In one of my previous post about "Advanced Cheat Sheet", I wrote at the end how to make a change in a set of files using the args list (:h args):

1. Fill the args with the list of file to modify: :args **/*.cpp.
2. Apply the substitution: :argdo %s/old/new/g.

This is straightforward but not really efficient since Vim has to apply the substitution on all files even if there is no match.

A more convenient and scalable way to achieve a massive substitution is to fill the quickfix list (:h quickfix) with the set of file in which there are pattern matches and then apply the substitution on this set of file with the command :cdo.

1. Look for a pattern match: :grep! fooBar
2. Apply and save the changes: :cdo %s/fooBar/foo_bar/ge | update

The problem with this is that the substitute is done once per entry in the quickfix list. So, if there are 2 'foo' partterns in a file an you want to replace 'foo' with 'foofoo', 'foo' will be replaced with 'foofoofoofoo'.

From vim 8.1, the command :cfdo (see :h :cfdo) was added to apply the command once per file (and not once per quickfix entry): the 'f' of the command stands for 'filter'. So from vim 8.1, the procedure is:

1. Look for a pattern match: :grep! fooBar
2. Apply and save the changes: :cfdo %s/fooBar/foo_bar/ge | update