absl::HashState

Declared in <absl/hash/hash.h>

A type erased version of the hash state concept, for use in user‐defined AbslHashValue implementations that can't use templates (such as PImpl classes, virtual functions, etc.). The type erasure adds overhead so it should be avoided unless necessary.

Note: This wrapper will only erase calls to combine_contiguous(H, const unsigned char*, size_t) RunCombineUnordered(H, CombinerF)

All other calls will be handled internally and will not invoke overloads provided by the wrapped class.

Users of this class should still define a template AbslHashValue function, but can use absl::HashState::Create(&state) to erase the type of the hash state and dispatch to their private hashing logic.

This state can be used like any other hash state. In particular, you can call HashState::combine() and HashState::combine_contiguous() on it.

Example:

class Interface { public: template <typename H> friend H AbslHashValue(H state, const Interface& value) { state = H::combine(std::move(state), std::type_index(typeid(*this))); value.HashValue(absl::HashState::Create(&state)); return state; } private: virtual void HashValue(absl::HashState state) const = 0; };

class Impl : Interface { private: void HashValue(absl::HashState state) const override { absl::HashState::combine(std::move(state), v1_, v2_); } int v1_; std::string v2_; };

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