Silly C++ Question

PatrickNew · 04-24-2007, 10:39 PM

Okay, so I started to learn to program a few years ago in C++. I couldn't get my head around OO, so I went to C, where I've done most of my work. Through my University, I've also done some work in Java, so I fear that I'm falling prey to a subtle syntactical difference.

I define the following class:

Code:

class Preferences{

private:
  static GHashTable* thePrefs;
  static char *localLoc;
  static char *globalLoc;

public:
  //Bunch of methods
};

Everything in the class is declared static. The class is designed to encapsulate the user preferences for an application, allowing other parts of the app a simple, consistent way of accessing the settings.

My trouble is that I declare a number of static methods, and every single reference of any of my static data members in any of those methods triggers this error:

Code:

test.cpp:(.text+0x9e): undefined reference to `Preferences::thePrefs'

except with the variable name referring to whichever variable I tried to access.

Yes, I am directly using the C glib rather than glibmm. I just learned glib in C, and would prefer it that way. Could that cause any problems? It still gives me this trouble when all the static variables are primitives.

nadroj · 04-24-2007, 10:52 PM

could you post the complete code of the class?

PatrickNew · 04-24-2007, 10:55 PM

It's rough, probably pre-pre-pre alpha.
I get the same errors if I comment any of the functions totally out of the code, so it's not any one function causing it.

Preferences.h

Code:

#include <glib.h>

class Preferences
{
 public:
  static char* lookupKey(char*);
  static void storeKey(char*, char*);*/

  static void initialize();

  /*TODO Adapt File I/O to VFS if/when one is implemented*/
  static gboolean loadFromFiles(char* global, char* local);
  static gboolean reloadFromFiles();*/

private:
  static GHashTable* thePrefs;
  static char *localLoc;
  static char *globalLoc;

};

Preferences.cpp

Code:

#include <string.h>
#include "Preferences.h"


void Preferences::initialize()
{
  thePrefs = g_hash_table_new_full((GHashFunc)g_str_hash, (GEqualFunc)strcmp,
      (GDestroyNotify)g_free, (GDestroyNotify)g_free);
  localLoc = NULL;
  globalLoc = NULL;
}

char* Preferences::lookupKey(char* theKey)
{
  return (char*) g_hash_table_lookup(thePrefs, theKey);
}

void Preferences::storeKey(char* theKey, char* theData)
{
  g_hash_table_insert(thePrefs, g_strdup(theKey), g_strdup(theData));
}

gboolean Preferences::loadFromFiles(char* global, char* local)
{
  GKeyFile* globalKeys = g_key_file_new();
  GKeyFile* localKeys = g_key_file_new();
  char** keys;
  char** tmp;

  gboolean rtnVal = g_key_file_load_from_file(globalKeys, global,
					      G_KEY_FILE_NONE, NULL);
  char* groupName;
  gboolean localWorked;
  if(rtnVal)
  {
    groupName = g_key_file_get_start_group(globalKeys);

    keys = g_key_file_get_keys(globalKeys,
			       groupName,NULL, 
			       NULL);
    tmp = keys;
    while(*tmp)
    {
      g_hash_table_insert(thePrefs, g_strdup(*tmp), 
			  g_strdup(g_key_file_get_value(globalKeys ,groupName,
							*tmp, NULL)));
      tmp++;
    }
    g_strfreev(keys);
  }
  g_key_file_free(globalKeys);

  localWorked = g_key_file_load_from_file(localKeys, local, G_KEY_FILE_NONE,
					  NULL);
  rtnVal = rtnVal || localWorked;
  if(localWorked)
  {
    groupName = g_key_file_get_start_group(localKeys);
    keys = g_key_file_get_keys(localKeys,
			       groupName,NULL, 
			       NULL);
    tmp = keys;
    while(*tmp)
    {
      g_hash_table_insert(thePrefs, g_strdup(*tmp), 
			  g_strdup(g_key_file_get_value(localKeys,groupName,
							*tmp, NULL)));
      tmp++;
    }
    g_strfreev(keys);
  }
  g_key_file_free(localKeys);

  if(localLoc)
    g_free(localLoc);
  if(globalLoc)
    g_free(globalLoc);

  localLoc = g_strdup(local);
  globalLoc = g_strdup(global);

  return rtnVal;
}

 gboolean Preferences::reloadFromFiles()
{
  return loadFromFiles(globalLoc, localLoc);
}

nadroj · 04-24-2007, 11:16 PM

what command did you use when you compiled it? (so i can try the same and attempt to debug)

PatrickNew · 04-24-2007, 11:25 PM

g++ `pkg-config --cflags --libs glib-2.0` -o test test.cpp

So I'm including the test file, which doesn't do much of anything.

