[boost] swappable user defined types and STLport libraries

From: Alisdair Meredith (alisdair.meredith_at_[hidden])
Date: 2003-08-04 20:58:38

There is a problem with the Borland BCB6 compiler specializing std::swap
for user defined types when using the STLport standard library.  This
may apply to other compilers using the library as well, but only have
experience with Borland.

An example is the clearest demonstration:

//  -----  Begin example
#include <boost/shared_ptr.hpp>

class Swappable
{
public:
    Swappable() {}

    void Swap( Swappable & ) {  }
};

namespace   std
{
template<>
void swap( Swappable &lhs, Swappable &rhs )
{
    lhs.Swap( rhs );
}

}

int main(int argc, char* argv[])
{
    Swappable   a, b;
    std::swap( a, b );

    boost::shared_ptr<Swappable> pA( new Swappable() );
    boost::shared_ptr<Swappable> pB( new Swappable() );
# if BOOST_WORKAROUND( __BORLANDC__, >= 0x0560 )
    _STL::swap( pA, pB );
# else
    std::swap( pA, pB );
# endif
    return 0;
}
//  -----  End example

If the workaround is disabled, the compiler cannot find the
specialization in namespace std as the swap algorithm is implemented in
namespace _STL and introduced by a namespace alias.

What does this mean for Boost?
i/  There are no test cases for this in the test suite, or BCB would be
failing more tests!
ii/ Anywhere we use std::swap, we need a borland hack as above (although
better tuned to STLport version as well)

So far this bug has bitten us in the smart pointers, function and Spirit
libraries but I doubt this is an exhaustive list!

What is the best way forward with this?  
Should I submit separate patches (and test cases) for each library as I
find them?  
Is it worth introducing a new config macro for the problem?
Is there a neater fix I'm missing?
Do we want to consider other algorithms that may be specialized on user
defined types, such as std::less?

I'd certainly like to have the 'right' boiler-plate version of the patch
to apply before submitting patches.

-- 
AlisdairM