Accessing Singleton from across the Qt app
-
Hi,
I'm still in my learning process of both C++ and Qt. Recently I learned the Singleton pattern and tried out an example with a few modification to the code. I tested the code as a plain C++ program and it worked as expected. I now want to make this singleton be part of my Qt app. Where should I create/initialize the singleton to be able to access the same instance from across the Qt app.
I used qt designer to create my app. It has main, mainwindow and several Qdialogs. I want to be able to access the Singleton instance from all of them.
-
You access your singleton class in a C++ program using Qt exactly the same way as in a C++ program that does not use Qt: through its static "instance" function. That function will either cause the instance to be created, or provided a pointer/reference to the one that already exists.
-
@WhatIf
My advice: just don't use the singleton anti-pattern!
Here's some relevant discussions:
http://forum.qt.io/topic/64346/qtextdocument-and-multithreading
http://forum.qt.io/topic/62652/const-global-variable-outside-of-constructors
http://forum.qt.io/topic/62451/message-handler-crashes-if-installed-on-sepparate-class -
You access your singleton class in a C++ program using Qt exactly the same way as in a C++ program that does not use Qt: through its static "instance" function. That function will either cause the instance to be created, or provided a pointer/reference to the one that already exists.
Well, in my plain c++ program, the only place where I accessed the singleton was from inside the main function.
Now, in a Qt program, I will have to access the singleton from mainwindow and a few QDialogs. If I include (#include "Singelton") in mainwindow and the few QDialogs, won't that create multiple objects of the singleton each has a single instance. For example,
-
I call, Singleton::getInstance()->anyFunction();, from mainwindow, if it's the first code to call getInsatnce(), the instance (instance #1) will be created and only that single instance will be used anywhere inside of mainwindow.
-
I call, Singleton::getInstance()->anyFunction();, from any QDialog, if it's the first code to call getInsatnce(), the instance (instance #2) will be created and only that single instance will be used anywhere inside of a specific QDialog.
Is it one instance per class or app?
-
-
Hi
If the singleton is programmed correctly, it is intended to be used in whole program and
will only 1 instance of the class will exists for whole application.so one instance pr app.
but as @kshegunov points out, its not a good pattern as it very near to just
using global a variable. -
Hi
If the singleton is programmed correctly, it is intended to be used in whole program and
will only 1 instance of the class will exists for whole application.so one instance pr app.
but as @kshegunov points out, its not a good pattern as it very near to just
using global a variable. -
Is there another pattern that provide the same functionality the Singleton pattern does that is safer to use?
@WhatIf
yes you can use dependency injection
https://en.wikipedia.org/wiki/Dependency_injection
https://www.youtube.com/watch?v=IKD2-MAkXyQIt sounds so funky but in its base idea, you just give the object
(that would have been the singleton) to the other classes that need it via
the constructor.So instead of it somehow knows of a global function, u give it to the classes when u construct them. While is slightly more work, it also make it easier to
create test version of the "service" class(es) u pass around or other such
functions. Also u can then give different classes different version of the service if needed. something not possible with singleton. -
@WhatIf
yes you can use dependency injection
https://en.wikipedia.org/wiki/Dependency_injection
https://www.youtube.com/watch?v=IKD2-MAkXyQIt sounds so funky but in its base idea, you just give the object
(that would have been the singleton) to the other classes that need it via
the constructor.So instead of it somehow knows of a global function, u give it to the classes when u construct them. While is slightly more work, it also make it easier to
create test version of the "service" class(es) u pass around or other such
functions. Also u can then give different classes different version of the service if needed. something not possible with singleton.yes you can use dependency injection
https://en.wikipedia.org/wiki/Dependency_injectionThis is a thing?! Unbelievable! Everything is a pattern now. :D
@WhatIf
Do it as @mrjj suggested, it's the most clean way to accomplish what you want. Just have a set method (and/or constructor) accepting your external object and use that.Read and abide by the Qt Code of Conduct
-
yes you can use dependency injection
https://en.wikipedia.org/wiki/Dependency_injectionThis is a thing?! Unbelievable! Everything is a pattern now. :D
@WhatIf
Do it as @mrjj suggested, it's the most clean way to accomplish what you want. Just have a set method (and/or constructor) accepting your external object and use that.This is a thing?! Unbelievable! Everything is a pattern now. :D
he he.
yes yes. They saw pattern putting parameters in constructors.. :)
They even made whole frameworks for it.
https://sites.google.com/site/fruitlib/ -
Look at this article.
