Undefined reference to Singleton::instance

Question

I am trying to code a Singleton. When I compile it I get many undefined errors, like instance and mutex_ in the getSingleton()

undefined reference to 'Singleton::instance'
undefined reference to 'Singleton::mutex_'

#include<iostream>
#include<string>
#include<mutex>
using namespace std;
class Singleton{
public:
    static Singleton* getSingleton(){
        if(instance == NULL){
            mutex_.lock();
            if(instance == NULL){
                instance = new Singleton();
            }
            mutex_.unlock();
        }
        return instance;
    }
private:
    Singleton(){}
    Singleton& operator =(const Singleton& ){}
    static Singleton *instance;
    static mutex mutex_;
};
int main(){
    Singleton* singleton = Singleton::getSingleton();
    return 0;
}

Possible duplicate of [Undefined reference to static class member](https://stackoverflow.com/questions/272900/undefined-reference-to-static-class-member) — Michael Veksler, Mar 20 '19 at 05:02
Instead of this, look at https://stackoverflow.com/q/1661529/4955498 — Michael Veksler, Mar 20 '19 at 05:14

score 0 · Answer 1 · answered Mar 20 '19 at 07:30

You should place define this static fields to *.cpp file like there

class Singleton{
public:
    static Singleton* getSingleton(){
        if(instance == NULL){
            mutex_.lock();
            if(instance == NULL){
                instance = new Singleton();
            }
            mutex_.unlock();
        }
        return instance;
    }
private:
    Singleton(){}
    Singleton& operator =(const Singleton& ){}
    static Singleton *instance;
    static mutex mutex_;
};

mutex Singleton::mutex_;
Singleton * Singleton::instance;

You can remove the mutex if you use singleton myers and c++11:

class Singleton {
public:
    static Singleton& Instance() {
        static Singleton S;
        return S;
    }

private:
    Singleton();
    ~Singleton();
};

I need initialize the variable like this ```Singleton* Singleton::instance = NULL; mutex Singleton::mutex_;``` — test, Mar 27 '19 at 01:08

Undefined reference to Singleton::instance

1 Answers1