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

Le mardi 17 mars 2026 à 05:25 +0100, Rainer Deyke via Boost a écrit :
> On 3/16/26 11:59, Tobias Loew via Boost wrote:
> 
> 
> While better support for bit flags based on scoped enums is
> undoubtedly 
> useful, I don't like how verbose the example code is.
> 
> 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:
> 
>    enum class pizza_topping { tomato, cheese, salami, olives, garlic
> };
> 
>    void order_pizza(enum_bitset<pizza_topping> const &toppings) {
>      std::cout << "Pizza ordered with\n";
>      if (toppings[pizza_toppings::tomato]) {
>          std::cout << "- tomato\n";
>      }
>      if (toppings[pizza_toppings::cheese]) {
>          std::cout << "- cheese\n";
>      }
>      if (toppings[pizza_toppings::salami]) {
>          std::cout << "- salami\n";
>      }
>      if (toppings[pizza_toppings::olives]) {
>          std::cout << "- olives\n";
>      }
>      if (toppings[pizza_toppings::garlic]) {
>          std::cout << "- garlic\n";
>      }
>      std::cout << "\n";
>    }
> 
> Isn't that so much more readable and compact?
> 

Much more. This is one of the reason i included bitset support in my
indexed_array library
( https://github.com/Julien-Blanc-tgcm/indexed_array ), the bitset
doing exactly that : access individual bits via an enum value.

I've come to the conclusion that any representation of enum flags with
C++ enums is a design mistake by itself (a pretty widespread one, but
still a mistake). Enums are individual values, enum flags are a set of
values, they are different by nature. In C++, different things deserve
a different type, call it an enum_bitset, an enum_set or whatever, but
donc make it a plain enum / enum class.

Regards,

Julien
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