How do I share C++ source code files between projects in Visual Studio?

You can add source code from a common location to multiple projects. I do that a lot; I have code in a common directory that is at the same level in the directory hierarchy as most of my project files. It's simply a matter of adding .h and .cpp files from the common directory to your various projects.

I did notice that VisualStudio gets a little cranky and difficult if you use a network drive for common source, so I don't do that. But so long as all of your source code is on local disks, and the IDE knows where to find them, there is no problem having the same source file in very many projects.