Using command line to communicate with a program in C++

You can't use the commandline to do that. You could make your program provide a "command window" of it's own that would allow you to have it "do stuff" when you type something in.

You could also write another commandline app that you could pass parameters to that would then instruct your alreay running application to do something. There are a few ways to go about ddoing this, and I suggest you google the phrase "MFC interprocess communications" to get an idea of what's available to you.

I think I still have some code at home that uses registered messages to transfer data back and forth between applications. If I remember, I'll see what I have, and send it to you if you're interested.

You could do this, have the app handle the command line. If an instance is already running, then the current instance will send a message to that instance telling it what to do and then exit. If an instance is not running, the current instance will start and proceed to execute the "command".

As to how the 2nd instance can communicate with the first, there are several options you can choose from. The simplest would be WM_COPYDATA, and if your app is a console app, you could still have a hidden window merely to handle messages.

A quick and dirty way would be using a simple text file to issue directives to your application.
:)