hashable.cpp

// https://coliru.stacked-crooked.com/a/0ba6c5762255a8e3
#include <iostream>
#include <utility>
#include <cstdint>

class XorShifter
{

public:

    XorShifter(uint64_t seed)
    {
        _state[0] = seed;    
    }

    uint64_t operator()()
    {
        uint64_t& s0 = _state[0];
        uint64_t& s1 = _state[1];
        
        s0 ^= s0 >> 17;
	    s0 ^= s0 << 23;
	    s0 ^= s0 >> 29;
	    
	    _state[1] += 0x61C8864680B583EB;
	    
	    return s0 + s1;
    }

private:

    uint64_t _state[2];
};

template <class Type>
union Hashable
{

public:

    template<class... Args>
    Hashable(Args&&... args) :
        hash{_xorShifter()}
    {
        new (static_cast<void*>(&_value)) Type(std::forward<Args>(args)...);
    }
    
          Type&  value()       &  { return           _value  ; }
          Type&& value()       && { return std::move(_value) ; }
    const Type&  value() const &  { return           _value  ; }
    
    const uint64_t hash;

private:

    Type           _value;
    static XorShifter _xorShifter;
};

template <class Type>
XorShifter Hashable<Type>::_xorShifter{typeid(Type).hash_code()};

int main()
{
    Hashable<int> hashableInt = 42;
    
    std::cout << hashableInt.hash << " " << hashableInt.value() << std::endl;
    
    return 0;    
}