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Subject: Re: [boost] Review Request: impl_ptr (pimpl)
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-08-22 08:09:23

2017-08-22 2:34 GMT+02:00 Vladimir Batov via Boost <boost_at_[hidden]>:

> On 08/21/2017 10:38 PM, Andrzej Krzemienski via Boost wrote:
>> 2017-08-15 2:51 GMT+02:00 Vladimir Batov via Boost <boost_at_[hidden]
>> >:
>>> After much procrastination I would like to request a formal review for
>>> the pimpl library...
>> I find it disturbing that the first example describes using shared_ptr-s
>> and defensive null checks. Boost has always been promoting the STL-like
>> value-semantic style of programming, and this usage of shared_ptr's looks
>> counter to the trend. When I am defining type Book -- unless I have to hide
>> implementation, I will just put the members directly in order to achieve
>> value semantics, in particular, I want to guarantee the following property:
>> Book b1 {"title", "author"}; Book b2 = b1; Book b3 = b1; modify(b1); assert
>> (b2 == b3); assert (b2 != b1); The shared_ptr does no seem to guarantee the
>> last assertion. Also, there are these additional members testing if pimpl
>> is null, which appear suspicious to me. How can it be null? Do I have to
>> implement the default constructor now? Does this also mean than in my types
>> that I want to pimpl-ize I cannot use operator bool for my own
>> business-logic-related purpose, because it is already taken by the
>> implementation of boost::pimpl? There may be good use cases for
>> flyweight-like implementation, but it should not necessarily be the first
>> example we see in the docs.
> Andrzej,
> To be honest I am somewhat surprised... in part by the fact that you find
> the first example no less than "disturbing". I do hear your view and I find
> it is a perfectly valid view. Still, I am sure you've been around the bush
> for long enough to know that there are as many views/opinions/preferences
> as there are people. Then different projects have different requirements.
> So, there should not be anything "disturbing" if you see someone doing
> something his way rather than yours to satisfy the requirements of their
> project rather than yours. Don't you agree?
> Then you go on to explain that the shared-properties pimpl does not suit
> your requirements and to describe your potential confusion (and even
> frustration) with such pimpls... Where all that is coming from? No one is
> forcing you to use the shared-properties pimpl, right? Exclusive-ownership
> pimpls are equal-rights' citizens in the implementation. They are equally
> presented in the documentation. What is there to be "disturbed" by? Indeed,
> in the documentation I do mention/describe the shared-properties pimpl
> first... because it is easier to introduce the Pimpl concept using
> std::shared_ptr. Different people have different skill/knowledge levels,
> different expectations, different priorities, different reading styles.
> There is no way to satisfy them all. Please do not be "disturbed" if you do
> not see things done exactly you'd do that.

Part of my complaint is indeed coming from my personal judgement of
different implementation strategies. The other part is me giving you a
feedback from the "user experience" of a potential user of your library. I
am an impatient programmer who is facing alternative: implement pimpl
manually or use a third-party library, there might be like 100 libraries
for this purpose with different quality, and I have very little time to
asses if it is better to use one of them, or to write my own. Having
encountered a library I need to be able to answer a couple of questions
quickly, like, is this library worth investing time in even reading the
full documentation. So, one of the first questions I would like being
answered is. Is the author of this library aware of the present programming
styles, like value semantics, zombie-object avoidance, or is (s)he confined
to these older OO-like techniques, where everything is pointers and
garbage-collected (or shared_ptr-collected) memory with null pointers
causing bugs?

I cannot figure this out from the docs of Boost.Pimpl quickly. Ultimately,
I can figue it out from studying the entire docs, or the implementations,
or private conversation with you. But: I cannot figure it out *quickly*
from the *initial pages* of the docs. First page: no information -- I know
what pimpl is already. Second page: "the basics" -- I do not want the
basics. I want answers, preferably in form of a short example. There is a
word "unique-ownership" highlighted, so this is something, but still some
questions unanswered: will I have null pointers/null states all around?

Third page: I get an example of something I would not like to get. I give
you that it your order of presenting policies is no worse than my expected
order of policies, but I can see these two functions in the interface for
testing for the null state. Note that separating interface from
implementation has nothing to do with offering an additional null state.
The two things are orthogonal; it is just that it is easy to provide a null
state when you are implementing a handle anyway -- but they are different
things, and they distract attention. It is not clear from the first glimpse
at the example whether I am forced to have the null state or whether I can
opt-in to have it.

By that time I will have exhausted the time I was willing to devote to
studying the library usefulness, and would move to the next library or to
implementing my own pimpl.

Note that my remarks are to the structure of the initial pages of the docs:
not to your design choices.

> > Also, there
> > are these additional members testing if pimpl is null, which appear
> > suspicious to me. How can it be null?
> std::optional has relational operators and can be "null", invalid.
> std::shared_ptr() has relational operators and can be "null", invalid.
> unique_ptr has relational operators and can be "null", invalid. The class I
> proposed -- impl_ptr -- has relational operators and can be "null",
> invalid. What is so suspicious about it? The name itself -- impl_ptr --
> highlights the fact that it is a smart pointer, a handle/body construct.
> So, the handle can be associated with a body... or the handle might not
> have a body, i.e. "invalid", "null".

Of optional, I expect a null state. This is what optional is for and this
is reflected in its type. Of shared_ptr I do not necessarily expect a null
state, and I treat it as necessary evil, and long for types with shared
semantics but without null state which is causing many bugs in my programs
I work with. Of "pimpl" library I expect separation of interface and
implementation: not a null state. If you provide it, I would expect a clear
indication that it is an opt-in feature.

> > Do I have to implement the default constructor now?
> No, you do not. Your frustration seems palpable. I fail to understand how
> possibly I could cause that.

It is not you causing it. Maybe I am just a nerd. I have bad experience
with having to spend lot of time in my company fixing bugs after people who
overused OO and shared_ptrs. I see now the docs of Boost.Pimpl
unnecessarily promoting these. Doing flyweighs requires taking care of oher
things like using shared_ptr<const T> rather than shared_ptr<mutable T>,
which the docs do not cover.

> > Does this also mean than in my types that I want to pimpl-ize
> > I cannot use operator bool for my own business-logic-related purpose,
> > because it is already taken by the implementation of boost::pimpl?
> No, it does not mean anything like you describe... and it not "taken". It
> is provided. If you want your "own business-logic", you implement it and
> you use it. There is nothing stopping you from doing that, right? Unless I
> missed, mis-understood something.

Myabe you could reflect that in the initial examples by not providing
`operator bool` and `operator!`? BTW, you probably do not need `operator!`
anyway. The negation works out of the box when you only define explicit
conversion to bool.


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