CODEWITHSUNDEEP
c++undefined-reference
darkpsychic
darkpsychic

Reputation: 188

Error in Singleton

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?

Upvotes: 0

Views: 1259

Answers (4)

Guillaume Racicot
Guillaume Racicot

Reputation: 41840

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.

Upvotes: 1

for_stack
for_stack

Reputation: 23041

There are two problems with your code.

  1. 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.

  2. 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

Upvotes: 3

Pandrei
Pandrei

Reputation: 4961

 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;

Upvotes: 1

Bo Persson
Bo Persson

Reputation: 92381

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

Please try

Singleton* Singleton::m_instance = nullptr;

Upvotes: 3

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