How to write a code predict the rest of a word a user is typing

Question

I want to write this code with c++

Answer 1

Set up a dictionary of words. Then when user has typed some letters look up if any of the known words in the dictionary start with those letters.
If many then narrow it down when more letters are added.
You can even make the dictionary dynamic, add words over time, or use statistical analysis to see which words are more likely.