From: Eric Niebler <eniebler@fb.com>
Date: Sun, 8 Jan 2017 05:44:02 +0000 (-0800)
Subject: add folly::as_const, like C++17's std::as_const
X-Git-Tag: v2017.03.06.00~119
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=3cec19d4760d9b4057c7e4a6464be86aad216a73;p=folly.git

add folly::as_const, like C++17's std::as_const

Summary: A one-liner from C++17, but a useful one. Const-qualifies an lvalue reference, avoiding the need for an awkward const_cast in some cases.

Reviewed By: yfeldblum

Differential Revision: D4389929

fbshipit-source-id: 1650c4901489eb0dd62fd9fa633b4a0da9f01954
---

diff --git a/folly/Utility.h b/folly/Utility.h
index a7692505..385ed34a 100644
--- a/folly/Utility.h
+++ b/folly/Utility.h
@@ -68,4 +68,32 @@ constexpr typename std::decay<T>::type copy(T&& value) noexcept(
     noexcept(typename std::decay<T>::type(std::forward<T>(value)))) {
   return std::forward<T>(value);
 }
+
+/**
+ * A simple helper for getting a constant reference to an object.
+ *
+ * Example:
+ *
+ *   std::vector<int> v{1,2,3};
+ *   // The following two lines are equivalent:
+ *   auto a = const_cast<const std::vector<int>&>(v).begin();
+ *   auto b = folly::as_const(v).begin();
+ *
+ * Like C++17's std::as_const. See http://wg21.link/p0007
+ */
+#if __cpp_lib_as_const || _MSC_VER
+
+/* using override */ using std::as_const
+
+#else
+
+template <class T>
+constexpr T const& as_const(T& t) noexcept {
+  return t;
+}
+
+template <class T>
+void as_const(T const&&) = delete;
+
+#endif
 }
diff --git a/folly/test/UtilityTest.cpp b/folly/test/UtilityTest.cpp
index 337ade5a..df8fa5d2 100644
--- a/folly/test/UtilityTest.cpp
+++ b/folly/test/UtilityTest.cpp
@@ -59,3 +59,19 @@ TEST_F(UtilityTest, copy_noexcept_spec) {
   EXPECT_FALSE(noexcept(folly::copy(thr)));
   EXPECT_TRUE(noexcept(folly::copy(std::move(thr)))); // note: does not copy
 }
+
+TEST_F(UtilityTest, as_const) {
+  struct S {
+    bool member() {
+      return false;
+    }
+    bool member() const {
+      return true;
+    }
+  };
+  S s;
+  EXPECT_FALSE(s.member());
+  EXPECT_TRUE(folly::as_const(s).member());
+  EXPECT_EQ(&s, &folly::as_const(s));
+  EXPECT_TRUE(noexcept(folly::as_const(s)));
+}