Asymmetry of user-difined swap and distance

[Maxim Dementiev]

On Fri, 7 Aug 2009 19:19:03 +0100
  Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> 2009/8/7 Maxim Dementiev:
>>
>> It means that swap for user types could be defined either in std namespace
>> or in the user type namespace (argument-dependent name lookup).
> 
> Yes, this is intentional. swap() is a point of customisation point,
> see http://www.ddj.com/cpp/184401876 and the definition in
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1691.html

Ok. Thanks!

>> On the other hand, distance (advance, etc.) for user types must be defined
>> in std namespace.
> 
> There is no need to customise std::distance(), or certainly a lot less
> need, because the Standard Library can provide optimal implementations
> for all possible iterator categories.  The generic swap() function in

Dear Jonathan, what about structures like B-Trees where the speed is equal to random iterators on one node but is lesser when you 
iterates though different nodes?
So, for me the question isn't "Do I really need it?"
But "Shall I extend the std namespace?"

Yours faithfully,
Max