getSingletonInstance() for singletons in QML

Yihua Liu

I'm reading https://doc.qt.io/qt-6/qml-singleton.html#exposing-an-existing-object-as-a-singleton where there is a code snippet:

SingletonForeign::s_singletonInstance = getSingletonInstance();
QQmlApplicationEngine engine;
engine.loadFromModule("MyModule", "Main");

However, I did not find the definition or descriptions of the function getSingletonInstance() anywhere. What is this function and how can I define and use it? Thanks

Shrishashank

The function getSingletonInstance() is not a built-in Qt function. It is likely a custom function that the code author created to provide a singleton instance of the SingletonForeign class.

A singleton is a design pattern that ensures that a class has only one instance and provides a global point of access to that instance. The getSingletonInstance() function in this context is probably used to retrieve or create the single instance of the SingletonForeign class.

Here’s an example of how you can define the getSingletonInstance() function for a singleton class:

Step 1: Define the SingletonForeign class

class SingletonForeign {
public:
static SingletonForeign* getSingletonInstance() {
if (s_singletonInstance == nullptr) {
s_singletonInstance = new SingletonForeign();
}
return s_singletonInstance;
}

static void cleanup() {
    delete s_singletonInstance;
    s_singletonInstance = nullptr;
}

private:
SingletonForeign() {}

// Static member to hold the single instance
static SingletonForeign* s_singletonInstance;

};

SingletonForeign* SingletonForeign::s_singletonInstance = nullptr;

Step 2: Use the getSingletonInstance() method in your main code
In your case, you would use getSingletonInstance() to get the singleton instance of SingletonForeign:
SingletonForeign::s_singletonInstance = SingletonForeign::getSingletonInstance();
QQmlApplicationEngine engine;
engine.loadFromModule("MyModule", "Main");