Cleanup: unify idiosyncratic singletons

The way we handle singletons in QML, QML insists on allocating the
objects. This leads to a very idiosyncratic way of handling
singletons: The global instance pointer is set in the constructor.

Unify all these by implementing a "SillySingleton" template. All
of the weird singleton-classes can derive from this template and
don't have to bother with reimplementing the instance() function
with all the safety-checks, etc.

This serves firstly as documentation but also improves debugging
as we will now see wanted and unwanted creation and destruction
of these weird singletons.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

1 files changed, 74 insertions, 0 deletions

diff --git a/core/singleton.h b/core/singleton.h
new file mode 100644
index 000000000..9573b07e8
--- /dev/null
+++ b/core/singleton.h
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * We use singletons in numerous places. In combination with QML this gives
+ * us a very fundamental problem, because QML likes to allocate objects by
+ * itself. There are known workarounds, but currently we use idiosyncratic
+ * singleton classes, which initialize the global instance pointer in the
+ * constructor. Things get even more complicated if the same singleton is used
+ * on mobile and on desktop. The latter might want to simply use the classical
+ * instance() function without having to initialize the singleton first, as
+ * that would beat the purpose of the instance() method.
+ *
+ * The template defined here, SillySingleton, codifies all this. Simply derive
+ * a class from this template:
+ *	class X : public SillySingleton<X> {
+ *		...
+ *	};
+ * This will generate an instance() method. This will do the right thing for
+ * both methods: explicit construction of the singleton via new or implicit
+ * by calling the instance() method. It will also generate warnings if a
+ * singleton class is generated more than once (i.e. first instance() is called
+ * and _then_ new).
+ *
+ * In the long run we should get rid of all users of this class.
+ */
+#ifndef SINGLETON_H
+#define SINGLETON_H
+
+#include <typeinfo>
+#include <QtGlobal>
+
+// 1) Declaration
+template<typename T>
+class SillySingleton {
+	static T *self;
+protected:
+	SillySingleton();
+	~SillySingleton();
+public:
+	static T *instance();
+};
+
+template<typename T>
+T *SillySingleton<T>::self = nullptr;
+
+// 2) Implementation
+
+template<typename T>
+SillySingleton<T>::SillySingleton()
+{
+	if (self)
+		qWarning("Generating second instance of singleton %s", typeid(T).name());
+	self = static_cast<T *>(this);
+	qDebug("Generated singleton %s", typeid(T).name());
+}
+
+template<typename T>
+SillySingleton<T>::~SillySingleton()
+{
+	if (self == this)
+		self = nullptr;
+	else
+		qWarning("Destroying unknown instance of singleton %s", typeid(T).name());
+	qDebug("Destroyed singleton %s", typeid(T).name());
+}
+
+template<typename T>
+T *SillySingleton<T>::instance()
+{
+	if (!self)
+		new T;
+	return self;
+}
+
+#endif