函数重载会产生错误：对重载函数的调用是不明确的

Question

我试图为了两个不同的目的重载我的函数pointer_move。我通过参数的数量重载它，但它仍然给出错误，调用函数（重载）是不明确的。请帮我解决问题。

#include <iostream>
using namespace std;

int* pointer_move(int *arr, int index, int current_position=0) {

    if(current_position < index) {
        while(current_position != index) {
            arr++;
            current_position++;
        }
        return arr;
    } else if(current_position > index) {
        while(current_position != index) {
            arr--;
            current_position--;
        }
        return arr;
    } else {
        return arr;
    }
}

int* pointer_move(int *arr, int element, int* null, int current_position=0) {

    if(current_position < element) {
        while(current_position < element) {
            arr++;
            current_position++;
        }
        return arr;
    } else if(current_position > element) {
        while(current_position > element) {
            arr--;
            current_position--;
        }
        return arr;
    } else {
        return arr;
    }
}

int main() {

    int x[5]={1,32,123,2321,14356};

    int *xp=x;
    xp=pointer_move(xp, 3);
    cout << *xp << endl;

    xp=x;
    xp=pointer_move(xp, 5, NULL);
    cout << *xp << endl;

}