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