@pacmaninbw actually, I was trying to fix an error in my program that went unnoticed. This error was not mentioned in the answer.

Thanks for suggestions! When amalgamating coordinates, I want all of them take the same type (either int or double). But if I use double for x and y, there will be lots of conversions to int. I wonder if it will make the program slow.

Thanks for the review! I have a long way to go... What is RAII/RFID?

@JoachimRohde How? It works fine on my computer (Windows)