Re: [PATCH v2 3/8] qnum: QNumValue type for QNum value literals

[Markus Armbruster]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Tue, Nov 24, 2020 at 09:49:30AM +0100, Markus Armbruster wrote:
>> Eduardo Habkost <ehabkost@redhat.com> writes:
>> 
>> > On Mon, Nov 23, 2020 at 08:51:27AM +0100, Markus Armbruster wrote:
>> >> Eduardo Habkost <ehabkost@redhat.com> writes:
>> >> 
>> >> > On Fri, Nov 20, 2020 at 06:29:16AM +0100, Markus Armbruster wrote:
>> >> [...]
>> >> >> When the structure of a data type is to be kept away from its users, I
>> >> >> prefer to keep it out of the public header, so the compiler enforces 
>> >> >> the
>> >> >> encapsulation.
>> >> >
>> >> > I prefer that too, except that it is impossible when users of the
>> >> > API need the compiler to know the struct size.
>> >> 
>> >> There are cases where the structure of a data type should be
>> >> encapsulated, yet its size must be made known for performance (avoid
>> >> dynamic memory allocation and pointer chasing).
>> >> 
>> >> Need for encapsulation correlates with complex algorithms and data
>> >> structures.  The cost of dynamic allocation is often in the noise then.
>> >
>> > I don't know what we are talking about anymore.  None of this
>> > applies to the QNum API, right?
>> >
>> > QNum/QNumValue are not complex data structures, and the reason we
>> > need the compiler to know the size of QNumValue is not related to
>> > performance at all.
>> 
>> We started with the question whether to make QNumValue's members
>> private.  We digressed to the question when to make members private.
>> So back to the original question.
>> 
>> > We might still want to discourage users of the QNum API from
>> > accessing QNum.u/QNumValue.u directly.  Documenting the field as
>> > private is a very easy way to do it.
>> 
>> It's a complete non-issue.  QNum has been around for years, and we
>> haven't had any issues that could've been plausibly avoided by asking
>> people to refrain from accessing its members.
>> 
>> If there was an actual need to keep the members private, I'd move the
>> struct out of the header, so the compiler enforces privacy.
>
> Understood.  There's still a question I'd like to answer, to
> decide how the API documentation should look like:
>
>   Is QNum.u/QNumValue.u required to be part of the API
>   documentation?
>
> If accessing that field directly is not necessary for using the
> API, I don't think it should appear in the documentation (because
> it would be just noise).

The current patch's comment on QNumValue looks good to me.

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