Re: [Boost-users] [MultiIndex] Visual C++ 10 issue

Subject: Re: [Boost-users] [MultiIndex] Visual C++ 10 issue
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2010-05-11 09:23:34

On Tue, 11 May 2010, joaquin_at_[hidden] wrote:

> Jeremiah Willcock escribió:
>> On Mon, 10 May 2010, Richard Webb wrote:
>>
>> 
>>> Sounds like the same problem as 
>>> https://svn.boost.org/trac/boost/ticket/3594
>>> 
>> 
>> It seems like it is.  Is the workaround that complicated, though?  Is there 
>> a way to get some of that factored out to reuse in other Boost libraries? 
>> Would putting an explicit cast to ... std::pair::* in the template argument 
>> work instead?
>> 
>
> Hi Jeremiah,
>
> The problem is not a bug with VC++ 10, but a collateral effect of the 
> way std::pair is implemented in the stdlib provided for this compiler 
> --namely, by deriving from an implementation-specific _Pair_base class 
> where the members are actually defined. This reduced test case can help 
> explain the problem:

Could this be viewed as a library issue, then, or is the type of 
&std::pair<a, b>::first not considered to be part of the library's 
interface?  It does seem bad that any use of pointers-to-members is 
fragile to the use of mixins or hidden base classes as part of a class's 
implementation.

(snip)

> The workaround applied at https://svn.boost.org/trac/boost/ticket/3594 
> works, but it's probably overkill (and uses the objectionable 
> member_offset, which is really meant to be used as a last resort for 
> legacy compilers). A nicer workaround consists in providing a 
> user-defined key extractor:
>
> template<typename First,typename Second>
> class pair_first_extractor
> {
>   typedef std::pair<First,Second> value_type;
>
> public:
>   typedef First result_type;
>
>   const result_type& operator()(const value_type& x)const
>   {
>     return x.first;
>   }
>
>   result_type& operator()(value_type& x)const
>   {
>     return x.first;
>   }
> };
>
> typedef multi_index::multi_index_container<
>  std::pair<key_type, value_type>,
>  multi_index::indexed_by<
>    multi_index::sequenced<>,
>    multi_index::hashed_unique<
>      pair_first_extractor<key_type, value_type>
>    >
>  >
> > ghost_cells_type;

OK.  Should that code (for std::pairs) be part of multi_index anywhere? 
It would seem like other users would use parts of pairs as keys. 
Otherwise, can I copy your code into BGL (under the Boost license)?  Also, 
would just a simple cast on the pointer-to-member type work, or are those 
not allowed in non-type template arguments?

-- Jeremiah Willcock