compiles!
diff --git a/tools/doxygen/Doxyfile.c++.internal b/tools/doxygen/Doxyfile.c++.internal
index 05c3b4f..e63c9ae 100644
--- a/tools/doxygen/Doxyfile.c++.internal
+++ b/tools/doxygen/Doxyfile.c++.internal
@@ -840,6 +840,7 @@
 src/cpp/util/byte_buffer.cc \
 src/cpp/util/slice.cc \
 src/cpp/util/status.cc \
+src/cpp/util/string_ref.cc \
 src/cpp/util/time.cc
 
 # This tag can be used to specify the character encoding of the source files