commit cf4600e1582f3001a26b4430a5ffbfe23b3ff4d1
author Kelvin Zhang <zhangkelvin@google.com> Tue Oct 27 15:50:33 2020 -0400
committer Kelvin Zhang <zhangkelvin@google.com> Mon Jan 04 18:51:11 2021 -0500
tree 05b10c5bf52e457d20c93bf04ce9d2fe8fb205c4
parent 3265b317b39edea86b5351029092b7728ca8461b

Use string_view for pref interface to reduce copy

If you pass in a static string literal like "Hello World!", then with
parameter type of const string& you need to construct a new string
object, requiring a copy. It will also require a copy if your data is in
another container, for example std::vector<char> . In update_engine, we
store manifest bytes in std::vector, and sometimes we want to save that
manifest to disk. This CL can help us reduce copy of the manifest(up to
2MB).

Test: treehugger
Change-Id: I70feb4c0673c174fd47f02c4bd41994f74cda743

common/fake_prefs.cc

diff --git a/common/fake_prefs.cc b/common/fake_prefs.cc
index 275667e..ea6ea60 100644
--- a/common/fake_prefs.cc
+++ b/common/fake_prefs.cc
@@ -17,6 +17,7 @@
 #include "update_engine/common/fake_prefs.h"
 
 #include <algorithm>
+#include <utility>
 
 #include <gtest/gtest.h>
 
@@ -66,8 +67,8 @@
   return GetValue(key, value);
 }
 
-bool FakePrefs::SetString(const string& key, const string& value) {
-  SetValue(key, value);
+bool FakePrefs::SetString(const string& key, std::string_view value) {
+  SetValue(key, std::string(value));
   return true;
 }
 
@@ -149,10 +150,10 @@
 }
 
 template <typename T>
-void FakePrefs::SetValue(const string& key, const T& value) {
+void FakePrefs::SetValue(const string& key, T value) {
   CheckKeyType(key, PrefConsts<T>::type);
   values_[key].type = PrefConsts<T>::type;
-  values_[key].value.*(PrefConsts<T>::member) = value;
+  values_[key].value.*(PrefConsts<T>::member) = std::move(value);
   const auto observers_for_key = observers_.find(key);
   if (observers_for_key != observers_.end()) {
     std::vector<ObserverInterface*> copy_observers(observers_for_key->second);