Putting classes inside namespaces

Question

I see that Qt puts a class inside the Ui interface like this:

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    ...

Is this method the same as enclosing the entire class inside the namespace? It certainly looks cleaner.

Answer 1

No, in your example Ui::MainWindow is a different class than the MainWindow class defined in the global namespace.

Behold:

namespace Ui {
    class MainWindow;
}

class MainWindow
{

};

int main()
{
    Ui::MainWindow mw; // fails due to incomplete type
}

This code won't compile, since Ui::MainWindow is an incomplete type.

Most likely, the Qt code was just using a forward declaration. You can forward declare the class in a namespace, but you still must actually implement the class in the same namespace, or else it is not the same class.