAction::MyAction(QObject * parent, const QString & name) : QAction(parent)
{
	setObjectName(name);
}

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Anyway this code is not called.

[jpn]

It just... won't magically jump directly to QAction constructor if you create a MyAction.

#include <QtGui>
#include <QtDebug>
 
class MyAction : public QAction
{
public:
	MyAction(QObject* parent, const QString& name);
};
 
MyAction::MyAction(QObject* parent, const QString& name)
	: QAction(parent)
{
	qDebug() << "MyAction::MyAction()";
	setText(name);
	setObjectName(name);
}
 
class MainWindow : public QMainWindow
{
public:
	MainWindow(QWidget* parent = 0);
};
 
MainWindow::MainWindow(QWidget* parent)
	: QMainWindow(parent)
{
	qDebug() << "MainWindow::MainWindow()";
	MyAction* action = new MyAction(this, "Action");
	menuBar()->addAction(action);
}
 
int main(int argc, char* argv[])
{
	QApplication a(argc, argv);
	MainWindow w;
	w.show();
	return a.exec();
}

#include <QtGui>
#include <QtDebug>

class MyAction : public QAction
{
public:
	MyAction(QObject* parent, const QString& name);
};

MyAction::MyAction(QObject* parent, const QString& name)
	: QAction(parent)
{
	qDebug() << "MyAction::MyAction()";
	setText(name);
	setObjectName(name);
}

class MainWindow : public QMainWindow
{
public:
	MainWindow(QWidget* parent = 0);
};

MainWindow::MainWindow(QWidget* parent)
	: QMainWindow(parent)
{
	qDebug() << "MainWindow::MainWindow()";
	MyAction* action = new MyAction(this, "Action");
	menuBar()->addAction(action);
}

int main(int argc, char* argv[])
{
	QApplication a(argc, argv);
	MainWindow w;
	w.show();
	return a.exec();
}

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[Pepe]

I think I have found the problem. It wasn't calling QAction ( QObject * parent, const char * name ) but another constructor that I had: MyAction(QObject *, bool).

But why a "myaction *action = new MyAction(this, "name")" could call that constructor with a bool argument? I don't get it.

And the problem is that I really need that constructor.

Your code modified:

#include <QtGui>
#include <QtDebug>
 
 
class MyAction : public QAction
{
public:
    MyAction(QObject* parent, const QString& name);
	MyAction(QObject *parent, bool a);
};
 
MyAction::MyAction(QObject* parent, const QString& name)
    : QAction(parent)
{
    qDebug() << "MyAction::MyAction()";
    setText(name);
    setObjectName(name);
}
 
MyAction::MyAction(QObject *parent, bool a) : QAction(parent)
{
    qDebug() << "MyAction::MyAction(QObject, bool)";
}
 
class MainWindow : public QMainWindow
{
public:
    MainWindow(QWidget* parent = 0);
};
 
MainWindow::MainWindow(QWidget* parent)
    : QMainWindow(parent)
{
    qDebug() << "MainWindow::MainWindow()";
    MyAction * action = new MyAction(this, "Action");
    menuBar()->addAction(action);
}
 
int main(int argc, char* argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

#include <QtGui>
#include <QtDebug>


class MyAction : public QAction
{
public:
    MyAction(QObject* parent, const QString& name);
	MyAction(QObject *parent, bool a);
};
 
MyAction::MyAction(QObject* parent, const QString& name)
    : QAction(parent)
{
    qDebug() << "MyAction::MyAction()";
    setText(name);
    setObjectName(name);
}

MyAction::MyAction(QObject *parent, bool a) : QAction(parent)
{
    qDebug() << "MyAction::MyAction(QObject, bool)";
}

class MainWindow : public QMainWindow
{
public:
    MainWindow(QWidget* parent = 0);
};
 
MainWindow::MainWindow(QWidget* parent)
    : QMainWindow(parent)
{
    qDebug() << "MainWindow::MainWindow()";
    MyAction * action = new MyAction(this, "Action");
    menuBar()->addAction(action);
}
 
int main(int argc, char* argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

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After running it, console output:

MainWindow::MainWindow()
MyAction::MyAction(QObject, bool)

[jpn]

The most sensible choice for a human is not always the most sensible choice for a machine.

A literal "Action" is a const char*. A pointer can be directly used as a boolean value whereas for a QString to match, the compiler would have to implicitly convert const char* to a QString. This is why MyAction(QObject*, bool) gets chosen over MyAction(QObject*, const QString&). Now, if you add MyAction(QObject*, const char*) as mentioned in the first post, it gets naturally chosen as a first choice because it's the closest possible match.

[bpetty]

Makes me wonder what would happen if you called MyAction(this, QString("name"));

[wysota]

It just... won't magically jump directly to QAction constructor if you create a MyAction.

Actually in such situation it will call the default QAction constructor, won't it?

[jpn]

Actually in such situation it will call the default QAction constructor, won't it?

The question was about

new MyAction(this, "name");

new MyAction(this, "name");

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