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Add a comment explaining why to_underlying isn't used in hash_value for enums

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Peter Dimov
2022-07-03 21:08:14 +03:00
parent a3cac265b1
commit c14d3a1e2b
1 changed files with 14 additions and 0 deletions
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14
include/boost/container_hash/hash.hpp
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@@ -163,6 +163,20 @@ namespace boost
typename boost::enable_if_<boost::is_enum<T>::value, std::size_t>::type
hash_value( T v )
{
// This should in principle return the equivalent of
//
// boost::hash_value( to_underlying(v) );
//
// However, the C++03 implementation of underlying_type,
//
// conditional<is_signed<T>, make_signed<T>, make_unsigned<T>>::type::type
//
// generates a legitimate -Wconversion warning in is_signed,
// because -1 is not a valid enum value when all the enumerators
// are nonnegative.
//
// So the legacy implementation will have to do for now.
return static_cast<std::size_t>( v );
}