How can I redirect output of a Windows C++ program to another executable?

One could somewhat easily do something like:

 int main (int argc, char** argv)
 {
      for (;;) {
         /* do something */
         cout << clever_password << "\0";
      }
 }

… and then in the shell, simply:

 your-clever-password-guesser | \
     sed -e 's,'\'','\''"'\''"'\'',g' | \
     xargs -0 -n1 -r -t -I {} -- unrar e 'p{}' some-file.rar

Breaking that down:

• Print out each password guess with a terminating '\0' character. This allows the password to (potentially) contain things like spaces and tabs that might otherwise “mess up” in the shell.
• Ask the stream editor sed to protect you from apostrophes '. Each ' must be encoded as a sequence of '\'' (apos-backslash-apos-apos) or '"'"' (apos-quote-apos-quote-apos) to pass through the shell safely. The s///g pattern replaces every ' with '"'"', but the apostrophes that it, itself is passing to sed are written as '\''. (I mixed the styles of escaping the ' to make it easier for me to distinguish between the apostrophe-escaping for sed and the apostrophe-escaping which sed is adding to the stream of passwords.) One could, instead, alter the strings as they're being printed in the C++ program.
• Invoke xargs to run unrar with each password, with the options that mean:
  • Each password is delimited by \0 (-0)
  • Use only one at a time (-n1)
  • Don't run if there isn't anything to do (-r) — e.g. if your program didn't print out any possible passwords at all.
  • Show the command-line as it's going to be run (-t) — this lets you monitor the guesses as they fly past on your screen
  • Put the password in place of the somewhat traditional for that purpose symbol {} (-I {})
  • Then, run the command that follows --
  • Extract from the RAR file (unrar e …)
  • With the password given replacing the {} in 'p{}'; the ' here protect against spaces and things that may be in the password
  • Then, the filename to un-RAR

If you wanted to try to run multiple unrar instances in parallel, you could also insert -P4 into the xargs invocation (e.g. …-I {} -P4 --…) to run 4 instances at a time; adjust this until your machine gets too loaded down to gain any benefits. (Since this is likely disc I/O bound, you might want to make sure to copy the RAR file into a RAM filesystem like /tmp or /run before starting it, if it's a reasonable size, so that you're not waiting on disc I/O as much, but the OS will likely cache the file after a few dozen rounds, so that might not actually help much over the course of a long run.)

This is a brute-force way to do it, but doesn't require as deep a knowledge of programming as, say, using fork/exec/wait to launch unrar processes, or using a rar-enabled library to do it yourself (which would probably yield a significant improvement in speed over launching the executable hundreds or thousands of times)

PS

I realized afterwards that perhaps you're looking for interaction with the actual WinRAR™ program. The above isn't at all helpful for that; but it will enable you to run the command-line unrar repeatedly.

Also, if you're on a Windows system, you'd need to install some of the standard shell utilities — a POSIX-compatible Bourne shell like BASH, sed, and xargs — which might imply something like Cygwin being needed. I don't have any practical experience with Windows systems to give good advice about how to do that, though.