diff --git a/cpp/ycm/ClangCompleter/UnsavedFile.h b/cpp/ycm/ClangCompleter/UnsavedFile.h
index e0565d74..aecb355a 100644
--- a/cpp/ycm/ClangCompleter/UnsavedFile.h
+++ b/cpp/ycm/ClangCompleter/UnsavedFile.h
@@ -18,6 +18,8 @@
 #ifndef UNSAVEDFILE_H_0GIYZQL4
 #define UNSAVEDFILE_H_0GIYZQL4
 
+#include <cstddef>
+
 struct UnsavedFile
 {
   UnsavedFile() : filename_( NULL ), contents_( NULL ), length_( 0 ) {}