diff --git a/cpp/ycm/CandidateRepository.h b/cpp/ycm/CandidateRepository.h
index 0674cfc3..7ad8ec0d 100644
--- a/cpp/ycm/CandidateRepository.h
+++ b/cpp/ycm/CandidateRepository.h
@@ -33,6 +33,15 @@ struct CompletionData;
 typedef boost::unordered_map< std::string, const Candidate * >
 CandidateHolder;
 
+
+// This singleton stores already built Candidate objects for candidate strings
+// that were already seen. If Candidates are requested for previously unseen
+// strings, new Candidate objects are built.
+//
+// This is shared by the identifier completer and the clang completer so that
+// work is not repeated.
+//
+// This class is thread-safe.
 class CandidateRepository : boost::noncopyable {
 public:
   static CandidateRepository &Instance();