updated is_avalanching trait protocol

benchmark/string.cpp

@@ -1,5 +1,5 @@
 // Copyright 2021 Peter Dimov.
-// Copyright 2023 Joaquin M Lopez Munoz.
+// Copyright 2023-2024 Joaquin M Lopez Munoz.
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
@@ -25,6 +25,7 @@
 #include <iostream>
 #include <iomanip>
 #include <chrono>
+#include <type_traits>
 
 using namespace std::chrono_literals;
 
@@ -341,7 +342,7 @@ template<> struct fnv1a_hash_impl<64>
 
 struct fnv1a_hash: fnv1a_hash_impl< std::numeric_limits<std::size_t>::digits >
 {
-    using is_avalanching = void;
+    using is_avalanching = std::true_type;
 };
 
 template<class K, class V> using std_unordered_map_fnv1a =

benchmark/string_stats.cpp

@@ -18,6 +18,7 @@
 #include <iostream>
 #include <iomanip>
 #include <chrono>
+#include <type_traits>
 
 using namespace std::chrono_literals;
 
@@ -262,7 +263,7 @@ template<> struct fnv1a_hash_impl<64>
 
 struct fnv1a_hash: fnv1a_hash_impl< std::numeric_limits<std::size_t>::digits >
 {
-    using is_avalanching = void;
+    using is_avalanching = std::true_type;
 };
 
 template<class K, class V> using boost_unordered_flat_map_fnv1a =
@@ -272,7 +273,7 @@ template<class K, class V> using boost_unordered_flat_map_fnv1a =
 
 struct slightly_bad_hash
 {
-    using is_avalanching = void;
+    using is_avalanching = std::true_type;
 
     std::size_t operator()( std::string const& s ) const
     {
@@ -295,7 +296,7 @@ template<class K, class V> using boost_unordered_flat_map_slightly_bad_hash =
 
 struct bad_hash
 {
-    using is_avalanching = void;
+    using is_avalanching = std::true_type;
 
     std::size_t operator()( std::string const& s ) const
     {

benchmark/string_view.cpp

@@ -1,5 +1,5 @@
 // Copyright 2021 Peter Dimov.
-// Copyright 2023 Joaquin M Lopez Munoz.
+// Copyright 2023-2024 Joaquin M Lopez Munoz.
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
@@ -26,6 +26,7 @@
 #include <iostream>
 #include <iomanip>
 #include <chrono>
+#include <type_traits>
 
 using namespace std::chrono_literals;
 
@@ -342,7 +343,7 @@ template<> struct fnv1a_hash_impl<64>
 
 struct fnv1a_hash: fnv1a_hash_impl< std::numeric_limits<std::size_t>::digits >
 {
-    using is_avalanching = void;
+    using is_avalanching = std::true_type;
 };
 
 template<class K, class V> using std_unordered_map_fnv1a =

doc/unordered/changes.adoc

@@ -10,6 +10,7 @@
 
 * Added container `pmr` aliases when header `<memory_resource>` is available. The alias `boost::unordered::pmr::[container]` refers to `boost::unordered::[container]` with a `std::pmr::polymorphic_allocator` allocator type.
 * Equipped open-addressing and concurrent containers to internally calculate and provide statistical metrics affected by the quality of the hash function. This functionality is enabled by the global macro `BOOST_UNORDERED_ENABLE_STATS`.
+* Avalanching hash functions must now be marked via an `is_avalanching` typedef with an embedded `value` constant set to `true` (typically, defining `is_avalanching` as `std::true_type`). `using is_avalanching = void` is deprecated but allowed for backwards compatibility.
 
 == Release 1.85.0
 

doc/unordered/hash_quality.adoc

@@ -33,7 +33,7 @@ follows:
 ----
 struct my_string_hash_function
 {
-  using is_avalanching = void; // instruct Boost.Unordered to not use post-mixing
+  using is_avalanching = std::true_type; // instruct Boost.Unordered to not use post-mixing
 
   std::size_t operator()(const std::string& x) const
   {

doc/unordered/hash_traits.adoc

@@ -32,9 +32,14 @@ large changes in the returned hash code —ideally, flipping one bit in the
 the input value results in each bit of the hash code flipping with probability 50%. Approaching
 this property is critical for the proper behavior of open-addressing hash containers.
 
