You can use
git show for that:
git show HEAD^:main.cpp > old_main.cpp
(Note that there is colon [
:] character between
<revision>:<path> syntax is described in git rev-parse manpage, next to last point in the "Specifying revisions" section:
:followed by a path names the blob or tree at the given path in the tree-ish object named by the part before the colon.
:path(with an empty part before the colon) is a special case of the syntax described next: content recorded in the index at the given path. A path starting with
../is relative to the current working directory. The given path will be converted to be relative to the working tree’s root directory. This is most useful to address a blob or tree from a commit or tree that has the same tree structure as the working tree.
<path> here is FULL path relative to the top directory of your project, i.e. the directory with
.git/ directory. (Or, to be more exact, to "<revision>", which in general can be any <tree-ish>, i.e. something that represents tree.)
If you want to use path relative to the current directory, you need to use
./<path> syntax (or
../path to go up from current directory).
Edit 2015-01-15: added information about relative path syntax
You can get in most cases the same output using low-level (plumbing)
git cat-file command:
git cat-file blob HEAD^:main.cpp > old_main.cpp