STD::__DETAIL::_HASH_CODE_BASE(3cxx)

💖 NAME 💖

okay bestie, gather round, because today we're meeting std::__detail::_Hash_code_base< _Key, _Value, _ExtractKey, _H1, _H2, _Hash, __cache_hash_code > — and yes that is genuinely the whole name, she's a libstdc++ internal, she's from the (3cxx) manual, and she is not here to be simple. ✨

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🦋 SYNOPSIS 🦋

so like… she's the blueprint, and these three are the ones who inherit from her:

• std::__detail::_Hashtable_base< _Key, _Tp, _Pred, allocator< std::pair< const _Key, _Tp > >, _H1, _H2, _Hash, _Traits >
• std::__detail::_Hashtable_base< _Key, _Tp, equal_to< _Key >, allocator< std::pair< const _Key, _Tp > >, _H1, _H2, _Hash, _Traits >
• std::__detail::_Hashtable_base< _Value, hash< _Value >, allocator< _Value >, _Equal, _H1, _H2, _Hash, _Traits >

no cap, that's the whole inheritance tea. 💅

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✨ Detailed Description ✨

first, the fit check — here's her declaration, exactly as it is:

template<typename _Key, typename _Value, typename _ExtractKey, typename _H1,
    typename _H2, typename _Hash, bool __cache_hash_code>
    struct std::__detail::_Hash_code_base< _Key, _Value, _ExtractKey, _H1,
    _H2, _Hash, __cache_hash_code >

Primary class template _Hash_code_base.

so what does she actually do? she wraps up two policy decisions that — and this is the important part, bestie — aren't quite orthogonal. two whole issues, live and in the building:

1. the difference between using a ranged hash function and using the combination of a hash function and a range-hashing function. and here's the gag: when you're in the ranged-hash-function case, we don't even have things like hash codes?? so we just put a dummy type in as a placeholder. iconic workaround honestly.
2. whether or not we cache hash codes. and lowkey — caching hash codes is meaningless if we have a ranged hash function. like, why would you. 🙈

she also keeps the key extraction objects in here, purely for convenience. and then, the plot twist: each specialization derives from one or more of the template parameters to benefit from Ebo. and that matters, fr fr — because this type is inherited in some cases by the _Local_iterator_base type used to implement local_iterator and const_local_iterator. and like with any iterator type, the whole goal is to make it as small as possible. main character energy = minimal byte footprint. 👑

⚠️ and now the disclaimer: Primary template is unused except as a hook for specializations. she's a vibe, not a workhorse. she's the placeholder queen. periodt.

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🎀 Author 🎀

• Generated automatically by Doxygen for libstdc++ from the source code. she did NOT hand-write this herself, she was born this way. 💀✨

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and that's the whole page, bestie 🫶 this lil writeup came to you from the libstdc++ source, straight off that footer — the one that mangles itself into libstdstd::__detail:...che_hash_code >(3cxx) like it got shy halfway through. no date on it, just vibes and a Doxygen autograph. take care of her, she's small on purpose 💜🌸