Undefined refernce static member C++

Question

I'm sick of this, I'm really exhausted. I am new in C++. But I have read a lot about that ,but still doesn't understand what is the correct to implement this.
I have just copied all code from this article Signal Handling in C++

BUT it still doesn't work !!!! I created new class Signal Handler.... Copy everything from this article BUT still this .... error message "Undefined reference .......". I am really confused about this. Please explain how to do this correctly and why I always get this message. Here is just code copied from the article.
SignalHandler.h

#ifndef SIGNALHANDLER_H
#define SIGNALHANDLER_H
#include "eventhandler.h"
#include <signal.h>
class SignalHandler
{
public:
 // Entry point.
 static SignalHandler *getInstance (void);

 // Register an event handler <eh> for <signum>
 // and return a pointer to any existing <Event_Handler>
 // that was previously registered to handle <signum>.
 EventHandler* registerHandler (int signum,EventHandler *eh);

 // Remove the <Event_Handler> for <signum>
 // by setting the slot in the <signal_handlers_>
 // table to NULL.
 int removeHandler (int signum);

private:
 // Ensure we're a Singleton.
 SignalHandler (void);

 // Singleton pointer.
 static SignalHandler *instance;

 // Entry point adapter installed into <sigaction>
 // (must be a static method or a stand-alone
 // extern "C" function).
 static void dispatcher (int signum);

 // Table of pointers to concrete <Event_Handler>s
 // registered by applications.  NSIG is the number of
 // signals defined in </usr/include/sys/signal.h>.
 static EventHandler *signalHandlers[NSIG];
};
#endif // SIGNALHANDLER_H

SignalHandler.cpp

#include "signalhandler.h"

EventHandler* SignalHandler::registerHandler (int signum,EventHandler *eh)
{
  // Copy the <old_eh> from the <signum> slot in
  // the <signal_handlers_> table.
  EventHandler *oldEh = SignalHandler::signalHandlers[signum];

  // Store <eh> into the <signum> slot in the
  // <signal_handlers_> table.
  SignalHandler::signalHandlers[signum] = eh;

  // Register the <dispatcher> to handle this
  // <signum>.
  struct sigaction sa;
  sa.sa_handler = SignalHandler::dispatcher;
  sigemptyset (&sa.sa_mask);
  sa.sa_flags = 0;
  sigaction (signum, &sa, 0);

  return oldEh;
}
void SignalHandler::dispatcher (int signum)
{
  // Perform a sanity check...
  if (SignalHandler::signalHandlers[signum] != 0) {
    // Dispatch the handler's hook method.
   SignalHandler::signalHandlers[signum]->handleSignal(signum);
  }
}

WHY am I getting this messages ...............

 error: undefined reference to `SignalHandler::signalHandlers'
error: undefined reference to `SignalHandler::signalHandlers'

Note that I have just copied this from the article. I really hope for your help.Thanks.

If you're new in C++, you should read about how static variables are initialized in a C++ class. Any static variable must be explicitly initialized in the respective cpp source file... — Zyend, Dec 04 '14 at 15:39
http://stackoverflow.com/questions/743203/accessing-static-class-variables-in-c — Jeff Watkins, Dec 04 '14 at 15:39

score 0 · Answer 1 · answered Dec 04 '14 at 15:42

0

You have to define the static array. Inside the class definition it is only declared. Write in the cpp module

EventHandler * SignalHandler::signalHandlers[NSIG];

answered Dec 04 '14 at 15:42

Vlad from Moscow

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Thanks for answer it really works. Can you give any good article to read about this. Why this should be defined both in header and source files . – braohaufngec Dec 04 '14 at 16:03
@braohaufngec I am sorry. I do not know an appropriate article. It is better to read the C++ Standard. – Vlad from Moscow Dec 04 '14 at 16:11

score 0 · Answer 2 · answered Dec 04 '14 at 15:43

0

Every function and object which is ODR-used must be defined.

And static SignalHandler *instance; in class-scope only declares that member, it does not define it.

Add the definition to exactly one translation-unit (.cpp-file).

SignalHandler* Signalhandler::instance;

Same for all the other static members.

answered Dec 04 '14 at 15:43

Deduplicator

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Thanks for answer it really works. Can you give any good article to read about this. Why this should be defined both in header and source files . – braohaufngec Dec 04 '14 at 16:13
@braohaufngec: It should *not* be defined in both. The line in the class-definition is only a forward-declaration, which is only a promise for a definition somewhere else, like in another translation-unit. – Deduplicator Dec 04 '14 at 16:22

Undefined refernce static member C++

2 Answers2