How to count characters in a C_String in C++?

Question

I'm a new Computer Science student, and I have a homework question that is as follows:

Write a Function that passes in a C-String and using a pointer determine the number of chars in the string.

Here is my code:

#include <iostream>
#include <string.h>
using namespace std;
const int SIZE = 40;

int function(const char* , int, int);

int main()
{
     char thing[SIZE];
     int chars = 0;

     cout << "enter string. max " << SIZE - 1 << " characters" << endl;
     cin.getline(thing, SIZE);
     int y = function(thing, chars, SIZE);
     cout << y;
}


int function(const char *ptr, int a, int b){
    a = 0;
    for (int i = 0; i < b; i++){
        while (*ptr != '\0'){
            a++;
        }
    }
    return a;
}