Re: [boost] [utility/swap] Okay to add array support to Boost.Swap (sandbox/swap)?

From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-06-23 05:42:11

>> Would it be okay to you if we would add support for array types to
>> the Boost.Swap utility that is located in the sandbox?

> Whether or not this is a good idea depends on whether you think swap
> is fundamentally an O(1) and/or nonthrowing operation.

>> The array support I would like to add to boost::swap is
>> non-throwing, as long as the array element type has a
>> non-throwing swap.

David Abrahams wrote:
> Sure, but it is not O(1) no matter what the element type does.

For a particular template instantiation of boost::swap, for a particular 
type of arrays, the number of element swaps would be constant.  Doesn't 
that make it O(1)?  :-)

Maybe I just don't understand the definition of O(1) well enough. Do you 
consider the following swap_pod<T> function O(1)?  Or is it O(n), having
n = sizeof(T)?  It would support swapping arrays, as long as their 
element type is POD.

  template<typename T>
  void swap_pod(T & lhs, T & rhs)
  {
    unsigned char tmp[sizeof(T)];
    std::memcpy(tmp, lhs, sizeof(T));
    std::memcpy(lhs, rhs, sizeof(T));
    std::memcpy(rhs, tmp, sizeof(T));
  }

(BTW, my proposal wasn't to use std::memcpy, but instead, to do the swap 
elementwise, using the custom swap function of the element type, 
whenever found by ADL.)

So do you think that adding array support to boost::swap is /not/ a good 
idea?

Note that boost::array<T,N> /does/ have a swap function, even though it 
is documented to have no constant complexity. 
www.boost.org/doc/libs/1_35_0/doc/html/array.html  Do you think that 
boost::array<T,N> should not have one?

Kind regards, Niels