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

From: Markus Armbruster
Subject: Re: [PATCH v2 3/8] qnum: QNumValue type for QNum value literals
Date: Tue, 24 Nov 2020 09:49:30 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)

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.

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