+3  A: 

grep -R <some_string> <directory>

ezod
Add the --files-with-matches option if you only want a list of files and not the text printed from the grep call.
Ian C.
this is the flag i was looking for - thanks!
amitlicht
A: 

Another option is:

find . -name "*.cpp" | xargs grep some_string

if you want to restrict the search to particular file patterns or types. With find, you can also specify the maximum depth to search.

Amit Kumar