How to both run a program or read from a .txt file at the same time with a single command?

Wondering how in C++ it is possible to run either a shell as a program or just read from a .txt file and use the inputs from this .txt file in my program. If there are any prebuilt functions that do this that would be awesome. Both of these, either reading from a .txt file or running a shell as a program argument into this function. Right now I'm experimenting with the exec family of commands.