Question

I have come across the singleton pattern.
I was unable to understand the difference between

singletonobj.getinstance().dosomething() //1st one

and

singletonobj.dosomething() //2nd one

What does getinstance() do, that isn’t being done in the second case?

Answer 1

First, singletonobj is a wrong/confusing name. Singletons are called on a class basis, like:

Log::getInstance().doSomething();

and not

Log log = new Log;
log.getInstance().doSomething();

So this should answer the question, but I’ll detail anyway 🙂

Log::doSomething();

would force doSomething() to be a static method, while

Log::getInstance().doSomething();

has doSomething() as an instance method.

Why use getInstance()? Because a singleton, by its very definition, should only have zero or one instances. By making the constructor for a singleton private and publishing the getInstance() method, it allows you to control how many instances of this class there are. The private constructor is simply to avoid other classes to instance this class, which would defeat the purpose of this class being a singleton, as you wouldn’t be able to control how many instances of this class there are.

class SingletonExample {

    private:
    static SingletonExample* instance = NULL;

    SingletonExample() { }

    public:

    static SingletonExample getInstance() {

        if (instance == NULL) {
            instance = new SingletonExample;
        }

        return *instance;
    }

    void doSomething() {            
        // Do something :)
    }


}