From 44b4c2db49f9d3283e04d4c34b5f4d22e66d0af4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C3=A9o=20Lam?= <leo@leolam.fr>
Date: Sat, 15 Feb 2020 21:13:29 +0100
Subject: Common: Avoid std::function overhead in ScopeGuard

So far in all our uses of ScopeGuard, the type of the callable is
usually just a lambda or a function pointer, so there is no need
to rely on std::function's type erasure.

While the cost of using std::function is probably negligible, it still
causes some unnecessary overhead that can be avoided by making
ScopeGuard a templated class. Thanks to class template argument
deduction in C++17 most existing usages do not even need to be changed.

See https://godbolt.org/z/KcoPni for a comparison between
a ScopeGuard that uses std::function and one that doesn't
---
 Source/Core/Common/ScopeGuard.h | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

(limited to 'Source/Core/Common')

diff --git a/Source/Core/Common/ScopeGuard.h b/Source/Core/Common/ScopeGuard.h
index d95ef8e4ae..c1cad06dd7 100644
--- a/Source/Core/Common/ScopeGuard.h
+++ b/Source/Core/Common/ScopeGuard.h
@@ -4,17 +4,15 @@
 
 #pragma once
 
-#include <functional>
+#include <optional>
 
 namespace Common
 {
+template <typename Callable>
 class ScopeGuard final
 {
 public:
-  template <class Callable>
-  ScopeGuard(Callable&& finalizer) : m_finalizer(std::forward<Callable>(finalizer))
-  {
-  }
+  ScopeGuard(Callable&& finalizer) : m_finalizer(std::forward<Callable>(finalizer)) {}
 
   ScopeGuard(ScopeGuard&& other) : m_finalizer(std::move(other.m_finalizer))
   {
@@ -22,13 +20,13 @@ public:
   }
 
   ~ScopeGuard() { Exit(); }
-  void Dismiss() { m_finalizer = nullptr; }
+  void Dismiss() { m_finalizer.reset(); }
   void Exit()
   {
     if (m_finalizer)
     {
-      m_finalizer();  // must not throw
-      m_finalizer = nullptr;
+      (*m_finalizer)();  // must not throw
+      m_finalizer.reset();
     }
   }
 
@@ -37,7 +35,7 @@ public:
   void operator=(const ScopeGuard&) = delete;
 
 private:
-  std::function<void()> m_finalizer;
+  std::optional<Callable> m_finalizer;
 };
 
 }  // Namespace Common
-- 
cgit v1.2.3