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