C++ Singleton Problem

I have the code for a singleton below, and I am getting a linker error "multiple definitions " when i include the singleton.h file in more than one C++ file. This singleton code should cope with up to 5 singletons in a process and report error above that.

The singleton clas is all in the .h file:


class CMySingleton
{
private:
CMySingleton(){}; // Private Constructor

static int nCount; // Current number of instances
static int nMaxInstance; // Maximum number of instances

public:
~CMySingleton(); // Public Destructor

static CMySingleton* CreateInstance(); // Construct Indirectly

//Add whatever members you want
};

#endif // !defined(AFX_MYSINGLETON_H__8AA85488_875D_11DF_985E_0090F508B061__INCLUDED_)

CMySingleton::~CMySingleton()
{
--nCount; // Decrement number of instances
}


CMySingleton* CMySingleton::CreateInstance()
{
CMySingleton* ptr = NULL;

if(nMaxInstance > nCount)
{
ptr = new CMySingleton;
++nCount; // Increment no of instances
}
return ptr;
}

int CMySingleton::nCount = 0;
int CMySingleton::nMaxInstance = 5; // When maxInstance is 1, we have a pure singleton class

// end CMySingleton.h

The multiple definition link errors occur on :
CMySingleton::nCount
int CMySingleton::nMaxInstance

for eg.

NOTE: This code works if i include it once , in main.cpp, in that case it compiles and links and runs.
But i do need to include it in more than one *.cpp file:

Any Advice appreciated.

Thanks

Marco

Move the definition part into a cpp file and leave just the declaration part in the header.

Hello Sir/madam~, I am find there are many links over google about the singleton, can you give me typical example of singletoon in windows XP coding Micosoft window?

Thank you

Quote:

---

Originally Posted by S_M_A

Move the definition part into a cpp file and leave just the declaration part in the header.

---

That is what I had in the first place, and it just would not compile or link then.

I only got it working by moving everything into one header file. And it worked for one include into main.cpp;

But when i created another test.cpp file to check whether it would limit the number of singletons as designed, it gave the link errors I originally posted.

Try this: main.cpp

Code:

---

#include "singleton.h"
// Didn't bother to create a header
void CreateSingleton();
void DeleteSingleton();
int main(int argc, char* argv[])
{
  CMySingleton* ptr = CMySingleton::CreateInstance();
  CreateSingleton();
  delete ptr;
  DeleteSingleton();
  return 0;
}

---

singleton.h

Code:

---

#ifndef _SINGLETON_H
#define _SINGLETON_H

class CMySingleton
{
  private:
  CMySingleton(){}; // Private Constructor

  static int nCount; // Current number of instances
  static int nMaxInstance; // Maximum number of instances

public:
  ~CMySingleton(); // Public Destructor
  static CMySingleton* CreateInstance(); // Construct Indirectly

//Add whatever members you want
};

#endif

---

singleton.cpp

Code:

---

#include "singleton.h"

CMySingleton* CMySingleton::CreateInstance()
{
  CMySingleton* ptr = NULL;

  if(nMaxInstance > nCount)
  {
    ptr = new CMySingleton;
    ++nCount; // Increment no of instances
  }
  return ptr;
}

CMySingleton::~CMySingleton()
{
  --nCount; // Decrement number of instances
}

int CMySingleton::nCount = 0;
int CMySingleton::nMaxInstance = 5; // When maxInstance is 1, we have a pure singleton class

---

otherfile.cpp

Code:

---

#include "singleton.h"

static CMySingleton* ptr;
void CreateSingleton()
{
  ptr = CMySingleton::CreateInstance();
}

void DeleteSingleton()
{
  delete ptr;
}

---