diff --git a/cpp/ycm/ClangCompleter/ClangCompleter.cpp b/cpp/ycm/ClangCompleter/ClangCompleter.cpp
index d03684a7..c7239c44 100644
--- a/cpp/ycm/ClangCompleter/ClangCompleter.cpp
+++ b/cpp/ycm/ClangCompleter/ClangCompleter.cpp
@@ -59,7 +59,7 @@ ClangCompleter::~ClangCompleter() {
 
 
 std::vector< Diagnostic > ClangCompleter::DiagnosticsForFile(
-  std::string filename ) {
+  const std::string &filename ) {
   shared_ptr< TranslationUnit > unit = translation_unit_store_.Get( filename );
 
   if ( !unit )
@@ -83,9 +83,9 @@ bool ClangCompleter::UpdatingTranslationUnit( const std::string &filename ) {
 
 
 void ClangCompleter::UpdateTranslationUnit(
-  std::string filename,
-  std::vector< UnsavedFile > unsaved_files,
-  std::vector< std::string > flags ) {
+  const std::string &filename,
+  const std::vector< UnsavedFile > &unsaved_files,
+  const std::vector< std::string > &flags ) {
   bool translation_unit_created;
   shared_ptr< TranslationUnit > unit = translation_unit_store_.GetOrCreate(
       filename,
@@ -114,11 +114,11 @@ void ClangCompleter::UpdateTranslationUnit(
 
 std::vector< CompletionData >
 ClangCompleter::CandidatesForLocationInFile(
-  std::string filename,
+  const std::string &filename,
   int line,
   int column,
-  std::vector< UnsavedFile > unsaved_files,
-  std::vector< std::string > flags ) {
+  const std::vector< UnsavedFile > &unsaved_files,
+  const std::vector< std::string > &flags ) {
   shared_ptr< TranslationUnit > unit =
       translation_unit_store_.GetOrCreate( filename, unsaved_files, flags );
 
@@ -132,11 +132,11 @@ ClangCompleter::CandidatesForLocationInFile(
 
 
 Location ClangCompleter::GetDeclarationLocation(
-  std::string filename,
+  const std::string &filename,
   int line,
   int column,
-  std::vector< UnsavedFile > unsaved_files,
-  std::vector< std::string > flags ) {
+  const std::vector< UnsavedFile > &unsaved_files,
+  const std::vector< std::string > &flags ) {
   shared_ptr< TranslationUnit > unit =
       translation_unit_store_.GetOrCreate( filename, unsaved_files, flags );
 
@@ -149,11 +149,11 @@ Location ClangCompleter::GetDeclarationLocation(
 
 
 Location ClangCompleter::GetDefinitionLocation(
-  std::string filename,
+  const std::string &filename,
   int line,
   int column,
-  std::vector< UnsavedFile > unsaved_files,
-  std::vector< std::string > flags ) {
+  const std::vector< UnsavedFile > &unsaved_files,
+  const std::vector< std::string > &flags ) {
   shared_ptr< TranslationUnit > unit =
       translation_unit_store_.GetOrCreate( filename, unsaved_files, flags );
 
@@ -165,7 +165,7 @@ Location ClangCompleter::GetDefinitionLocation(
 }
 
 
-void ClangCompleter::DeleteCachesForFile( std::string filename ) {
+void ClangCompleter::DeleteCachesForFile( const std::string &filename ) {
   translation_unit_store_.Remove( filename );
 }
 
diff --git a/cpp/ycm/ClangCompleter/ClangCompleter.h b/cpp/ycm/ClangCompleter/ClangCompleter.h
index 5218f4e5..70fb3252 100644
--- a/cpp/ycm/ClangCompleter/ClangCompleter.h
+++ b/cpp/ycm/ClangCompleter/ClangCompleter.h
@@ -37,48 +37,43 @@ struct Location;
 typedef std::vector< CompletionData > CompletionDatas;
 
 
-// TODO: document that all filename parameters must be absolute paths
+// All filename parameters must be absolute paths.
 class ClangCompleter : boost::noncopyable {
 public:
   ClangCompleter();
   ~ClangCompleter();
 
-  std::vector< Diagnostic > DiagnosticsForFile( std::string filename );
+  std::vector< Diagnostic > DiagnosticsForFile( const std::string &filename );
 
   bool UpdatingTranslationUnit( const std::string &filename );
 
-  // NOTE: params are taken by value on purpose! With a C++11 compiler we can
-  // avoid internal copies if params are taken by value (move ctors FTW), and we
-  // need to ensure we own the memory.
-  // TODO: Change some of these params back to const ref where possible after we
-  // get the server up.
   void UpdateTranslationUnit(
-    std::string filename,
-    std::vector< UnsavedFile > unsaved_files,
-    std::vector< std::string > flags );
+    const std::string &filename,
+    const std::vector< UnsavedFile > &unsaved_files,
+    const std::vector< std::string > &flags );
 
   std::vector< CompletionData > CandidatesForLocationInFile(
-    std::string filename,
+    const std::string &filename,
     int line,
     int column,
-    std::vector< UnsavedFile > unsaved_files,
-    std::vector< std::string > flags );
+    const std::vector< UnsavedFile > &unsaved_files,
+    const std::vector< std::string > &flags );
 
   Location GetDeclarationLocation(
-    std::string filename,
+    const std::string &filename,
     int line,
     int column,
-    std::vector< UnsavedFile > unsaved_files,
-    std::vector< std::string > flags );
+    const std::vector< UnsavedFile > &unsaved_files,
+    const std::vector< std::string > &flags );
 
   Location GetDefinitionLocation(
-    std::string filename,
+    const std::string &filename,
     int line,
     int column,
-    std::vector< UnsavedFile > unsaved_files,
-    std::vector< std::string > flags );
+    const std::vector< UnsavedFile > &unsaved_files,
+    const std::vector< std::string > &flags );
 
-  void DeleteCachesForFile( std::string filename );
+  void DeleteCachesForFile( const std::string &filename );
 
 private: