You can use a shell loop that runs diff for each file, though this will not catch the cases where d2 contains a file, but d1 doesn't. It might be sufficient though.

for file in d1/*.cpp; do

    diff "$file" "d2/${file##*/}"

done

Or all on one line:

for file in d1/*.cpp; do diff "$file" "d2/${file##*/}"; done

The ${file##*/} part is a special parameter expansion.

If the file variable contains d1/hello.cpp, then "${file##*/}" will expand to hello.cpp (the value of file, but with everything up to, and including, the last / removed).

So "d2/${file##*/}" will result in d2/hello.cpp and the resulting diff command is thus diff d1/hello.cpp d2/hello.cpp

See http://mywiki.wooledge.org/BashFAQ/100 for more on string manipulations in bash.

On a side note, a version control system (such as subversion, git, mercurial etc...) would make this type of diffing much easier.