Singleton Attempt, Static Variable, "Undefined reference" error.

[Motoma]

Good evening everyone.
I am starting to re-explore C++, and I wanted to build a singleton class. Unfortunately, when I set things up as I do in PHP, it doesn't work out for me. I hope that the problem is something small and obvious, but here is what I have:

#include <iostream>
 
using namespace std;
 
class ClientManager
{
public:
  static ClientManager* Begin()
  {
    if(ClientManager::instance == NULL)
      {
        cout<<"Creating a new ClientManager.\n";
        ClientManager::instance = new ClientManager();
      }
    else
      {
        cout<<"Retrieving the existing ClientManager.\n";
      }
 
    return (ClientManager*) ClientManager::instance;
  }
 
  ~ClientManager()
  {
    delete(ClientManager::instance);
    cout<<"ClientManager destroyed.\n";
  }
 
  void PerformBasicFunction()
  {
    cout<<"Performing a generic function.\n";
  }
 
private:
  static void* instance;
  ClientManager()
  {
    cout<<"ClientManager created.\n";
  }
};
 
int main()
{
  cout<<"Main loop called.\n";
  ClientManager* cm = ClientManager::Begin();
  cout<<"Main loop ended.\n";
  return 0;
}
 
 

When I attempt to compile this I receive the following errors:

main.cpp:(.text._ZN13ClientManager5BeginEv[ClientManager::Begin()]+0x8): undefined reference to `ClientManager::instance'
main.cpp:(.text._ZN13ClientManager5BeginEv[ClientManager::Begin()]+0x42): undefined reference to `ClientManager::instance'
main.cpp:(.text._ZN13ClientManager5BeginEv[ClientManager::Begin()]+0x7c): undefined reference to `ClientManager::instance'

I have looked at everything that I could think of, and everything seems to the in place. I am sure that it is my inexperience that is preventing me from seeing the issue, so if anyone has an idea, I would be very appreciative.

Thanks,
Motoma

[gpraghuram]

Good evening everyone.
I am starting to re-explore C++, and I wanted to build a singleton class. Unfortunately, when I set things up as I do in PHP, it doesn't work out for me. I hope that the problem is something small and obvious, but here is what I have:

#include <iostream>
 
using namespace std;
 
class ClientManager
{
public:
  static ClientManager* Begin()
  {
    if(ClientManager::instance == NULL)
      {
        cout<<"Creating a new ClientManager.\n";
        ClientManager::instance = new ClientManager();
      }
    else
      {
        cout<<"Retrieving the existing ClientManager.\n";
      }
 
    return (ClientManager*) ClientManager::instance;
  }
 
  ~ClientManager()
  {
    delete(ClientManager::instance);
    cout<<"ClientManager destroyed.\n";
  }
 
  void PerformBasicFunction()
  {
    cout<<"Performing a generic function.\n";
  }
 
private:
  static void* instance;
  ClientManager()
  {
    cout<<"ClientManager created.\n";
  }
};
 
int main()
{
  cout<<"Main loop called.\n";
  ClientManager* cm = ClientManager::Begin();
  cout<<"Main loop ended.\n";
  return 0;
}
 
 

When I attempt to compile this I receive the following errors:



I have looked at everything that I could think of, and everything seems to the in place. I am sure that it is my inexperience that is preventing me from seeing the issue, so if anyone has an idea, I would be very appreciative.

Thanks,
Motoma

When the instance variable is defined inside the Begin method then the error will be gone
I tried implementing a singleton class using the approcah which i explained you.

static ClientManager* Begin()
  {
    static void * instance;
....
}
 

Raghuram

[JosAH]

Did you *define* your static variable 'instance' somewhere in a .cpp file?

kind regards,

Jos

[Motoma]

Did you *define* your static variable 'instance' somewhere in a .cpp file?

kind regards,

Jos

Does line 35 not count?

In other circumstances, "defining" variables this way seems to work correctly:

#include <iostream>
#include <string>
 
using namespace std;
 
class toasty
{
public:
  string s;
  void shout()
  {
    cout<<s<<endl;
  }
};
 
int main()
{
  toasty* t = new toasty();
  t->s = "Hello Bytes.com!";
  t->shout();
}
 
 

[Banfa]

Does line 35 not count?

