C++ Programming - OOPS Concepts - Discussion

Singleton design pattern is a software design principle that is used to restrict the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. For example, if you are using a logger, that writes logs to a file, you can use a singleton class to create such a logger. You can create a singleton class using the following code.

Example
#include <iostream>

using namespace std;

class Singleton
{
static Singleton *instance;
int data;

// Private constructor so that no objects can be created.
Singleton()
{
data = 0;
}

public:
static Singleton *getInstance()
{
if (!instance)
instance = new Singleton;
return instance;
}

int getData()
{
return this -> data;
}

void setData(int data)
{
this -> data = data;
}
};


Initialize pointer to zero so that it can be initialized in the first call to getInstance.

Singleton *Singleton::instance = 0;

int main(){
Singleton *s = s->getInstance();
cout << s->getData() << endl;
s->setData(100);
cout << s->getData() << endl;
return 0;
}
output
This will give the output:

0
100

Singleton class allow only one object to be created.

Please explain this.

If a class has only one object created and that is the only object of the class. Then the object is known as the singleton object.

The Singleton class exactly means is that, if we create many objects in a class then it will point to the first object which has been created.

It will not point to other objects.

Why is virtual class not accessed through any class? Please tell me.

Why only singleton class is allowed? Please explain me.

I cannot understand this. Please, anyone explain to me.

Singleton class is a designed pattern that restricts the instantiation of a class to one object.

Ex: By giving double click on the Application of any video player (Windows,VLC...etc) it will open the Application. if we try to open new application by minimizing previous one, it will display minimized one only, it wont create another new application. Because, It's restricted to the instantiation of a class to only one object.

I can't understand this. Please, anyone explain to me.