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This attempts to simplify and clean up our std::hash code. The primary benefit is improved diagnostics for users when they do something wrong involving std::hash or unordered containers. An additional benefit is that for the unstable ABI (--enable-symvers=gnu-versioned-namespace) we can reduce the memory footprint of several std::hash specializations. In the current design, __hash_enum is a base class of the std::hash primary template, but the partial specialization of __hash_enum for non-enum types is disabled. This means that if a user forgets to specialize std::hash for their class type (or forgets to use a custom hash function for unordered containers) they get error messages about std::__hash_enum not being constructible. This is confusing when there is no enum type involved: why should users care about __hash_enum not being constructible if they're not trying to hash enums? This change makes the std::hash primary template only derive from __hash_enum when the template argument type is an enum. Otherwise, it derives directly from a new class template, __hash_not_enabled. This new class template defines the deleted members that cause a given std::hash specialization to be a disabled specialization (as per P0513R0). Now when users try to use a disabled specialization, they get more descriptive errors that mention __hash_not_enabled instead of __hash_enum. Additionally, adjust __hash_base to remove the deprecated result_type and argument_type typedefs for C++20 and later. In the current code we use a __poison_hash base class in the std::hash specializations for std::unique_ptr, std::optional, and std::variant. The primary template of __poison_hash has deleted special members, which is used to conditionally disable the derived std::hash specialization. This can also result in confusing diagnostics, because seeing "poison" in an enabled specialization is misleading. Only some uses of __poison_hash actually "poison" anything, i.e. cause a specialization to be disabled. In other cases it's just an empty base class that does nothing. This change removes __poison_hash and changes the std::hash specializations that were using it to conditionally derive from __hash_not_enabled instead. When the std::hash specialization is enabled, there is no more __poison_hash base class. However, to preserve the ABI properties of those std::hash specializations, we need to replace __poison_hash with some other empty base class. This is needed because in the current code std::hash<std::variant<int, const int>> has two __poison_hash<int> base classes, which must have unique addresses, so sizeof(std::hash<std::variant<int, const int>>) == 2. To preserve this unfortunate property, a new __hash_empty_base class is used as a base class to re-introduce du0plicate base classes that increase the class size. For the unstable ABI we don't use __hash_empty_base so the std::hash<std::variant<T...>> specializations are always size 1, and the class hierarchy is much simpler so will compile faster. Additionally, remove the result_type and argument_type typedefs from all disabled specializations of std::hash for std::unique_ptr, std::optional, and std::variant. Those typedefs are useless for disabled specializations, and although the standard doesn't say they must *not* be present for disabled specializations, it certainly only requires them for enabled specializations. Finally, for C++20 the typedefs are also removed from enabled specializations of std::hash for std::unique_ptr, std::optional, and std::variant. libstdc++-v3/ChangeLog: * doc/xml/manual/evolution.xml: Document removal of nested types from std::hash specializations. * doc/html/manual/api.html: Regenerate. * include/bits/functional_hash.h (__hash_base): Remove deprecated nested types for C++20. (__hash_empty_base): Define new class template. (__is_hash_enabled_for): Define new variable template. (__poison_hash): Remove. (__hash_not_enabled): Define new class template. (__hash_enum): Remove partial specialization for non-enums. (hash): Derive from __hash_not_enabled for non-enums, instead of __hash_enum. * include/bits/unique_ptr.h (__uniq_ptr_hash): Derive from __hash_base. Conditionally derive from __hash_empty_base. (__uniq_ptr_hash<>): Remove disabled specialization. (hash): Do not derive from __hash_base unconditionally. Conditionally derive from either __uniq_ptr_hash or __hash_not_enabled. * include/std/optional (__optional_hash_call_base): Remove. (__optional_hash): Define new class template. (hash): Derive from either (hash): Conditionally derive from either __optional_hash or __hash_not_enabled. Remove nested typedefs. * include/std/variant (_Base_dedup): Replace __poison_hash with __hash_empty_base. (__variant_hash_call_base_impl): Remove. (__variant_hash): Define new class template. (hash): Conditionally derive from either __variant_hash or __hash_not_enabled. Remove nested typedefs. * testsuite/20_util/optional/hash.cc: Check whether nested types are present. * testsuite/20_util/variant/hash.cc: Likewise. * testsuite/20_util/optional/hash_abi.cc: New test. * testsuite/20_util/unique_ptr/hash/abi.cc: New test. * testsuite/20_util/unique_ptr/hash/types.cc: New test. * testsuite/20_util/variant/hash_abi.cc: New test. |
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file: libstdc++-v3/README New users may wish to point their web browsers to the file index.html in the 'doc/html' subdirectory. It contains brief building instructions and notes on how to configure the library in interesting ways.