[boost] [compressed_pair] using compressed_pair to substitute a possibly empty member

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2008-01-05 08:11:28

Hi,

I'm using compressed_pair to substitute a possibly empty member whose 
type is specified by a template argument and experience difficulties:

     template< typename T >
     class A
     {
         T x;
       public:

         // [ctor, ...]

         void foo() const
         {
             // if T is a reference type, non-const access to the
             // object referenced by x is possible, here
         }

     };

     // but:

     template< typename T >
     class changed_A
       : boost::compressed_pair<T,boost::blank>
     {
       public:

         // [ctor, ...]

         void foo() const
         {
             // first() always returns a reference to const
             // even if T is a reference to non-const
         }

     };

So: Did I find a subtle design flaw or bug in compressed_pair or is it 
just the use case I'm throwing at it?

In case of the latter we should probably introduce a compressed_member 
(or so) and have it implement const semantics consistent with members.

Regards,
Tobias