Re: [boost] Is there any interest in type-safe container of bool flags with noexcept guarantees?

Subject: Re: [boost] Is there any interest in type-safe container of bool flags with noexcept guarantees?
From: Roman Orlov (compmaniak_at_[hidden])
Date: 2017-03-22 21:20:37

On 22.03.2017 20:14, Richard Hodges via Boost wrote:
> I agree that bitfields are not the way to go. They suffer from a number of
> deficiencies - not least that each bit is *not a separate object* so they
> are problematic i multithreaded environments. They also of course carry no
> type information.
>
> However, I am struggling to see how the proposed class is any more useful
> than a std::tuple classes that support a bool conversion operator.
>
> Can you demonstrate a use case where a tuple is inadequate?
>
std::tuple and proposed class typed_flags are completely different.
Via std::tuple you can emulate typed bit storage like this

   class p1; //
   class p2; // incomplete types
   class p3; //

   template<typename T>
   struct bit {
     unsigned char value;
   };

   std::tuple<bit<p1>, bit<p2>, bit<p3>> b1;
   assert( sizeof(b1) == 3 );

std::tuple can be parameterized only with *complete* types because it
stores instances of specified types. Thus in this example the
size of b1 is 3 bytes. We can't write std::tuple in this way

   std::tuple<p1, p2, p3> b2;

typed_flags can be parameterized with *incomplete* types because types
are used as tags for identifying positions of corresponding bits.

   typed_flags<p1, p2, p3> b3;
   assert( sizeof(b3) == 1 );

Actual bit values are stored in std::array<uint8_t, N> where N is
calculated depending on the number of template parameters.
So typed_flags is a templated frontend to the raw bit storage.
When we test a certain flag (or bit)

   b3.test<p2>();

the type p2 is resolved to the item in the backing array and offset
inside the item. typed_flags API is similar to std::bitset.

I can't post verbose examples here.
For better understanding, please, take a look at my repo
https://github.com/compmaniak/typed_flags