CODEWITHSUNDEEP
c++singleton
fizzbuzz
fizzbuzz

Reputation: 157

How to understand Singleton in c++?

Please tell me why the singleton code below works? Every time When Singleton::instance() is called, an INSTANCE will be created, so are two INSTANCE supposed to be created in the following code?

#include <bits/stdc++.h>

using namespace std;

class Singleton
{
    private:
       Singleton() = default;

    public:
       static Singleton& instance()
       {
          static Singleton INSTANCE;
          return INSTANCE;
       }
};

int main()
{
    Singleton &s1 = Singleton::instance();
    Singleton &s2 = Singleton::instance();

    return 0;
}

Upvotes: 0

Views: 92

Answers (3)

ΦXocę 웃 Пepe&#250;pa ツ
ΦXocę 웃 Пepe&#250;pa ツ

Reputation: 48288

you can go line by line:

1. 
Singleton &s1 = Singleton::instance();

you call here the static method instance(), inside that method you see the line

 static Singleton INSTANCE;

the trick here is to understand that variables declared static inside a method will only declared once, no matter how many times you call the method after. So it creates the instance of Singleton and it gets returned

 2.
Singleton &s2 = Singleton::instance();

the second call will actually skip the delaration of INSTANCE and will return that immediatly.

for more clarifying you can put some meaningful messages in the constructor

something like

std::cout << "Contructor called!" << std::endl;

even better if you use a static variable as a counter just for testing purposes.

Upvotes: 2

eerorika
eerorika

Reputation: 238401

Every time When Singleton::instance() is called, an INSTANCE will be created, so are two INSTANCE supposed to be created in the following code?

No. Only one instance is created.

each time when expression static Singleton INSTANCE; executed. Isn't different INSTANCE created?

No.

I know what static means

Evidently, you don't know what you think you know.

Upvotes: 0

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122830

You should read about the static keyword. It can have different meaning and here you have two of them:

   static Singleton& instance()
   // ^ ---------------------------- (1)
   {
      static Singleton INSTANCE;
      // ^ ------------------------- (2)
      return INSTANCE;
   }

The first static declares the method instance as a "class-method" (or simply "static method"). You can call the method without having an instance.

The second static declares that INSTANCE has static storage duration. That is: There is only one. It is initialized once and when you reenter the function you get the exact same instance.

Upvotes: 3

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