Error in Singleton pattern implementation

Question

I see errors like

src/singleton.cxx:16:error: invalid use of member 'Singleton::instance' in static member function src/singleton.cxx:28:error: from this location src/singleton.cxx:16:error: invalid use of member 'Singleton::instance' in static member function src/singleton.cxx:29:error: from this location src/singleton.cxx:16:error: invalid use of member 'Singleton::instance' in static member function src/singleton.cxx:31:error: from this location src/singleton.cxx: In function 'int main()':

Now after making changes I get the following errors

singleton-rhel6.3.o: In function Singleton::get_instance()': src/singleton.cxx:27: undefined reference toSingleton::instance'

#include <cstddef>

class Singleton {

private:
  Singleton();
  static Singleton * instance;
  int m_num;
  int incr_call();

public :
  static Singleton * get_instance();

};

Singleton * Singleton::get_instance() {
  if(instance == NULL)
    instance = new Singleton();
  return instance;
}

Singleton::Singleton():
  m_num(0)
{
}

int Singleton::incr_call() {
  return m_num++;
}

int main() {
  Singleton * s = Singleton::get_instance();
  return 0;
}

not the problem but `Singleton * instance;` should not be public — NathanOliver, Oct 21 '16 at 16:06
instance should be static. Although you may want to consider Bjarne Stroustrup's implementation of this instead. — drescherjm, Oct 21 '16 at 16:07
This may be of use: http://stackoverflow.com/a/1008289/3807729 — Galik, Oct 21 '16 at 16:16
@Galik the link you show me has the static instance declared in the public section too — Kim Kardashian, Oct 21 '16 at 17:31
After the edit that fixed the original problem, I assume the errors are different. With that said you are not supposed to modify the code after it has been answered since it invalidates the answer. And makes the question confusing to future readers. — drescherjm, Oct 21 '16 at 18:00
so whats the protocol, should i post an answer, with edited code ? i have provided the new errors ...btw — Kim Kardashian, Oct 21 '16 at 18:02
Normally you ask a different question (since it is a different problem) and leave the original question alone. — drescherjm, Oct 21 '16 at 18:02
For this case I my advice is to use the code @Jarod42 just posted. That is what I meant on my first comment (about the alternate implementation)! — drescherjm, Oct 21 '16 at 18:04

score 1 · Answer 1 · answered Oct 21 '16 at 16:10

1

instance should be static since you want to be able to call it in get_instance. Also, instance should be private:

class Singleton {

public :
  static Singleton * get_instance();


private:
  Singleton();
  static Singleton * instance;
  int m_num;
  int incr_call();

};
Singleton* Singleton::instance;

You should change your constructor too, to not initialize instance:

Singleton::Singleton():
  m_num(0)
{ }

Because instance is static default initialization is done and it will be NULL / nullptr.

answered Oct 21 '16 at 16:10

Hatted Rooster

35,759
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i made these changes, and it still complains when instance is compared to NULL – Kim Kardashian Oct 21 '16 at 16:54
You probably forgot `Singleton* Singleton::instance;` – drescherjm Oct 21 '16 at 18:15

score 1 · Answer 2 · answered Oct 21 '16 at 18:00

1

If you have to use singleton, use Meyers' one:

class Singleton {
private:
    Singleton() = default;
    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;

public :
    static Singleton& get_instance()
    {
        static Singleton instance;
        return instance;
    }

// Extra stuff
};

answered Oct 21 '16 at 18:00

Jarod42

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i dont understand this. I read the concept of singleton, saw some examples and trying to implement it. what does meyers do thats fixing the problem im having ? – Kim Kardashian Oct 21 '16 at 18:04
This code is simpler than yours and avoids the static initialization order fiasco https://isocpp.org/wiki/faq/ctors#static-init-order – drescherjm Oct 21 '16 at 18:09

Error in Singleton pattern implementation

2 Answers2