function as a parameter to another function error - c++

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I have the following function:

CompareType CompareByCitizensNum(const City& c1, const City& c2) {
        if (c1.getNumCitizens() > c2.getNumCitizens()
                || ((c1.getNumCitizens() == c2.getNumCitizens())
                        && (c1.getCityId() > c2.getCityId()))) {
            return BIGGER;
        } else if (c1.getCityId() == c2.getCityId()) {
            return EQUALS;
        }
        return SMALLER;
    }

and this is the method that needs to use this function:

avlTreeVertix(City newKey, avlTreeVertix* fatherToBe,
            CompareType (*compare)(const City&, const City&)) :
            bf(0), key(newKey), RightHigh(0), LeftHigh(0), vertexesInSubTree(1), father(
                    fatherToBe), childL(NULL), childR(
            NULL), compare(compare) {
        CHECK_NULL(father,);
        if (compare(key, father->key) == BIGGER) {
            //if (isGreater(father)) {
            father->childR = this;
        } else {
            father->childL = this;
        }
    }

I tried to call it by the following line:

rankTree = new avlTreeVertix(c, NULL, CompareByCitizensNumx);

But it says: no matching function for call to 'wet2::avlTreeVertix::avlTreeVertix(wet2::City&, NULL, )'

Any ideas? Thanks!

asked 9 secs ago

user2320431

34

function as a parameter to another function error - c++