In main I use a variable say "char user_input;" then I no longer have a need for that particular variable. However, I do need another variable say "char function_result;"

The way I see it I can easily do two things:
 1) make a new variable
 2) or use the old variable
but is there a way to rename the old variable with the new name?

Nope. Make a new one -- it's stylistically more pleasing for a tiny (and temporary) impact on memory consumption.

----
Scott J. Kleper
Author, "Professional C++"
(Wrox, 2005)

There is a way, but it might be clumsy.

You can do something like this:

...
char my_first_char;
// do other things
// then use the old variable
// to use the space of the old variable
char & another_char(my_first_char);

Dean Michael C. Berris
Orange and Bronze Technologies, Inc.
Mobile +639287291459

Its not renaming...it's giving another name. :)