[Boost-bugs] [Boost C++ Libraries] #9948: remove use of const_cast in intrusive containers

Subject: [Boost-bugs] [Boost C++ Libraries] #9948: remove use of const_cast in intrusive containers
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2014-04-24 16:40:08

#9948: remove use of const_cast in intrusive containers
-----------------------------------------+------------------------
 Reporter:  Matei David <matei@…>        |      Owner:  igaztanaga
     Type:  Feature Requests             |     Status:  new
Milestone:  To Be Determined             |  Component:  intrusive
  Version:  Boost 1.55.0                 |   Severity:  Problem
 Keywords:  intrusive custom references  |
-----------------------------------------+------------------------
 In intrusive containers, the existing value traits customization mechanism
 described
 [http://www.boost.org/doc/libs/1_55_0/doc/html/intrusive/value_traits.html

 here] allows for value `pointer` to be a custom class. Moreover, the
 existing pointer traits mechanism allows for the user to:

 - Define and use a custom `pointer_traits<pointer>::reference` class
 instead of raw references.
 - Provide custom conversions between `pointer` and `reference`, and
 between `pointer` and `const_pointer`.

 That being said, in `list.hpp`, `slist.hpp`, `bstree.hpp` the C++ operator
 `const_cast` operator is used to convert a `const_reference` (defined as
 `pointer_traits<const_pointer>::reference`) into a `reference` (defined as
 `pointer_traits<pointer>::reference`). This happens in `[s_]iterator_to()`
 methods. This is unfortunate because `const_cast` cannot be overridden,
 and it cannot work with anything other than real pointers and real
 references. So even though most of the existing code base (through value
 traits & pointer traits mechanisms) can support custom references, that
 possibility is removed by the use of `const_cast`.

 I created a pull request in that sense, where I replaced calls of the
 form:

 `const_cast<reference>(value)` (where `value` is a
 `pointer_traits<const_pointer>::reference`)

 by:

 `*pointer_traits<pointer>::const_cast_from(pointer_traits<const_pointer>::pointer_to(value))`

 This relies on the existence of:

 1. `const_pointer
 pointer_traits<const_pointer>::pointer_to(const_reference)`;
 2. `pointer pointer_traits<pointer>::const_cast_from(const_pointer)`;
 3. `reference pointer::operator * ()`.

 I think it is reasonable to expect all of these to be met. A user of raw
 references and raw pointers will be unaffected, because the default
 definitions of `pointer_traits` work as expected. As explained earlier,
 there is no existing code that uses custom references. That leaves only
 code with custom pointers and raw references to worry about breaking.

 Regarding 1, the `pointer_to()` method is used heavily by the boost
 intrusive code, so it is reasonable to expect any existing specialization
 of `pointer_traits` to provide that.

 Regarding 2, the `const_cast_from()` method, even though clearly
 documented in the pointer traits description, is not otherwise used in
 boost intrusive. So it is possible for existing code to specialize
 `pointer_traits` and not provide `const_cast_from`, which would result in
 broken code with the current modifications. I think the error message
 would be quite informative though: "missing const_cast_from", and easy to
 diagnose and fix. It would also be a step in the right direction: if a
 user really needs custom pointer traits, then they should really be
 providing the custom casts.

 As for 3, the `operator * ()` method, I think it's reasonable to expect a
 custom pointer class would provide that. Standard smart pointer classes
 provide that. In the future, I would also suggest adding this conversion
 explicitly in `pointer_traits`, e.g., require specializations to define
 `pointer_traits<pointer>::reference
 pointer_traits<pointer>::reference_from(pointer)`.

 The pull request is [https://github.com/boostorg/intrusive/pull/4 here].
 As explained, this removes `const_cast` from `list.hpp`, `slist.hpp`, and
 `bstree.hpp`. It is still used in `hashtable.hpp`, whose code looks a bit
 different than the other containers, so I left it alone.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/9948>
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