How can I change Windows shell (cmd.exe) environment variables from C++?

Question

I would like to write a program that sets an environment variable in an instance of the shell (cmd.exe) it was called from. The idea is that I could store some state in thi

Answer 1

A common techniques is the write an env file, that is then "call"ed from the script.

del env.var
foo.exe ## writes to env.var
call env.var