C++ Programming - OOPS Concepts - Discussion

Singleton class allow only one object to be created.

Why only singleton class is allowed? Please explain me.

Please explain this.

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

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.

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

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

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

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.

Singleton class can create only one object, all others class we can't create object.