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From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-06-25 05:13:03

> Howard Hinnant wrote:
> swap1 time is 0.172796 seconds
> swap2 time is 1.3e-05 seconds

David Abrahams wrote:
> Your program gives about the results I'd have expected for the case
> you tested, but it doesn't address what I was concerned about.

Howard's example shows that a swap overload for std::tr1::array
effectively reduces the complexity of swapping a
tr1::array<vector<int>,N> from O(M*N) to O(N), when the array has N
vectors, each having size M. And it provides no-throw. Aren't those
the issues you were concerned about?

Any type that is both CopyConstructible and Assignable is Swappable, by
definition. Within generic code, people typically use the well-known
idiom to swap a pair of Swappable objects:

  // Allow argument-dependent lookup to find a custom swap.
  using std::swap;
  swap(lhs, rhs);

As long as boost::swap hasn't come out of the sandbox, of course
(hint)! Don't you think it /always/ makes sense to provide a custom
swap overload for a Swappable type, as long as it outperforms the
default std::swap, in one way or the other?

Kind regards, Niels

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