Error in Singleton

Question

I was implementing a singleton class for my game but every time I try to compile it, it gives a compilation error. The code is as follows :

    //Singleton.h
    #pragma once

    class Singleton{
    public:
        static Singleton* Instance();

    private:
        Singleton(){};

        static Singleton* m_instance;
    };

    Singleton* SingletonInstance = Singleton::Instance();

Singleton.cpp

//Singleton.cpp
#include "Untitled1.h"

Singleton* m_instance = nullptr;


Singleton* Singleton::Instance(){
    if(m_instance == nullptr){
        m_instance = new Singleton;
    }
    return m_instance;
}

I am getting the following errors:

||=== Build: Debug in df (compiler: GNU GCC Compiler) ===|
obj\Debug\Untitled1.o||In function `ZN9Singleton8InstanceEv':|
C:\Users\Samsung R519\Desktop\Untitled1.cpp|7|multiple definition of `SingletonInstance'|
obj\Debug\main.o:C:\Users\Samsung R519\Desktop\main.cpp|4|first defined here|
obj\Debug\Untitled1.o:Untitled1.cpp|| undefined reference to `Singleton::m_instance'|
obj\Debug\Untitled1.o:Untitled1.cpp|| undefined reference to `Singleton::m_instance'|
||=== Build failed: 4 error(s), 0 warning(s) (0 minute(s), 10 second(s)) ===|

What am I doing wrong?

Reconsider (a) why you need a singleton: it’s almost always an anti-pattern; and (b) why you are using pointers here. Using a simple object with automatic storage and return it by reference is almost certainly superior, unless you need deferred initialisation. — Konrad Rudolph, Aug 02 '16 at 13:16
Possible duplicate of [Singleton Inheritance Linker Error](http://stackoverflow.com/questions/7750832/singleton-inheritance-linker-error) — anatolyg, Aug 02 '16 at 13:16
Another (better?) way to implement a singleton in C++: http://stackoverflow.com/a/1008289/3052438 — Piotr Siupa, Aug 02 '16 at 13:20

score 3 · Answer 1 · answered Aug 02 '16 at 13:15

3

In the .cpp file you are defining a new m_instance that is not a member.

Please try

Singleton* Singleton::m_instance = nullptr;

answered Aug 02 '16 at 13:15

Bo Persson

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It worked but now this error is coming: Singleton.cpp|7|multiple definition of `SingletonInstance' – darkpsychic Aug 02 '16 at 13:23

score 3 · Accepted Answer · answered Aug 02 '16 at 13:25

There are two problems with your code.

Singleton* SingletonInstance = Singleton::Instance(); You shouldn't put the definition in header file. Instead, you should declare SingletonInstance in the header and define it in .cpp file. In fact, there's no need to have this global variable. You can always call Singleton::Instance() to get the singleton.
m_instance is a member of Singleton, so you should define it with the class name: Singleton::m_instance

The solution:

// xxx.h
// other code...
extern Singleton* SingletonInstance; // declaration

// xxx.cpp
// other code...
Singleton* Singleton::m_instance = nullptr;   // specify the class name

Singleton* SingletonInstance = Singleton::Instance(); // definition

score 1 · Answer 3 · answered Aug 02 '16 at 13:22

 class Singleton{
           ....
           static Singleton* m_instance;
  }

and

Singleton* m_instance = nullptr;

Even though have the same name, they are different pointers because they have different scopes: the first one is a declaration of a class member and the second one is both a declaration and a definition of a pointer in the global namespace.

So, what you are looking for is the definition of the class member pointer, which is:

Singleton* Singleton::m_instance = nullptr;

score 1 · Answer 4 · answered Aug 02 '16 at 13:39

I suggest you to use the Scott Meyers singleton. I will avoid any error related to order of initialisation and any memory management error related to the singleton.

Here's how you implement it:

struct Singleton {
    static Singleton& instance() {
        static Singleton myInstance;

        return myInstance;
    }
};

If you want to return a pointer instead of a reference, this is equally fine.

Error in Singleton

4 Answers4