Re: [boost] [swap] Renaming boost_swap_impl::swap_impl and/or its namespace?

From: Niels Dekker - mail address until 2008-12-31 (nd_mail_address_valid_until_2008-12-31_at_[hidden])
Date: 2008-08-14 17:27:49

>> How could moving boost_swap_impl::swap_impl to the
>> boost namespace possibly cause a stack overflow?

David Abrahams wrote:
> Sounds like an infinite recursion to me.

Yes, it is...  The following little test program works fine when using
the current trunk version of utility/swap.hpp.  But it gets into
infinite recursion when swap_impl is moved to the boost namespace:

  ////////////////////////////////////////
  #include <boost/swap.hpp>

  int main() {
    int i1 = 0;
    int i2 = 9;
    boost::swap(i1, i2);
    return i2;
  }
  ////////////////////////////////////////

After moving swap_impl into the boost namespace, Visual Studio 2008
shows me the following call stack, before the overflow:

  boost::swap_impl<int>(int & left=0, int & right=9)
  boost::swap<int,int>(int & left=0, int & right=9)
  ...
  boost::swap_impl<int>(int & left=0, int & right=9)
  boost::swap<int,int>(int & left=0, int & right=9)
  boost::swap_impl<int>(int & left=0, int & right=9)
  boost::swap<int,int>(int & left=0, int & right=9)
  boost::swap_impl<int>(int & left=0, int & right=9)
  boost::swap<int,int>(int & left=0, int & right=9)
  main()
  __tmainCRTStartup()
  mainCRTStartup()

So apparently, within boost::swap_impl, boost::swap<int,int> would be
preferred over std::swap<int>, even while std::swap<T> is more
specialized than boost::swap<T1,T2>.  Isn't that weird?  Note that the
current version of swap_impl has a using-directive:

  template<class T>
  void swap_impl(T& left, T& right)
  {
    using namespace std;
    swap(left,right);
  }

If swap_impl would have a using-declaration instead, "using std::swap",
the infinite recursion would not occur. (But we've just replaced the
using-declaration by a using-directive, to work around some ADL compiler
bugs!)

I still wonder if this infinite recursion is a compiler bug as well. But
anyway, it's clear to me now that there's a good reason to keep
swap_impl out of the boost namespace.  :-)

Kind regards, Niels