hash state

group turbo_hash_state

class HashState : public hash_internal::HashStateBase<HashState>

#include <hash.h>

HashState is a type erased version of the hash state concept, for use in user-defined hash_value 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) run_combine_unordered(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 hash_value function, but can use turbo::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 hash_value(H state, const Interface& value) {
      state = H::combine(std::move(state), std::type_index(typeid(*this)));
      value.HashValue(turbo::HashState::Create(&state));
      return state;
}
private:
virtual void HashValue(turbo::HashState state) const = 0;
};

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