How to use output from Arduino in C++ code?
Unfortunately, C++ doesn't have native support for serial I/O.

You could use the Windows API directly if you're feeling adventurous. It can be a little complicated to get to grips with, but not insurmountable. You can get basic information about it here. There's an example of using it with Arduino here.

As an alternative, you could use a 3rd party library. This one looks good, although I haven't actually used it myself.