How to modify a txt file even when it is open in a c++ program?

Answer 1

You can only modify a file by opening it first.

Maybe you meant when it is open in another program? In that case, you can't and shouln't be able to anyway. Several programs modifying a file at the same time would just erase the changes of the other when saving if this was allowed.

Answer 2

What Zen mentioned is the classic reader/writer problem from computer science. You'll encounter it not only writing to a file. The file can be considered shared memory between two operation system processes. You'll pretty much encounter it every time you have to share memory.