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

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 :^)

Because your getInstance() sets the value to zero every time you call it? — Sami Kuhmonen, Apr 27 '17 at 18:06
You initialize it in the constructor as any other values that need to be initialized when the object is created. — Sami Kuhmonen, Apr 27 '17 at 18:42
Is it necessary to invoke the ``getInstance()`` method every time that I need access to the object, or is there a simpler way? — kmiklas, Apr 27 '17 at 19:06

score 2 · Answer 1 · answered Jan 06 '21 at 17:12

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...

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

1 Answers1