void swap(int *a, int *b) ; Why does swap(a,b) work?

Question

The prototype of my swap function is void swap(int *a, int *b); why is it invoked when I call swap(a, b) where a and b are integers?

Full program

#include <iostream>
void swap(int *a, int *b)
{
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main()
{
    using namespace std;
    int a = 9, b = 8;
    cout << "before swap:" << endl;
    cout << a << '\t' << b << endl;
    swap(a, b);
    cout << "after swap:" << endl;
    cout << a << '\t' << b << endl;
    cin.get();
    return 0;
}

Observed behavior

before swap:
9    8
after swap:
8    9

I know it should have been swap(&a,&b) to call the function swap. Why does it work nonetheless?