Re: [boost] [STL + allocators] Bug?

Subject: Re: [boost] [STL + allocators] Bug?
From: Phil Bouchard (phil_at_[hidden])
Date: 2016-02-18 22:29:49

On 2016-02-18 7:09 PM, Phil Bouchard wrote:
> On 02/18/2016 06:30 AM, Phil Endecott wrote:
>> Phil Bouchard wrote:
>>> I think [std::list etc] should make use of the allocator's specific
>>> pointer
>>
>> In C++03, containers were allowed to assume that allocator::pointer
>> == value_type*.  This is not true since C++11, where it should use
>> allocator::pointer via allocator_traits.
>>
>> A subtlety is that in the code you posted you're dealing with node
>> pointers, not value_type pointers.  I'm not sure what's going on there.
>> My first thought was that you were looking at a pre-11 version of
>> libstdc++, but I see a #if that suggests otherwise.  Maybe others
>> can comment.
>>
>> You might like to look at how Boost.Container works.  It should use
>> allocator::pointer even on C++03.
>>
>> Note that trying to use a smart pointer for allocator::pointer is
>> unlikely to work in general, e.g. you can't use shared_ptr, because
>> the semantics are not what the container expects.  The most common
>> use for redefining allocator::pointer that I've seen is to store
>> offset in memory-mapped files and similar.
>
> Thanks for your input...  I am trying out boost::container::... right
> now and it is not a trivial task to use a smart pointers inside
> containers but I think it is possible.
>
> I saw functions like to_raw_pointer() were user before calling the
> allocator's constructors & destructors... I believe this should be
> cleaned up sooner or later but for now I think I can survive by
> overloading only the allocate & deallocate functions.

I managed to make it compile with MSVC 2015 with a smart pointer inside 
containers:

template <typename T, typename UserPool = system_pool<system_pool_tag, 
sizeof(char)> >
     class block_allocator
     {
         typedef T                               element_type;

     public:
         typedef element_type                    value_type;
         typedef size_t                          size_type;
         typedef ptrdiff_t                       difference_type;
         typedef block_ptr<element_type>         pointer;
         typedef block_ptr<const element_type>   const_pointer;
         typedef element_type &                  reference;
         typedef const element_type &            const_reference;
[...]

My example block_ptr_test2.cpp doesn't work yet but at least I can go 
from here.  That's the real stress test I was looking for.