Code:

#include "Preferences.cpp"
#include <iostream>

using namespace std;

int main(int argc, char** argv)
{
  Preferences::initialize();
}

nadroj · 04-25-2007, 12:01 AM

i havent worked in C++ in a long time this is why im so confused! also, ive never made a class (in C++) like you are trying to, with all the statics, i mean.

ok, adding these lines to Preferences.cpp just after the includes seems to allow it to compile (dont know why though):

Code:

GHashTable* Preferences::thePrefs;
char* Preferences::localLoc;
char* Preferences::globalLoc;

PatrickNew · 04-25-2007, 12:08 AM

Allright, I give that a try and see what comes up in testing. Seems so odd, it feels like that should fail to compile because we're redefining variables. Well, thanks for your help!

nadroj · 04-25-2007, 12:11 AM

it seems that way, i know. if you get rid of the 'Preferences::' it will give you that error. this just must be some extra step you have to do when you have this situation.

my guess is that its 'reminding' the compiler about the private static variables that really exist in that class.

g4j31a5 · 04-25-2007, 12:29 AM

For static member variable, you should redefine it in the source .cpp

So, for your Preferences.cpp, you should add (the bold ones)

Code:

#include <string.h>
#include "Preferences.h"

GHashTable* Preferences::thePrefs;
char * Preferences::localLoc;
char * Preferences::globalLoc;

void Preferences::initialize()
{
  thePrefs = g_hash_table_new_full((GHashFunc)g_str_hash, (GEqualFunc)strcmp,
      (GDestroyNotify)g_free, (GDestroyNotify)g_free);
  localLoc = NULL;
  globalLoc = NULL;
}

char* Preferences::lookupKey(char* theKey)
{
  return (char*) g_hash_table_lookup(thePrefs, theKey);
}

void Preferences::storeKey(char* theKey, char* theData)
{
  g_hash_table_insert(thePrefs, g_strdup(theKey), g_strdup(theData));
}

gboolean Preferences::loadFromFiles(char* global, char* local)
{
  GKeyFile* globalKeys = g_key_file_new();
  GKeyFile* localKeys = g_key_file_new();
  char** keys;
  char** tmp;

  gboolean rtnVal = g_key_file_load_from_file(globalKeys, global,
					      G_KEY_FILE_NONE, NULL);
  char* groupName;
  gboolean localWorked;
  if(rtnVal)
  {
    groupName = g_key_file_get_start_group(globalKeys);

    keys = g_key_file_get_keys(globalKeys,
			       groupName,NULL, 
			       NULL);
    tmp = keys;
    while(*tmp)
    {
      g_hash_table_insert(thePrefs, g_strdup(*tmp), 
			  g_strdup(g_key_file_get_value(globalKeys ,groupName,
							*tmp, NULL)));
      tmp++;
    }
    g_strfreev(keys);
  }
  g_key_file_free(globalKeys);

  localWorked = g_key_file_load_from_file(localKeys, local, G_KEY_FILE_NONE,
					  NULL);
  rtnVal = rtnVal || localWorked;
  if(localWorked)
  {
    groupName = g_key_file_get_start_group(localKeys);
    keys = g_key_file_get_keys(localKeys,
			       groupName,NULL, 
			       NULL);
    tmp = keys;
    while(*tmp)
    {
      g_hash_table_insert(thePrefs, g_strdup(*tmp), 
			  g_strdup(g_key_file_get_value(localKeys,groupName,
							*tmp, NULL)));
      tmp++;
    }
    g_strfreev(keys);
  }
  g_key_file_free(localKeys);

  if(localLoc)
    g_free(localLoc);
  if(globalLoc)
    g_free(globalLoc);

  localLoc = g_strdup(local);
  globalLoc = g_strdup(global);

  return rtnVal;
}

 gboolean Preferences::reloadFromFiles()
{
  return loadFromFiles(globalLoc, localLoc);
}

EDIT: Oops, didn't realize that nadroj has answered it before.

PatrickNew · 04-25-2007, 07:37 AM

out of curiosity, is this true in general or is it just what I have to do to get this to compile? Redefining static member variables that is?

xhi · 04-25-2007, 01:38 PM

Quote:

Originally Posted by PatrickNew

out of curiosity, is this true in general or is it just what I have to do to get this to compile? Redefining static member variables that is?

in general

graemef · 04-25-2007, 05:07 PM

As an aside I get the feeling that your class is not really following the OO paradigm. If as I understand everything is static then really you should change the class to be a singleton class. There are lots of tutorials on the web explaining this. I would recommend looking at that and on the way learn a little more about the "OO way"

PatrickNew · 04-25-2007, 06:37 PM

Well, the class really isn't very OO due to the nature of its task - that is one application should never have multiple sets of settings at one time. So, I guess the non-OOness is intentional. But I will look into singleton clasees, and thanks for the tip!

PatrickNew · 04-25-2007, 06:55 PM

Wow, a singleton class is *exactly* what I really wanted. Thanks!

PatrickNew · 04-25-2007, 10:48 PM

Many thanks to all. I did actually convert to a singleton design and it turned out to be a smaller, cleaner solution that looks as though it will be more maintainable.