Add boost::hash_value for std::nullptr_t. Refs #29.

include/boost/container_hash/hash.hpp

@@ -27,6 +27,7 @@
 #include <boost/type_traits/enable_if.hpp>
 #include <boost/type_traits/conjunction.hpp>
 #include <boost/type_traits/is_union.hpp>
+#include <boost/type_traits/is_same.hpp>
 #include <boost/describe/bases.hpp>
 #include <boost/describe/members.hpp>
 #include <boost/cstdint.hpp>
@@ -494,6 +495,19 @@ namespace boost
         return seed;
     }
 
+#endif
+
+    // std::nullptr_t
+
+#if !defined(BOOST_NO_CXX11_NULLPTR)
+
+    template <typename T>
+    typename boost::enable_if_<boost::is_same<T, std::nullptr_t>::value, std::size_t>::type
+        hash_value( T const& v )
+    {
+        return boost::hash_value( static_cast<void*>( v ) );
+    }
+
 #endif
 
     // std::optional

test/Jamfile.v2

@@ -115,3 +115,5 @@ run described_class_test.cpp
 
 run hash_is_avalanching_test.cpp ;
 run hash_is_avalanching_test2.cpp ;
+
+run hash_nullptr_test.cpp ;

test/hash_nullptr_test.cpp

@@ -0,0 +1,41 @@
+// 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.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <cstddef>
+
+#if defined(BOOST_NO_CXX11_NULLPTR)
+
+BOOST_PRAGMA_MESSAGE( "Test skipped, BOOST_NO_CXX11_NULLPTR is defined" )
+int main() {}
+
+#else
+
+template<class T> std::size_t hv( T const& x )
+{
+    return boost::hash<T>()( x );
+}
+
+int main()
+{
+    {
+        BOOST_TEST_EQ( hv((void*)0), hv(nullptr) );
+    }
+
+    {
+        int x = 0;
+
+        BOOST_TEST_EQ( hv((int*)0), hv(nullptr) );
+        BOOST_TEST_NE( hv(&x), hv(nullptr) );
+
+        (void)x;
+    }
+
+    return boost::report_errors();
+}
+
+#endif