Hello, I'm completley new at programming and trying to learn C++

When I change the main.cpp file on XCode & save it, why doesn't my a.out file update when I run it through the mac terminal?

I'm typing: ./a.out

For example if my old a.out shows "Hello World", then I go to main.cpp and change it to "Hi", I save it and run it in XCode and it works fine. But when I go to Mac terminal and run the a.out file it will still show the old "Hello World" message.