Let's know what is Singletons :

Sometimes we need to have only one instance of our class for example a single DB connection shared by multiple objects as creating a separate DB connection for every object may be costly. Similarly, there can be a single configuration manager or error manager in an application that handles all problems instead of creating multiple managers and hence , Singletons comes ;

Definition :

The singleton pattern is a design pattern that restricts the instantiation of a class to one object.

Simple Implementation :

#include "iostream"
class Singleton{
    public:
        Singleton(const Singleton& ) = delete ;  // Singletons should not be cloneable.
        static Singleton& getInstance(){
            return s_instance;
        };
    private:
        Singleton(){};
        static Singleton s_instance;
};

Singleton Singleton::s_instance;

int main(){
    auto& ins = Singleton::getInstance();
    return 0;
};

A Simple Example based on Singletons :

#include "iostream"
class Random{
    public:
        Random(const Random& ) = delete ;       
        static Random& getInstance(){
            static Random s_instance;
            return s_instance;
        };
        static float Float(){ return getInstance().IFloat(); };
    private:    
        Random(){};
        float IFloat(){ return getRandom; };
        float getRandom = 0.5f; 
};
int main(){
    auto random = Random::Float();
    std::cout << random;  
    return 0;
};