First Differing Line

Write a function int firstDifference(const string& a, const string& b) that compares two text files line by line and returns the 1-based number of the first line where they differ, or 0 if the files are identical. If one file ends before the other, the first line that exists in only one of them is the first difference.

Examples (each test creates both files):
- lines x, y, z versus x, Q, z returns 2
- lines x, y, z versus x, y, z returns 0
- lines a, b versus a, b, c returns 3

The test prints the returned number on its own line, for example (see Expected Output).

Do NOT write a main function.