using a singleton when you will need more than one instance

[mikro]

hi,
i'd like to use a singleton for one of my classes. The class has an attribute named oid and i will need one instance for every value of oid - i just need to make sure there is no second one.
so i tried:

class cAnlagenArt {
public:
	static cAnlagenArt* self( QString oid );
private:
	static QMap<QString, cAnlagenArt* > _self;
}

class cAnlagenArt {
public:
	static cAnlagenArt* self( QString oid );
private:
	static QMap<QString, cAnlagenArt* > _self;
}

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and tried the following:

cAnlagenArt* cAnlagenArt::self(QString oid) {
    if ( ! _self.contains(oid) )
        _self[oid] = new cAnlagenArt( oid );
    return _self[oid];
}

cAnlagenArt* cAnlagenArt::self(QString oid) {
    if ( ! _self.contains(oid) )
        _self[oid] = new cAnlagenArt( oid );
    return _self[oid];
}

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which gets me lots of
undefined reference to `cAnlagenArt::_self'

how else can i do this?