error: does not name a type

#include <iostream>
#include <stack>
using namespace std;
class listSmall {
private:
    int index;
    listSmall* right;
    ListBig* self;
public:
    friend class ListBig;
    friend class Graph;
    explicit listSmall(int idx);
    //int getIndex();
    //void connectWithVertex(listSmall* b);
    //void disconnectFromVertex(int idx);
    //cell*getNeighbors();
};
class ListBig {
private:
    int index;
    listSmall* right;
    ListBig* down;
public:
    friend class Graph;
    friend class listSmall;
    explicit ListBig(int idx);
    void connectVertex(ListBig* s);
    listSmall* findRight(int idx);
};

ListBig* self; выдаёт ошибку - error: 'ListBig' does not name a type ListBig* self;