Please tell me what's wrong with the c++ operator overloading

#include <iostream>
using namespace std;

class IntegerSet {
    int numElements, maxNumElements;
    int *elements;
public:

    IntegerSet(int max) {
        numElements = 0;
        maxNumElements = max;
        elements = new int[max];
    }
    ~IntegerSet(); // to implement
    int addAnElement(int element); // add an integer element, to implement 
    IntegerSet operator+(const IntegerSet& other); // union, to implement
    IntegerSet operator-(const IntegerSet& other); // difference, to implement
    IntegerSet operator*(const IntegerSet& other); // intersection, to implement
    friend ostream& operator<<(ostream& os, const IntegerSet& integerSet);

};

IntegerSet::~IntegerSet() {
    delete(elements);
}

int IntegerSet::addAnElement(int element) {
    if (numElements < maxNumElements) {
        elements[numElements] = element;
        numElements++;
        return 0;
    }
    else
        return -1;
}

Why not here?
IntegerSet IntegerSet::operator+(const IntegerSet& other) {
    IntegerSet temp(5);
    temp.addAnElement(22);
    return temp;
}

ostream& operator<<(ostream& os, const IntegerSet& integerSet) {
    cout << "number of elements : " << integerSet.numElements << "\n";
    for (int i = 0;i<integerSet.numElements;i++)
        cout << integerSet.elements[i]<<" ";
    cout << "\n";
    return os;
}

int main() {
    IntegerSet a(10), b(5);
    int i;
    for (i = 0;i < 5;i++) {
        a.addAnElement(i);
        b.addAnElement(10 - i);
    }
    cout << a << endl;
    cout << b << endl;

    IntegerSet c = a + b;

    return 0;
}