Boost C++ Libraries

boost::hash is written to be as portable as possible, but unfortunately, several older compilers don't support argument dependent lookup (ADL) - the mechanism used for customization. On those compilers custom overloads for hash_value need to be declared in the boost namespace.

On a strictly standards compliant compiler, an overload defined in the boost namespace won't be found when boost::hash is instantiated, so for these compilers the overload should only be declared in the same namespace as the class.

Let's say we have a simple custom type:

namespace foo
{
    struct custom_type
    {
        int value;

        friend inline std::size_t hash_value(custom_type x)
        {
            boost::hash<int> hasher;
            return hasher(x.value);
        }
    };
}

On a compliant compiler, when hash_value is called for this type, it will look at the namespace inside the type and find hash_value but on a compiler which doesn't support ADL hash_value won't be found.

So on these compilers define a member function:

#ifndef BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP
        friend inline std::size_t hash_value(custom_type x)
        {
            boost::hash<int> hasher;
            return hasher(x.value);
        }
#else
        std::size_t hash() const
        {
            boost::hash<int> hasher;
            return hasher(value);
        }
#endif

which will be called from the boost namespace:

#ifdef BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP
namespace boost
{
    std::size_t hash_value(foo::custom_type x)
    {
        return x.hash();
    }
}
#endif

Full code for this example is at /libs/functional/hash/examples/portable.cpp.

Other Issues

On Visual C++ versions 6.5 and 7.0, hash_value isn't overloaded for built in arrays. boost::hash, boost::hash_combine and boost::hash_range all use a workaround to support built in arrays so this shouldn't be a problem in most cases.

On Visual C++ versions 6.5 and 7.0, function pointers aren't currently supported.

boost::hash_value(long double) on GCC on Solaris appears to treat long doubles as doubles - so the hash function doesn't take into account the full range of values.