Re: [boost] Checking interest in std::get for PODs

Subject: Re: [boost] Checking interest in std::get for PODs
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2016-06-03 15:50:17

Le 03/06/2016 à 08:27, Antony Polukhin a écrit :
> Hi,
>
> There's a C++14 library that is able to do basic reflections for PODs
> without any macro magic or markup: https://github.com/apolukhin/magic_get
Hi Anthony, this is really magic ;-)

Using

     T tmp{ ubiq_val{types + get<I>(offsets)}... };

as a mechanism to detect that T is constructible from N *ubiquitous* 
values convertible to its parts and store at the same time the size is 
simply brilliant.
I suspect that the compiler is suffering a lot, as it must try up to the 
size of the types isn't it?

I understand that it would be quite hard for the user to associate a 
unique number to a type, but it is just for curiosity, could the type 
registration be extended by the user so that we can avoid flattening?

struct make_my_life_harder { int a0; short a1; };
struct make_my_life_even_more_harder { unsigned int b0; unsigned short 
b1; make_my_life_harder cr;};
namespace detail {
namespace typeid_conversions {
     BOOST_MAGIC_GET_REGISTER_TYPE(make_my_life_harder , 24)
}
}

static_assert(flat_tuple_size<make_my_life_harder>::value==2,"");
static_assert(flat_tuple_size<make_my_life_even_more_harder>::value==3,"");

I have tried and it doesn't work :(
http://melpon.org/wandbox/permlink/i0r0Q5xPaiwYPQMo

I suspect that this is related to the way the constructions with k 
parameters are tried. Could you tell us more?
>
> Would it be interesting for Boost?
I think so.
> Could it pass Boost review if it has a
> reinterpret_cast in it?
>

Is this related to UB? I believe that the documentation should then 
mention the trick so that the user is aware of it.

It is worth signaling that this is not an implementation of std::get for 
PODs as the fields are flattened.

I like the pod_ops namespace with comparison and streaming. I believe 
that it will be worth adding them on the global namespace subject to 
some specific trait xxx::pod_equality_comparable, 
xxx::pod_less_than_comparable, xxx::pod_streamable. The user would need 
just to specialize these traits to enable the operators.

Can a Boost library add functions in the global namespace subject to 
SFINAE on some specific condition?

What about adding also a hash function?

BTW, what will be the name of the library?

Best,

Vicente