Singleton with two getInstance() methods handing over a parent pointer?

Question

I am still working on my Logger and I like the idea of a Singleton but my Logger derives frm QDialog, thus I would like to handle my Parent QWidget* MainWindow pointer when I call it first:

class Logger : public QDialog {
  Q_OBJECT

private:  
  explicit Logger(QWidget* parent = 0);

public:
  static Logger& firstInstance(QWidget* parent = 0) {
       static Logger theInstance(parent);
       return theInstance;
  }
  static Logger& instance() {
       return theInstance;
  }
  //..
}

So I would call Logger::firstInstance(this); from my MainWindow. And Logger::instance() from elsewhere. But my compiler mocks:

Error: 'theInstance' was not declared in this scope: return theInstance;

in the second instance() method.

Well, as the compiler says: There is no variable named `theInstance` in your second method. How is the compiler supposed to find it, then? — anderas, Aug 20 '15 at 14:20
Why is the logger a *dialog*? That sounds like a very strange thing for a logger to be. — molbdnilo, Aug 20 '15 at 14:27
@PiotrNycz When you cant justify your argument, it is sound and smoke — MarkZltr, Aug 21 '15 at 07:43
@MarkZir 1) See this http://stackoverflow.com/questions/137975/what-is-so-bad-about-singletons 2) Your question itself is a prof that world w/out singletons would be simpler... — PiotrNycz, Aug 21 '15 at 07:56

score 1 · Answer 1 · answered Aug 20 '15 at 14:18

You should actually call just firstInstance from instance, since you have static variable in firstInstance it will be initialized only on first call, then just returned already initialized variable.

  static Logger& instance() {
       return firstInstance();
  }

But actually, function firstInstance in public interface is bad idea, probably it will be better to make it private and declare MainWindow friend class.

Singleton with two getInstance() methods handing over a parent pointer?

1 Answers1

Linked