Re: [Interest] Flags: Non-intrusive, type-safe bitwise operators for enums

Am 17.03.26 um 05:25 schrieb Rainer Deyke via Boost:
> I've considered an alternate solution to the same problem based on a 
> wrapper around std::bitset.
>
>   template<typename Enum> class enum_bitset {
>   public:
>     auto operator[](Enum idx) {
>       return this->data_store[static_cast<std::size_t>(idx)];
>     }
>     // ...and all of the other member functions...
>   private:
>     std::bitset<...> data_store;
>   };
>
> This has the advantage of getting rid of all of the ugly 
> boost::flags::nth_bits and boost::flags::anys in the example code:
I was also thinking about that: Having to initialize an enum with 
powers-of-2 is a pitfall, although very common for flags. Maybe you 
already have flags.

Imagine writing a C++ wrapper around the Windows flags posted in the 
example. There being able to reuse the actual values can be very useful:
enum class BS {
  Notify = BS_NOTIFY,
  Foo = BS_FOO,
  Bar = BS_Bar
};

But for greenfield implementations I like not having the boilerplate of 
having to initialize enumerators.
On the other hand it makes them stick out as flags.

FWIW: Both approaches seem to get optimized to literally the same code. 
Random example: https://godbolt.org/z/Yjea7qzfv

What I am missing though are predefined bitsets:
With the pizza example you can have `Topping::Cheese` in an enum class 
but no good way to have `Topping::All` or `Topping::Vegetarian`, e.g. 
for `bool isVegan(... toppings) { return toppings & ~Topping::NonVegan; }
Of course you can define such as (constexpr) constants, but not anymore 
inside the bitset class, i.e. Topping, but only on the namespace as 
"AllToppings" or subclassing.
I have also used bits and bitsets in switch-case constructs, bit 
std::bitset::to_(u)ulong is only constexpr in C++23

So I had use for both variants.
Maybe they can be consolidated: template<typename Enum, bool 
isDense=true> class enum_bitset

I see use for that in Boost as it seems to be common enough to avoid 
having to reimplement it

Best,
Alex

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