[Boost-users] [MultiIndex] strange problem with simple use of member key extractor

Subject: [Boost-users] [MultiIndex] strange problem with simple use of member key extractor
From: John M. Dlugosz (mpbecey7gu_at_[hidden])
Date: 2013-01-17 19:28:15

namespace bmi = boost::multi_index;

....
        bmi::ordered_non_unique<
           bmi::member<
              ItemBase,
              unsigned,
              &ItemBase::my_member
              >
           >, // ordered_non_unique
....

I'm getting a compiler error on VS10 from

template<class Class,typename Type,Type Class::*PtrToMember>
struct non_const_member_base
{
....
   Type&
   operator()(const ChainedPtr& x)const
   {
     return operator()(*x);
   }

boost/multi_index/member.hpp(105): error C2440: 'return' : cannot convert from 'const 
unsigned int' to 'unsigned int &'

It also tells me that the ChainedPtr is a shared_ptr to a const ItemBase,
and I can see that _none_ of the operator()'s in that class return a non-reference.  But 
if the compiler is already thinking ref-becomes-lvalue and leaves that out, the next form

const Type& operator()(const Class& x,int=0)const
   {
     return x.*PtrToMember;
   }

is a perfect match to *x and so I expect the return value to be in fact const (because the 
object being dereferencd was const)

Any idea what's going on here, and what I might do about it?

Our project is using Boost 1.49, though I didn't see anything in the release notes about 
changes to MultiIndex in later releases.

—John