Singleton Correct Usage

Finding clear examples of singleton patterns isn't hard, but I am having trouble implementing them. Mainly, is there a way to set one at class level or am I completely off track?:

class SingletonUser
{
protected:
Singleton* singleton;

Public:
SingletonUser()
{
singleton = Singleton::GetInstance();
// error LNK2001: unresolved external symbol
// "private: static class Gravity * Gravity::m_instance"
}
}

You do not show your Singleton class, but I assume that in it you
have something like:

Code:

---

class Gravity
{
// other stuff

static Gravity* m_instance;        // declares a static variable
};

---

You declared a static class variable, but you still need to define it

Code:

---

Gravity* Gravity::m_instance= 0;

---

[ Redirected thread ]

I don't understand. The pointer is set in the GetInstance() method (if it hasn't been already). Why do you set it to zero, and where?

Here is my singleton:

Code:

---

// ***** Gravity.h *****
class Gravity
{

public:

        static Gravity* GetInstance();

        ~Gravity();

        double GetGravity();
        void SetGravity(double newVal);


protected:
        Gravity();
        Gravity(const Gravity& prm1);
        Gravity& operator=(const Gravity& prm1);

private:
        double m_gravity;

    static Gravity* m_instance; // Defined in the source file
};

// ***** Gravity.cpp *****

Gravity::Gravity()
: m_gravity(-9.81)
{}

Gravity::~Gravity(){}

Gravity* Gravity::GetInstance()
{
      if (!m_instance)
          {
                  static Gravity instance;//m_instance = new Gravity();
                  m_instance = &instance;         
          }
                  return m_instance;
}


// Properties
double Gravity::GetGravity(){

        return m_gravity;
}


void Gravity::SetGravity(SurfaceGravity newVal){

        m_gravity = newVal;
}

---

Any static class member variable must to defined.

So in gravity.cpp ...

Code:

---

include "gravity.h"

Gravity* Gravity::m_instance= 0;

---

Thank you, that nailed it.

Just wondering, is there no way to set a static variable within the class itself?

you can initialize it within the class itself only if it is of const integral
or const enumeration type ... but it still must be defined.

Code:

---

struct S
{
  static const int x = 10;
};

const int S::x; // still needed

---

Quote:

---

Originally Posted by Philip Nicoletti

you can initialize it within the class itself only if it is of const integral
or const enumeration type ... but it still must be defined.

Code:

---

struct S
{
  static const int x = 10;
};

const int S::x; // still needed

---

---

While this is correct by specification, it may cause problems with certain compilers, if you try to take the address of "x".....

Thank you Philip and Wizard, that really clarified the correct implementation of static member variables for me.