absl::HashState

Description

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_; };