View manipulator and is_reflectable trait

[denzor200]

#100

[Kojoley]

I don't want to devalue your work, after all thanks for at least trying to work on this. Probably you misunderstood me, it seems that is_reflectable is just a placeholder? It actually does nothing by default? Maybe I forgot to show you that there is already in the library an almost working trait

pfr/include/boost/pfr/detail/fields_count.hpp

Lines 77 to 92 in b7d839b

	template <class T, std::size_t N>
	struct is_aggregate_initializable_n {
	template <std::size_t ...I>
	static constexpr bool is_not_constructible_n(std::index_sequence<I...>) noexcept {
	return (!std::is_constructible<T, decltype(ubiq_lref_constructor{I})...>::value && !std::is_constructible<T, decltype(ubiq_rref_constructor{I})...>::value)
	|| is_single_field_and_aggregate_initializable<N, T>::value
	;
	}
	static constexpr bool value =
	std::is_empty<T>::value
	|| std::is_array<T>::value
	|| std::is_fundamental<T>::value
	|| is_not_constructible_n(detail::make_index_sequence<N>{})
	;
	};

your best bet is to fix some of it's blind spots and it solves the exact problem. Or you have tried to fix it and came to conclusion that it's not possible?

Also, a general rule: the less code is for a review the faster your PR is getting reviewed and more chances for it to be merged in a timely manner.

[apolukhin]

A few nitpicks

[apolukhin]

Change the table id to something unique, for example to table:libs_integration_comp

[apolukhin]

Some day this code snippet should be moved into cpp file and compiled like pfr_sample_adapting

[apolukhin]

Suggested change

	Use when you need to access values by library's provided for them reflection or via field-by-field reflection.
	Use when you need to access values by other library provided reflection or via field-by-field reflection from Boost.PFR.

?

[denzor200]

Probably you misunderstood me, it seems that is_reflectable is just a placeholder? It actually does nothing by default?

I do understand you about your issue automatic is_reflectable, this PR won't make to resolve issue impossible, issue will be resolved in the future version of PFR.

[denzor200]

Not actual. Actual PR here: #111