out_ptr -- alternatives

From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2019-06-28 08:08:06

Hi Everyone,
This is not a review of out_ptr, but I wanted to share my thoughts about
the motivation for this library.
I do not recall any time from my programming experience where I was in need
of passing a smart pointer to a T** interface, so maybe this is why I
cannot appreciate the value of out_ptr.

On the other hand, if I need to use a library with a C API, or other
non-C++ API (this happens when someone wants to naively port a Java library
to C++ and keep the Java interface) the first thing I do is to wrap the API
functions into a RAII class. It takes some time up front, but then whenever
it is used, I do not have to deal with the problems of the old API. `T**`
is only one of the problems.

The motivation focuses on only easily passing smart pointers to `init(T**)`
functions, but it does not address the problem of adapting the error
reporting of `init(T**)`to the error reporting of my program. I would never
want to only solve the former problem and omit the latter. Also, I would
never want to represent a handle to a resource with a smart pointer, as I
do not want to voluntarily inject the null-pointer state that I now have to
take care of everywhere. Instead I would prefer a dedicated type that is
not default-constructible, where one really needs to try hard to get the
special no-resource state (e.g. by moving from and not destroying the
object).

What I am missing from the motivation of out_ptr is the comparison with a
full RAII wrapper solution. You can argue that the scope of out_ptr is just
different: provide interpoperation between `init(T**)`functions and smart
pointers. But I would argue that this latter goal is wrong. In fact, I am
concerned that we might be encouraging a bad programming style: "need to
work with init(T**) APIs? use shared_ptr." instead of promoting RAII
wrappers that directly wrap the resource.

Does anyone else feel the same? Or am I missing something about the problem
domain? Anyway, I think that this problem should be covered in the
documentation.

Regards,
&rzej;