Singleton Implementation: counter not incrementing as expected with multiple pointers to instance - class

Question

The following program aims to instantiate and use the singleton pattern class proposed by Loki Astari and accepted as answer at the following link.
C++ Singleton design pattern
Note the addition of a simple counter, by way of the private counter variable, along with the increment() mutator, and getCtr() accessor methods.
Expected program output is:
0
1
Press any key to exit...
The actual output is
0
0
Press any key to exit...
Why is the counter in the singleton class not being incremented as expected?
What follows is a minimal, complete, and verifiable program, written to illustrate the issue.
#include "stdafx.h"
#include <iostream>
#include <string>
class S {
public:
static S & getInstance() {
static S instance;
instance.counter = 0; // initialize counter to 0
return instance;
}
S(S const &) = delete;
void operator = (S const &) = delete;
void increment() { ++counter; }
int getCtr() { return counter; }
private:
S() {}
int counter;
};
int main() {
S * s; // s is a pointer to the singleton object
S * t; // t is another pointer to the singleton object.
std::cout << s->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
std::cout << "Press any key to exit...";
std::cin.get();
return 0;
}
Thx, Keith :^)

Answer 1

your problem is you are initializing the counter inside the
getInstance() method
instead, initialize it inside the constructor
your code should be like the following,
#include <iostream>
#include <string>
class S {
public:
static S & getInstance() {
static S instance;
// instance.counter = 0; // initialize counter to 0
return instance;
}
S(S const &) = delete;
void operator = (S const &) = delete;
void increment() { ++counter; }
int getCtr() { return counter; }
private:
S() {counter =0;}
int counter;
};
int main() {
S * s; // s is a pointer to the singleton object
S * t; // t is another pointer to the singleton object.
std::cout << s->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
s->getInstance().increment(); // increment counter
std::cout << t->getInstance().getCtr() << std::endl;
std::cout << "Press any key to exit...";
std::cin.get();
return 0;
}
then the Output will be
0
1
2
3
4
5
Press any key to exit...