Re: Interest in a library for type-safe bitwise operations?

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2024-03-26 17:50:26

On 3/26/24 18:53, Tobias Loew via Boost wrote:
> Am 2024-03-25 11:38, schrieb Andrey Semashev via Boost:
>>
>> I don't really like the idea of operators in the global namespace. I
>> would rather prefer if the operators were defined in the library's
>> namespace, and the enablement was always through the macro. Where the
>> macro would not just import the operators for lookup but also "enable"
>> them for the given enum.
> 
> The library now has a macro-option to prevent importing all operators
> into the global
> namespace (BOOST_FLAGS_NO_GLOBAL_USING).

I think the no-global-namespace should be the only option, and there
should be no such macro. Otherwise, there is risk of configuration
mismatch and ODR violations.

Enums are often used in library interfaces, and offering this
configuration macro may pose a problem. For example, if a library A
wants to define its public enums with this macro defined, and its user B
wants to define its own enums without this macro then there will be a
compatibility issue.

> Additionally, there are macros to easily import all operators/utility
> functions
> into a namespace (BOOST_FLAGS_USING_OPERATORS(),
> BOOST_FLAGS_USING_UTILITIES()).
> 
> I'm not sure what you mean by your last sentence. Do you just ask for a
> macro, like
> 
> #define BOOST_FLAGS_ENABLE(Enum) \
> constexpr inline bool boost_flags_enable(Enum) { return true; } \
> BOOST_FLAGS_USING_OPERATORS()
> 
> or something else?

Yes, or something equivalent. I'm not sure this can be made working in
all contexts, e.g. for enums declared in user's namespaces as well as in
classes and templates, so there may be multiple such macros for
different use cases or there may be optional arguments or something. But
the effect should be the same - you apply one macro to the enum, and
that makes bitwise operators work for that enum.

I'm not sure what would be the purpose of the other macros you mention
(BOOST_FLAGS_USING_OPERATORS, BOOST_FLAGS_USING_UTILITIES). I mean, is
there a use case for importing the operators but not enabling them? And
what are the utilities and why one would want a macro to import them?