Asymmetry of user-difined swap and distance

["Maxim Dementiev" <max at e-soft dot ru>]

Hi,

Let's have a look at std::pair<>::swap and std::lower_bound<> implementations.

1. pair::swap in http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/include/bits/stl_pair.h?view=markup

      void
      swap(pair& __p)
      {
        using std::swap;
        swap(first, __p.first);
        swap(second, __p.second);
      }

2. lower_bound in http://gcc.gnu.org/viewcvs/trunk/libstdc%2B%2B-v3/include/bits/stl_algo.h?view=markup

      _DistanceType __len = std::distance(__first, __last);

Now we need to use user-defined versions of swap and distance for corresponding types when we work with STL.

It means that swap for user types could be defined either in std namespace or in the user type namespace (argument-dependent name 
lookup).

On the other hand, distance (advance, etc.) for user types must be defined in std namespace.

How come? Why this asymmetry?

Should we always extend std namespace?

Regards,
Maxim P. Dementiev