Re: [boost] Nasty compile time reflection and iteration for POD types using C++14 without macro

Subject: Re: [boost] Nasty compile time reflection and iteration for POD types using C++14 without macro
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2016-03-23 19:17:32

Antony Polukhin <antoshkka <at> gmail.com> writes:

> 
> Hello,
> 
> I've come up with a trick that may be useful in Boost.Hana,
> Boost.Serialization or some other libraries.

Definitely an interesting and novel trick. Although, as you said, it is a 
nasty trick, and such things would be better if supported in the language
(IMO). Until then, this trick might be useful.

> [...]
> * there's no known to me way to make get<>() method constexpr

If we computed the offset at which the n-th member lies inside the struct,
couldn't we use something like

    template <std::size_t I, class T>
    decltype(auto) get(const T& val) noexcept {
        std::size_t offsets[] = {}; // assume this is known
        using U = std::tuple_element_t<I, decltype(detail::as_tuple<T>())>;
        return *static_cast<U const*>( // <-- is this UB?
            static_cast<char const*>(std::addressof(val)) + offsets[I]
        );  
    }

Because if that works, computing the offsets is feasible.
    
    
> * does not detect constness (because of that get<>() always returns const
> reference)
> 
> Is there any interest in this feature?

That might be interesting for Hana, while we wait for a language feature to 
make this easier.

Louis