CODEWITHSUNDEEP
c++design-patternssingleton
Rasmi Ranjan Nayak
Rasmi Ranjan Nayak

Reputation: 11968

What exactly does getinstance() do in a singleton?

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?

Upvotes: 3

Views: 8021

Answers (3)

m0skit0
m0skit0

Reputation: 25874

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 :)
    }


}

Upvotes: 2

Luchian Grigore
Luchian Grigore

Reputation: 258628

Well, technically, singletonobj.getinstance() is redundant, because that means you already got a hold of the object.

The call would usually look something like:

SingletonClass::getinstance().dosomething();

or

singletonobj.dosomething()

in case you pre-fetched the object - i.e. previously did

SingletonClass& singletonobj = SingletonClass::getinstance();

Upvotes: 9

josh-cain
josh-cain

Reputation: 5226

It would appear that that the first example is calling a regular method, whereas the second example is calling a static method.

The Singleton design pattern ensures that only a single instance of a given object is instantiated at any given time. The first example returns said instance, and then calls a method using an instance of an object.

The second example appears to be using a static method that does not require an instance of an object and actually invalidates the singleton design pattern...

Upvotes: 1

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