In other circumstances, "defining" variables this way seems to work correctly:

No that is a declaration, that says it exists without actually creating it. The reason that static class variables and normal class variables are different is that when you declare a normal class variable it gets created when the instance of the class is created, but for static class variables where all instances share the same variable then the variable needs to be defined(created) outside of the creation of any instance of the class.

You need this in a cpp file somewhere

void *ClientManager::instance;

with any initialiser should you wish to have one.

[Motoma]

Thanks for that last hint Banfa.

JosAH, my apologies for being too base to see what you meant. I'll chalk this one up to lack of experience.

gpraghuram, thanks for the assistance as well. While your solution does work, it's not exactly what I am looking for, as I would like the instance variable to be accessible inside the class.

For anyone who happens to clamber by, here is my final code listing:

#include <iostream>
 
using namespace std;
 
class ClientManager
{
public:
  static ClientManager* Begin()
  {
    if(ClientManager::instance == NULL)
      {
        cout<<"Creating a new ClientManager.\n";
        ClientManager::instance = new ClientManager();
      }
    else
      {
        cout<<"Retrieving the existing ClientManager.\n";
      }
 
    return ClientManager::instance;
  }
 
  ~ClientManager()
  {
    ClientManager::instance = NULL;
    cout<<"ClientManager destroyed.\n";
  }
 
  void PerformBasicFunction()
  {
    cout<<"Performing a generic function.\n";
  }
 
private:
  static ClientManager* instance;
  ClientManager()
  {
    cout<<"ClientManager created.\n";
  }
};
 
ClientManager* ClientManager::instance;
 
void singletonTest()
{
  cout<<"Retrieving a ClientManager instance.\n";
  ClientManager* cm = ClientManager::Begin();
  cm->PerformBasicFunction();
}
 
int main()
{
  cout<<"Main loop called.\n";
  singletonTest();
  singletonTest();
  cout<<"Main loop ended.\n";
  return 0;
}
 
 

Thanks again for all of your help,
Motoma

[JosAH]

Thanks for that last hint Banfa.

JosAH, my apologies for being too base to see what you meant. I'll chalk this one up to lack of experience.

gpraghuram, thanks for the assistance as well. While your solution does work, it's not exactly what I am looking for, as I would like the instance variable to be accessible inside the class.

For anyone who happens to clamber by, here is my final code listing:

#include <iostream>
 
using namespace std;
 
class ClientManager
{
public:
  static ClientManager* Begin()
  {
    if(ClientManager::instance == NULL)
      {
        cout<<"Creating a new ClientManager.\n";
        ClientManager::instance = new ClientManager();
      }
    else
      {
        cout<<"Retrieving the existing ClientManager.\n";
      }
 
    return ClientManager::instance;
  }
 
  ~ClientManager()
  {
    ClientManager::instance = NULL;
    cout<<"ClientManager destroyed.\n";
  }
 
  void PerformBasicFunction()
  {
    cout<<"Performing a generic function.\n";
  }
 
private:
  static ClientManager* instance;
  ClientManager()
  {
    cout<<"ClientManager created.\n";
  }
};
 
ClientManager* ClientManager::instance;
 
void singletonTest()
{
  cout<<"Retrieving a ClientManager instance.\n";
  ClientManager* cm = ClientManager::Begin();
  cm->PerformBasicFunction();
}
 
int main()
{
  cout<<"Main loop called.\n";
  singletonTest();
  singletonTest();
  cout<<"Main loop ended.\n";
  return 0;
}
 
 

Thanks again for all of your help,
Motoma

... and now all your functions are 'inline' functions; don't do it this way; *define*
your class in an .hpp file; that way you *declare* all your functions and static
members. Elsewhere in a .cpp file *define* all those functions and static members.

Or do you really want to *declare* (and *define*) those functions as inline functions?
If you like 'code bloat' please do so ;-)

kind regards,

Jos

[weaknessforcats]

And, of course, there is this article http://bytes.com/forum/thread656124.html.

[Motoma]

... and now all your functions are 'inline' functions; don't do it this way; *define*
your class in an .hpp file; that way you *declare* all your functions and static
members. Elsewhere in a .cpp file *define* all those functions and static members.

Or do you really want to *declare* (and *define*) those functions as inline functions?
If you like 'code bloat' please do so ;-)

kind regards,

Jos

I am aware JosAH, when I was designing my simple test case, I felt that this was more readable than three separate sections of code...the whole trying to help the people who are trying to help me thing.

Perhaps I was wrong.