-`hash_is_avalanching<Hash>::value` is `true` if `Hash::is_avalanching` is a valid type,
-and `false` otherwise.
-Users can then declare a hash function `Hash` as avalanching either by embedding an `is_avalanching` typedef
+`hash_is_avalanching<Hash>::value` is:
+
+ * `false` if `Hash::is_avalanching` is not present,
+ * `Hash::is_avalanching::value` if this is present and convertible at compile time to a `bool`,
+ * `true` if `Hash::is_avalanching` is `void` (this usage is deprecated).
+ 
+The behavior is undefined if none of the three cases above is met.
+Users can then declare a hash function `Hash` as avalanching either by embedding an appropriate `is_avalanching` typedef
 into the definition of `Hash`, or directly by specializing `hash_is_avalanching<Hash>` to a class with
 an embedded compile-time constant `value` set to `true`.
 

include/boost/unordered/hash_traits.hpp

@@ -1,6 +1,6 @@
 /* Hash function characterization.
  *
- * Copyright 2022 Joaquin M Lopez Munoz.
+ * Copyright 2022-2024 Joaquin M Lopez Munoz.
  * Distributed under the Boost Software License, Version 1.0.
  * (See accompanying file LICENSE_1_0.txt or copy at
  * http://www.boost.org/LICENSE_1_0.txt)
@@ -19,12 +19,28 @@ namespace unordered{
 namespace detail{
 
 template<typename Hash,typename=void>
-struct hash_is_avalanching_impl: std::false_type{};
+struct hash_is_avalanching_impl:std::false_type{};
+
+template<typename IsAvalanching>
+struct avalanching_value
+{
+  static constexpr bool value=IsAvalanching::value;
+};
+
+/* may be explicitly marked as BOOST_DEPRECATED in the future */
+template<> struct avalanching_value<void>
+{
+  static constexpr bool value=true;
+};
 
 template<typename Hash>
-struct hash_is_avalanching_impl<Hash,
-  boost::unordered::detail::void_t<typename Hash::is_avalanching> >:
-    std::true_type{};
+struct hash_is_avalanching_impl<
+  Hash,
+  boost::unordered::detail::void_t<typename Hash::is_avalanching>
+>:std::integral_constant<
+  bool,
+  avalanching_value<typename Hash::is_avalanching>::value
+>{};
 
 } /* namespace detail */
 
@@ -32,8 +48,12 @@ struct hash_is_avalanching_impl<Hash,
  * when actual characterization differs from default.
  */
 
-/* hash_is_avalanching<Hash>::value is true when the type Hash::is_avalanching
- * is present, false otherwise.
+/* hash_is_avalanching<Hash>::value is:
+ *   - false if Hash::is_avalanching is not present.
+ *   - Hash::is_avalanching::value if this is present and constexpr-convertible
+ *     to a bool.
+ *   - true if Hash::is_avalanching is void (deprecated).
+ * UB otherwise.
  */
 template<typename Hash>
 struct hash_is_avalanching: detail::hash_is_avalanching_impl<Hash>::type{};

test/unordered/hash_is_avalanching_test.cpp

@@ -1,4 +1,5 @@
 // Copyright 2022 Peter Dimov
+// Copyright 2024 Joaquin M Lopez Munoz
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
@@ -26,6 +27,7 @@ namespace unordered
 
 #include <boost/unordered/hash_traits.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
 
 struct X1
 {
@@ -36,6 +38,27 @@ struct X2
     typedef void is_avalanching;
 };
 
+struct X4
+{
+    using is_avalanching = std::false_type;
+};
+
+struct X5
+{
+    using is_avalanching = std::true_type;
+};
+
+struct X6
+{
+    using is_avalanching = boost::false_type;
+};
+
+struct X7
+{
+    using is_avalanching = boost::true_type;
+};
+
+
 int main()
 {
     using boost::unordered::hash_is_avalanching;
@@ -43,6 +66,10 @@ int main()
     BOOST_TEST_TRAIT_FALSE((hash_is_avalanching<X1>));
     BOOST_TEST_TRAIT_TRUE((hash_is_avalanching<X2>));
     BOOST_TEST_TRAIT_TRUE((hash_is_avalanching<X3>));
+    BOOST_TEST_TRAIT_FALSE((hash_is_avalanching<X4>));
+    BOOST_TEST_TRAIT_TRUE((hash_is_avalanching<X5>));
+    BOOST_TEST_TRAIT_FALSE((hash_is_avalanching<X6>));
+    BOOST_TEST_TRAIT_TRUE((hash_is_avalanching<X7>));
 
     return boost::report_errors();
 }