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

11 May 2010


      On Tue, 11 May 2010, joaquin@tid.es 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::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
class pair_first_extractor
{
  typedef std::pair 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,
 multi_index::indexed_by<
   multi_index::sequenced<>,
   multi_index::hashed_unique<
     pair_first_extractor
...
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