Creating a static object

I'm having trouble creating a static object. Take a look at this snippet from a header file...

#include <stdlib.h>       #include "WProgram.h"     
#include "Logger.h" // this file
#include <PrivateLogger.h>


class Logger
{
  public:
    Logger();                        // Constructor
    void writeLine(char * line);     // Write a line of data to the log file
    void close();                    // Close the file

  private:
    // Member variables
    PrivateLogger m_logger;
};

This code currently works as shown. It will compile and run without problems. As soon as I try to make my object static like this.....

  private:
    // Member variables
    static PrivateLogger m_logger;

Then I get the follwoing error

Logger\Logger.cpp.o: In function Logger::close()': C:\all_apps\arduino-0021\libraries\Logger/Logger.cpp:57: undefined reference to Logger::m_logger'
C:\all_apps\arduino-0021\libraries\Logger/Logger.cpp:57: undefined reference to Logger::m_logger' Logger\Logger.cpp.o: In function Logger::writeLine(char*)':
C:\all_apps\arduino-0021\libraries\Logger/Logger.cpp:50: undefined reference to Logger::m_logger' C:\all_apps\arduino-0021\libraries\Logger/Logger.cpp:50: undefined reference to Logger::m_logger'

The code it's actually complaining about is listed here....

// Constructor
Logger::Logger()
{
}

void Logger::writeLine(char * line)    // Write a line of data to the log file
{
  m_logger.writeLine(line);
}

void Logger::close()    
{
  m_logger.close();
}

My Logger object is going to be used within lots of different classes within my sketch, so I'm trying to use the static keyword to force only a single instance of the logger class.

I'm trying to do this for two reasons, firstly since I only have one SD card and one log file it makes sense to only have a single object accessing it and, although it is possible to have multiple objects writing to the same file, whichever object closes the file first (and thus 'commits' whatever it's written) get to overwrite anything that any other object was writting.

Basically I have a couple of questions....

1. is it possible to implement a singleton class in arduino? If so, how?
2. Is my approach correct or not? and if it is why isn't it working and how do I fix it?

Cheers

Mike

Why does m_logger need to be static?

The m_logger variable should really be a pointer to an instance of PrivateLogger. The implementation file should create (if needed) or return (if existing) that instance.

Thanks for the reply Paul, I'm pushing (as usual) at the boundaries of my coding knowledge.

Why does m_logger need to be static

I understood that I need to make it static to ensure that there's only one instance of it? is this not the case?

The implementation file should create (if needed) or return (if existing) that instance

I know! This is what I'm trying to achieve, the question is, how do I do it?

Can you tell me how to do this, or give me a code example, or point me to some documentation (or another thread) than can give me some clues?

Any help appreciated!

Mike

I think what I would do is make m_logger a pointer, non-static. Then, make a static member to count the number of instances of the Logger class. When the first instance is created, create an instance of the PrivateLogger class. When subsequent instances are created, don't.

Thanks Paul,

can you point me in the direction of any code examples for this sort of thing?

Cheers

I have never gotten any static members of objects (even basic standard types like int) to compile successfuly... What I have done is put anything that needs to be a single instance for any class as a global variable scoped only to my code files which contain the object and methods... its a hack but it does work.