LWG Issue 2148: Hashing Enums

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189831 91177308-0d34-0410-b5e6-96231b3b80d8

include/functional

@@ -2399,6 +2399,22 @@ struct _LIBCPP_TYPE_VIS_ONLY hash<long double>
     }
 };
 
+#if _LIBCPP_STD_VER > 11
+template <class _Tp>
+struct _LIBCPP_TYPE_VIS_ONLY hash
+    : public unary_function<_Tp, size_t>
+{
+    static_assert(is_enum<_Tp>::value, "This hash only works for enumeration types");
+
+    _LIBCPP_INLINE_VISIBILITY
+    size_t operator()(_Tp __v) const _NOEXCEPT
+    {
+        typedef typename underlying_type<_Tp>::type type;
+        return hash<type>{}(static_cast<type>(__v));
+    }
+};
+#endif
+
 // struct hash<T*> in <memory>
 
 _LIBCPP_END_NAMESPACE_STD

test/utilities/function.objects/unord.hash/enum.fail.cpp

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <functional>
+
+//  Hashing a struct w/o a defined hash should fail.
+
+#include <functional>
+#include <cassert>
+#include <type_traits>
+
+struct X {};
+
+int main()
+{
+    X x;
+    size_t h = std::hash<X>{} ( x );
+}

test/utilities/function.objects/unord.hash/enum.pass.cpp

@@ -0,0 +1,63 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <functional>
+
+// make sure that we can hash enumeration values
+// Not very portable
+
+#include <__config>
+
+#if _LIBCPP_STD_VER > 11
+
+#include <functional>
+#include <cassert>
+#include <type_traits>
+#include <limits>
+
+enum class Colors { red, orange, yellow, green, blue, indigo, violet };
+enum class Cardinals { zero, one, two, three, five=5 };
+enum class LongColors : short { red, orange, yellow, green, blue, indigo, violet };
+enum class ShortColors : long { red, orange, yellow, green, blue, indigo, violet };
+enum class EightBitColors : uint8_t { red, orange, yellow, green, blue, indigo, violet };
+
+enum Fruits { apple, pear, grape, mango, cantaloupe };
+
+template <class T>
+void
+test()
+{
+    static_assert((std::is_base_of<std::unary_function<T, std::size_t>,
+                                   std::hash<T> >::value), "");
+    typedef typename std::underlying_type<T>::type under_type;
+    
+    std::hash<T> h1;
+    std::hash<under_type> h2;
+    for (int i = 0; i <= 5; ++i)
+    {
+        T t(static_cast<T> (i));
+        if (sizeof(T) <= sizeof(std::size_t))
+            assert(h1(t) == h2(static_cast<under_type>(i)));
+    }
+}
+
+int main()
+{
+    test<Cardinals>();
+
+    test<Colors>();
+    test<ShortColors>();
+    test<LongColors>();
+    test<EightBitColors>();
+
+    test<Fruits>();
+}
+#else
+int main () {}
+#endif

www/cxx1y_status.html

@@ -145,7 +145,7 @@
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-	<tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2148">2148</a></td><td>Hashing enums should be supported directly by std::hash</td><td>Bristol</td><td></td></tr>
+	<tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2148">2148</a></td><td>Hashing enums should be supported directly by std::hash</td><td>Bristol</td><td>Complete</td></tr>
 	<tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2149">2149</a></td><td>Concerns about 20.8/5</td><td>Bristol</td><td>Complete</td></tr>
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 	<tr><td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2163">2163</a></td><td>nth_element requires inconsistent post-conditions</td><td>Bristol</td><td>Complete</td></tr>