Comments on: Hash Functions for C++ Unordered Containers

By: tk

tk — Thu, 08 Mar 2012 22:13:19 +0000

Answered my own question – turns out I needed to use namespace tr1 even with -std=c++0x flag defined.

By: tk

tk — Thu, 08 Mar 2012 22:05:10 +0000

Hi Mark, first off - great article. Second, a question. In method 4 what happens if I want to call a function outside of the std namespace from inside the std::hash template specialization. For example:

PLAIN TEXT

namespace std {
template class hash
{
public:
size_t operator() (const order_id_ptr& x) const {
size_t hval = CUSTOM_HASH_FUNCTION_IN_GLOBAL_NAMESPACE(x->oid, UUID_LEN, 0);
return hval;
}
};
};

This yields tons of non-sensical compiler errors on g++ 4.4.5. Any insight would be hugely appreciated.

By: Mark Nelson

Mark Nelson — Mon, 07 Nov 2011 21:15:22 +0000

Thanks, be sure to link to it so others can find it!

By: FipS

FipS — Mon, 07 Nov 2011 21:09:13 +0000

Hi Mark, I've come across your page when needed to find quick info on how to form a custom hash function. This page is very informative. Thanks for your effort, FipS

By: jm

jm — Sun, 23 Oct 2011 02:46:02 +0000

You are right, Mark, I guess I missed the -std=c++0x in your comments. It should work just fine. I take my beef back ;) This post is the most thorough treatment of the whole issue I've seen online.

By: Mark Nelson

Mark Nelson — Sat, 22 Oct 2011 13:13:02 +0000

@jm:

Thanks jm - I believe, but am not positive, that when g++ is in C++11 mode you don't use the tr1 namespace. So versions that support the -std=c++0x option will work with my code as is.

Users with older versions of g++ may find that they have to hoist their function into namespace std::tr1, which is another issue that can very difficult to figure out from just reading compiler errors.

Any definitive correction to that is more than welcome.

- Mark

By: jm

jm — Sat, 22 Oct 2011 03:41:54 +0000

This is great. I came here for an example of Method 4 and this was the link (after many others) that finally proved worthwhile. My only beef is that in g++, I had to use nested namespaces to get it to work. Like this:
namespace std {
namespace tr1 {
/*
* your very special hash specialization
*/
}
}