Specialize boost::unordered::hash_is_avalanching for hash<string> and hash<string_view>

include/boost/container_hash/hash.hpp

@@ -63,6 +63,10 @@
 #include <variant>
 #endif
 
+#if !defined(BOOST_NO_CXX17_HDR_STRING_VIEW)
+# include <string_view>
+#endif
+
 namespace boost
 {
 
@@ -629,6 +633,30 @@ namespace boost
     };
 
 #endif
-}
+
+    // boost::unordered::hash_is_avalanching
+
+    namespace unordered
+    {
+        template<class T> struct hash_is_avalanching;
+        template<class Ch> struct hash_is_avalanching< boost::hash< std::basic_string<Ch> > >: boost::is_integral<Ch> {};
+
+        // boost::is_integral<char8_t> is false, but should be true (https://github.com/boostorg/type_traits/issues/175)
+#if defined(__cpp_char8_t) && __cpp_char8_t >= 201811L
+        template<> struct hash_is_avalanching< boost::hash< std::u8string > >: boost::true_type {};
+#endif
+
+#if !defined(BOOST_NO_CXX17_HDR_STRING_VIEW)
+
+        template<class Ch> struct hash_is_avalanching< boost::hash< std::basic_string_view<Ch> > >: boost::is_integral<Ch> {};
+
+#if defined(__cpp_char8_t) && __cpp_char8_t >= 201811L
+        template<> struct hash_is_avalanching< boost::hash< std::u8string_view > >: boost::true_type {};
+#endif
+
+#endif
+    } // namespace unordered
+
+} // namespace boost
 
 #endif // #ifndef BOOST_FUNCTIONAL_HASH_HASH_HPP

test/Jamfile.v2

@@ -112,3 +112,6 @@ run is_described_class_test3.cpp
 
 run described_class_test.cpp
   : : : <warnings>extra ;
+
+run hash_is_avalanching_test.cpp ;
+run hash_is_avalanching_test2.cpp ;

test/hash_is_avalanching_test.cpp

@@ -0,0 +1,52 @@
+// Copyright 2022 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/container_hash/hash.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX11_HDR_TYPE_TRAITS)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped, BOOST_NO_CXX11_HDR_TYPE_TRAITS is defined" )
+int main() {}
+
+#else
+
+#include <boost/unordered/hash_traits.hpp>
+#include <string>
+
+enum my_char { min = 0, max = 255 };
+
+int main()
+{
+    using boost::unordered::hash_is_avalanching;
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::string> > ));
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::wstring> > ));
+
+#if !defined(BOOST_NO_CXX11_CHAR16_T)
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::u16string> > ));
+
+#endif
+
+#if !defined(BOOST_NO_CXX11_CHAR32_T)
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::u32string> > ));
+
+#endif
+
+#if defined(__cpp_char8_t) && __cpp_char8_t >= 201811L
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::u8string> > ));
+
+#endif
+
+    BOOST_TEST_TRAIT_FALSE(( hash_is_avalanching< boost::hash<std::basic_string<my_char> > > ));
+
+    return boost::report_errors();
+}
+
+#endif

test/hash_is_avalanching_test2.cpp

@@ -0,0 +1,57 @@
+// Copyright 2022 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/container_hash/hash.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX11_HDR_TYPE_TRAITS)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped, BOOST_NO_CXX11_HDR_TYPE_TRAITS is defined" )
+int main() {}
+
+#elif defined(BOOST_NO_CXX17_HDR_STRING_VIEW)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped, BOOST_NO_CXX17_HDR_STRING_VIEW is defined" )
+int main() {}
+
+#else
+
+#include <boost/unordered/hash_traits.hpp>
+#include <string_view>
+
+enum my_char { min = 0, max = 255 };
+
+int main()
+{
+    using boost::unordered::hash_is_avalanching;
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::string_view> > ));
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::wstring_view> > ));
+
+#if !defined(BOOST_NO_CXX11_CHAR16_T)
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::u16string_view> > ));
+
+#endif
+
+#if !defined(BOOST_NO_CXX11_CHAR32_T)
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::u32string_view> > ));
+
+#endif
+
+#if defined(__cpp_char8_t) && __cpp_char8_t >= 201811L
+
+    BOOST_TEST_TRAIT_TRUE(( hash_is_avalanching< boost::hash<std::u8string_view> > ));
+
+#endif
+
+    BOOST_TEST_TRAIT_FALSE(( hash_is_avalanching< boost::hash<std::basic_string_view<my_char> > > ));
+
+    return boost::report_errors();
+}
+
+#endif