From e8527d2e4bfa7dc4538155f38f3374d997dd2f9b Mon Sep 17 00:00:00 2001
From: Yedidya Feldblum <yfeldblum@fb.com>
Date: Sat, 8 Jul 2017 20:59:42 -0700
Subject: [PATCH] In-place construction for Future

Summary:
[Folly] In-place construction for `Future`.

Using `in_place` as the first argument. Avoid constructing a contained object and then moving it into the `Future`, which may be more expensive, by providing an unambiguous way to specify perfect forwarding.

And a matching overload of `makeFuture` for no particularly good reason.

Differential Revision: D5362324

fbshipit-source-id: e612965f34ed0fae5fac17db631f7eab9984c696
---
 folly/futures/Future-inl.h        |  8 ++++++++
 folly/futures/Future.h            | 11 +++++++++--
 folly/futures/detail/Core.h       |  9 +++++++++
 folly/futures/test/FutureTest.cpp |  5 +++++
 4 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/folly/futures/Future-inl.h b/folly/futures/Future-inl.h
index ab70c38d..84bf33a1 100644
--- a/folly/futures/Future-inl.h
+++ b/folly/futures/Future-inl.h
@@ -184,6 +184,14 @@ template <typename T2>
 Future<T>::Future(typename std::enable_if<std::is_same<Unit, T2>::value>::type*)
     : core_(new detail::Core<T>(Try<T>(T()))) {}
 
+template <class T>
+template <
+    class... Args,
+    typename std::enable_if<std::is_constructible<T, Args&&...>::value, int>::
+        type>
+Future<T>::Future(in_place_t, Args&&... args)
+    : core_(new detail::Core<T>(in_place, std::forward<Args>(args)...)) {}
+
 template <class T>
 Future<T>::~Future() {
   detach();
diff --git a/folly/futures/Future.h b/folly/futures/Future.h
index e5e1fbe8..f4959ba7 100644
--- a/folly/futures/Future.h
+++ b/folly/futures/Future.h
@@ -25,10 +25,11 @@
 
 #include <folly/Optional.h>
 #include <folly/Portability.h>
-#include <folly/futures/DrivableExecutor.h>
-#include <folly/futures/Promise.h>
 #include <folly/Try.h>
+#include <folly/Utility.h>
+#include <folly/futures/DrivableExecutor.h>
 #include <folly/futures/FutureException.h>
+#include <folly/futures/Promise.h>
 #include <folly/futures/detail/Types.h>
 
 // boring predeclarations and details
@@ -90,6 +91,12 @@ class Future {
   /* implicit */ Future(
       typename std::enable_if<std::is_same<Unit, T2>::value>::type* = nullptr);
 
+  template <
+      class... Args,
+      typename std::enable_if<std::is_constructible<T, Args&&...>::value, int>::
+          type = 0>
+  explicit Future(in_place_t, Args&&... args);
+
   ~Future();
 
   /** Return the reference to result. Should not be called if !isReady().
diff --git a/folly/futures/detail/Core.h b/folly/futures/detail/Core.h
index 30d421c0..dc91729f 100644
--- a/folly/futures/detail/Core.h
+++ b/folly/futures/detail/Core.h
@@ -19,6 +19,7 @@
 #include <atomic>
 #include <mutex>
 #include <stdexcept>
+#include <utility>
 #include <vector>
 
 #include <folly/Executor.h>
@@ -27,6 +28,7 @@
 #include <folly/Optional.h>
 #include <folly/ScopeGuard.h>
 #include <folly/Try.h>
+#include <folly/Utility.h>
 #include <folly/futures/FutureException.h>
 #include <folly/futures/detail/FSM.h>
 
@@ -87,6 +89,13 @@ class Core final {
       fsm_(State::OnlyResult),
       attached_(1) {}
 
+  template <typename... Args>
+  explicit Core(in_place_t, Args&&... args) noexcept(
+      noexcept(::new (nullptr) T(std::declval<Args&&>()...)))
+      : result_(in_place, in_place, std::forward<Args>(args)...),
+        fsm_(State::OnlyResult),
+        attached_(1) {}
+
   ~Core() {
     DCHECK(attached_ == 0);
   }
diff --git a/folly/futures/test/FutureTest.cpp b/folly/futures/test/FutureTest.cpp
index e886a21a..4dd3f2dc 100644
--- a/folly/futures/test/FutureTest.cpp
+++ b/folly/futures/test/FutureTest.cpp
@@ -810,6 +810,11 @@ TEST(Future, ImplicitConstructor) {
   //auto f2 = []() -> Future<Unit> { }();
 }
 
+TEST(Future, InPlaceConstructor) {
+  auto f = Future<std::pair<int, double>>(in_place, 5, 3.2);
+  EXPECT_EQ(5, f.value().first);
+}
+
 TEST(Future, thenDynamic) {
   // folly::dynamic has a constructor that takes any T, this test makes
   // sure that we call the then lambda with folly::dynamic and not
-- 
2.34.1