David Abrahams wrote:
I haven't formed an opinion yet, but the fact remains that swapping a boost::array would have very different efficiency characteristics from swapping the analogous std::vector, so we ought to think it through carefully.
Howard Hinnant wrote:
If it helps, here is a test program using gcc 4.0.1: ... template <class T, std::size_t N> void swap1(std::tr1::array<T, N>& a, std::tr1::array<T, N>& b) { std::tr1::array<T, N> temp(a); a = b; b = temp; }
template <class T, std::size_t N> void swap2(std::tr1::array<T, N>& a, std::tr1::array<T, N>& b) { std::swap_ranges(a.begin(), a.end(), b.begin()); } ... On my machine:
$ g++ -O3 test.cpp $ ./a.out swap1 time is 0.172796 seconds swap2 time is 1.3e-05 seconds
Cool! Thanks, Howard. The swap member function of boost::array also calls std::swap_ranges, like your swap2: // swap (note: linear complexity) void swap (array<T,N>& y) { std::swap_ranges(begin(),end(),y.begin()); } Unfortunately std::swap_ranges doesn't always pick the custom swap function of T. At least, the STL implementation that comes with MSVC 2008 has swap_ranges calling std::iter_swap, which calls std::swap, without doing argument-dependent lookup. :-( Luckily, once boost::swap would be in the trunk, boost::array::swap could be implemented simply by swapping its one and only data member, elems (a built-in array of T): void swap (array<T,N>& y) { // Assuming that boost::swap supports built-in arrays: boost::swap(this->elems, y.elems); } Having boost::array::swap call boost::swap would have two advantages: it would support a boost::array containing built-in arrays as elements (boost::array<T[M],N>), and it would use ADL to pick the custom swap function of T. What do you think? Kind regards, Niels