"Functional programming" means that you primarily use functions (not C functions, but mathematical pure functions) to solve your problems.

This means you won't do IO in your computation because you can't do that. It also means you won't modify data, because you can't do that either. Also you might have access to first class functions, and can pass them around as values.

If you do procedural programming in C++ but your functions don't do IO or modify (not local) values, then congrats, you're doing functional programming.

Excellent! You will encounter 0 friction in using an FP then.

To the extent that programmers find friction using Haskell, it's usually because their computations unintentionally update the state of the world, and the compiler tells them off for it.

Normally what functional programmers will do is pull their state and side effects up as high as they can so that most of their program is functional