No need for a wrapping structure for posixTimeToDuration

[folly.git] / folly / Unit.h

diff --git a/folly/Unit.h b/folly/Unit.h
index cb6a6566df41a57ead7ea60d940e338f639bc61c..52f38fcfaf3caaa325bae8810b89b76c21bfd124 100644 (file)
--- a/folly/Unit.h
+++ b/folly/Unit.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,21 +21,42 @@
 namespace folly {
 
 /// In functional programming, the degenerate case is often called "unit". In
-/// C++, "void" is often the best analogue, however because of the syntactic
-/// special-casing required for void it is a liability for template
-/// metaprogramming. So, instead of e.g. Future<void>, we have Future<Unit>.
-/// You can ignore the actual value, and we port some of the syntactic
-/// niceties like setValue() instead of setValue(Unit{}).
+/// C++, "void" is often the best analogue. However, because of the syntactic
+/// special-casing required for void, it is frequently a liability for template
+/// metaprogramming. So, instead of writing specializations to handle cases like
+/// SomeContainer<void>, a library author may instead rule that out and simply
+/// have library users use SomeContainer<Unit>. Contained values may be ignored.
+/// Much easier.
+///
+/// "void" is the type that admits of no values at all. It is not possible to
+/// construct a value of this type.
+/// "unit" is the type that admits of precisely one unique value. It is
+/// possible to construct a value of this type, but it is always the same value
+/// every time, so it is uninteresting.
 struct Unit {
+  // These are structs rather than type aliases because MSVC 2017 RC has
+  // trouble correctly resolving dependent expressions in type aliases
+  // in certain very specific contexts, including a couple where this is
+  // used. See the known issues section here for more info:
+  // https://blogs.msdn.microsoft.com/vcblog/2016/06/07/expression-sfinae-improvements-in-vs-2015-update-3/
+
+  template <typename T>
+  struct Lift : std::conditional<std::is_same<T, void>::value, Unit, T> {};
+  template <typename T>
+  using LiftT = typename Lift<T>::type;
   template <typename T>
-  using Lift = std::conditional<std::is_same<T, void>::value, Unit, T>;
+  struct Drop : std::conditional<std::is_same<T, Unit>::value, void, T> {};
   template <typename T>
-  using Drop = std::conditional<std::is_same<T, Unit>::value, void, T>;
+  using DropT = typename Drop<T>::type;
 
-  bool operator==(const Unit& /*other*/) const { return true; }
-  bool operator!=(const Unit& /*other*/) const { return false; }
+  constexpr bool operator==(const Unit& /*other*/) const {
+    return true;
+  }
+  constexpr bool operator!=(const Unit& /*other*/) const {
+    return false;
+  }
 };
 
 constexpr Unit unit {};
 
-}
+} // namespace folly