tmp/map.o: In function `Map::unloadMap(Map*)':
map.cpp:(.text+0x6e): undefined reference to `Map::maps'
map.cpp:(.text+0xa9): undefined reference to `Map::maps'
tmp/map.o: In function `Map::getMap(QString)':
map.cpp:(.text+0x10b): undefined reference to `Map::maps'
map.cpp:(.text+0x136): undefined reference to `Map::maps'
map.cpp:(.text+0x146): undefined reference to `Map::maps'
tmp/map.o:map.cpp:(.text+0x167): more undefined references to `Map::maps' follow
collect2: ld returned 1 exit status

//-----------------------------------------------------------------------
//-----------------------------------------------------------------------
// map.h

#ifndef MAP_H
#define MAP_H

#include <QMap>
#include <QString>

class Map
{
public:
    static Map *getMap(QString name);
    static void unloadMap(QString name);
    static void unloadMap(Map *map);
    void used() { gamesUsing++; };
    void notUsed() { gamesUsing--; if(!gamesUsing) unloadMap(this); };
    
protected:
    static QMap<QString, Map *> maps;
    Map(QString name);
    const QString name;
    unsigned short gamesUsing;
};

#endif

//-----------------------------------------------------------------------
//-----------------------------------------------------------------------
// map.cpp

#include "map.h"

Map::Map(QString name)
  : name(name),
    gamesUsing(0)
{

}

Map *Map::getMap(QString name)
{
    if(maps.contains(name))
        return maps[name];
    
    // TODO: What if the map file doesn't exist? Need to return 0 if that happens
    maps[name] = new Map(name);
}

void Map::unloadMap(QString name)
{
    if(!maps.contains(name))
        return;
    if(maps[name]->gamesUsing)
        return; // Don't unload if a game is using it
    
    delete maps[name];
    maps.remove(name);
}

void Map::unloadMap(Map *map)
{
    if(map->gamesUsing)
        return; // Don't unload if a game is using it
    
    QString name = maps.key(map);
    delete map;
    maps.remove(name);
}