Some out_ptr concerns

From: Glen Fernandes (glen.fernandes_at_[hidden])
Date: 2019-06-28 12:08:14

Thanks to JeanHeyd for going through the Boost review process, and to
Zach for managing the review.

Briefly: I do not have enough reasons to recommend acceptance for
out_ptr just yet --- in its current form.  (I'd like to see it return
for a new review when the feedback from this review shapes the design
so that some of these concerns are resolved).

Second: This excludes "inout_ptr" (for which I had even more concerns)
and just focusing on "out_ptr"  - which is the simpler and more common
use case of the two.

Below comparing out_ptr vs the hand written code that a user would
write to use a /typical/ C out-pointer API with smart pointers, this
is what I'm comparing:

[Typical code user would write]:

    Type* raw;
    Result result = TheCApi(args..., &raw);
    if (!Success(result)) {
        // failure (e.g. return, or branch and handle error)
    }
    Smart<Type, Deleter> smart(raw, etc...);

[What happens with out_ptr now]:

    /* 'smart' already has to exist here before TheCApi is called.
       Either as Smart<Type, Deleter> smart;
       or Smart<Type, Deleter> smart(nullptr, etc...); depending on deleter */
    /* User writes: Result result = TheCApi(args..., out_ptr(smart));
       // or Result result = TheCApi(args..., out_ptr(smart, etc...));
       depending on deleter  */

    Type* raw = nullptr;
    Result result = TheCApi(args..., &raw);
    if (raw != nullptr) {
        smart.reset(raw);   // or smart.reset(raw, etc...); depending on deleter
    }
    if (!Success(result)) {
        // failure (e.g. return, or branch and handle error)
    }

Concern 1. Having to materialize the smart pointer before TheCApi is
called. Ordinarily this might be fine for some smart pointers and
stateful deleters, but it may not be for others.

Concern 2. The original user code initializes the smart pointer based
on Success(result). The out_ptr version would do so based on whether
the raw out pointer was changed to something non-null by TheCApi. This
might be fine for some cases, but it may not be for others. That it
isn't the same as fundamental logic as the user code bothers me,
though.

Concern 3. The equivalent of the hand written user code is less
overhead. Personally I'm of the opinion that maybe this doesn't matter
too, but here the author does say he has users who cannot abide by it,
and that this facility needs support from e.g. std::unique_ptr to be
viable. (And this leads to concern 4 below).

[I'm not a fan of needing unique_ptr implementations to add support
for this to make it comparable to some user's hand written code].

Concern 4. The UB optimization that the author wrote because of
Concern 3 is a real concern to him. The UB optimization has no place
in Boost, and one of which doesn't even work with many C++ library
vendors' unique_ptr. [Fortunately the Boost reviews have already
echoed this sentiment and I believe this is going away].

Concern 5. Supporting other custom smart pointers requires effectively
re-implementing out_ptr_t by specialization. I have come to see this
as not viable. [Already mentioned in other reviews]

Concern 6. C APIs where the deleter (its state, logic, etc.) are
governed by some other result from TheCApi. This is can be worked
around in many cases, but the workaround is clumsy. Or insupportable,
in other cases. i.e. The stateful deleter is always initialized with
'etc...' either before-hand via Smart<Type> smart(nullptr, etc...) or
at call site with: TheCApi(args, out_ptr(etc...));  before analyzing
other results of calling TheCApi.

(The 7th concern I had made notes about pre-review has already been
eliminated, when the unconditionally noexcept destructor is now only
conditionally noexcept).

Other thoughts:  The suggestion by Emil lead to an interesting
alternative idea to this problem domain, but I had no time to fully
explore it. I don't think it would address all of these concerns. It
has some other greater appeal, e.g. a way to call C APIs that
materializes the smart pointers as results (along with C API return
value) of an "Invoke-style" thing. Or even a way to transform a C API
with out-pointers into a C++ callable that returns smart pointers
(along with C API return value) via  a "Bind-style" thing.

Glen