From 622fe822372560c525bd8029d6e68d06d81d24d6 Mon Sep 17 00:00:00 2001
From: Marcelo Juchem <marcelo@fb.com>
Date: Tue, 2 Apr 2013 20:22:58 -0700
Subject: [PATCH] Implementing a traits class to check for incomplete types

Summary: A traits class to check for incomplete types

Test Plan: unit tests added

Reviewed By: delong.j@fb.com

FB internal diff: D760676
---
 folly/Traits.h            | 28 ++++++++++++++++++++++++++++
 folly/test/TraitsTest.cpp |  8 ++++++++
 2 files changed, 36 insertions(+)

diff --git a/folly/Traits.h b/folly/Traits.h
index bc0a0bf6..ebdb84d1 100644
--- a/folly/Traits.h
+++ b/folly/Traits.h
@@ -277,6 +277,34 @@ struct IsOneOf<T, T1, Ts...> {
   enum { value = std::is_same<T, T1>::value || IsOneOf<T, Ts...>::value };
 };
 
+/**
+ * A traits class to check for incomplete types.
+ *
+ * Example:
+ *
+ *  struct FullyDeclared {}; // complete type
+ *  struct ForwardDeclared; // incomplete type
+ *
+ *  is_complete<int>::value // evaluates to true
+ *  is_complete<FullyDeclared>::value // evaluates to true
+ *  is_complete<ForwardDeclared>::value // evaluates to false
+ *
+ *  struct ForwardDeclared {}; // declared, at last
+ *
+ *  is_complete<ForwardDeclared>::value // now it evaluates to true
+ *
+ * @author: Marcelo Juchem <marcelo@fb.com>
+ */
+template <typename T>
+class is_complete {
+  template <unsigned long long> struct sfinae {};
+  template <typename U>
+  constexpr static bool test(sfinae<sizeof(U)>*) { return true; }
+  template <typename> constexpr static bool test(...) { return false; }
+public:
+  constexpr static bool value = test<T>(nullptr);
+};
+
 /*
  * Complementary type traits to check for a negative/non-positive value.
  *
diff --git a/folly/test/TraitsTest.cpp b/folly/test/TraitsTest.cpp
index a2760ca8..7a80c6cc 100644
--- a/folly/test/TraitsTest.cpp
+++ b/folly/test/TraitsTest.cpp
@@ -96,6 +96,14 @@ TEST(Traits, is_negative) {
   EXPECT_FALSE(folly::is_non_positive(1u));
 }
 
+struct CompleteType {};
+struct IncompleteType;
+TEST(Traits, is_complete) {
+  EXPECT_TRUE((folly::is_complete<int>::value));
+  EXPECT_TRUE((folly::is_complete<CompleteType>::value));
+  EXPECT_FALSE((folly::is_complete<IncompleteType>::value));
+}
+
 int main(int argc, char ** argv) {
   testing::InitGoogleTest(&argc, argv);
   google::ParseCommandLineFlags(&argc, &argv, true);
-- 
2.34.1