boost::unordered_map — Need to specify a custom hash function for hashing std::set<int>?

Question

I'd like use boost::unordered_map<key,value>, where key is a std::set<int>. Since a set of integers is no built-in type, I assumed I had to supply my own hash function (or, rather, I was thinking of using boost's hash_range).

However, now I tried initializing a hash map like this, neither supplying a hash function nor an equality predicate -- and gcc didn't complain. What is happening here? Is boost clever enough to hash STL containers all on its own? Is this going to be slower than if I used a custom hash function? What about using boost::hash_range?

Thanks in advance.

Answer 1

According to the Boost documentation:

the default hash function is Boost.Hash

And, according to the documentation for Boost.Hash, default hash functions are provided for the standard containers. Consequently, there is already a hash function written for std::set. The Boost hash containers aren't smart enough to know how to hash sets automatically, but they are smart enough to use the implementation that's already provided.

Hope this helps!

Answer 2

The default boost::hash< Key > function is being chosen. According to its documentation

As it is compliant with TR1, it will work with:

integers
floats
pointers
strings

It also implements the extension proposed by Peter Dimov in issue 6.18 of the Library Extension Technical Report Issues List (page 63), this adds support for:

arrays
std::pair
the standard containers.
extending boost::hash for custom types.

http://www.boost.org/doc/html/hash.html

So yes, boost is clever enough to hash STL containers. Unless you know something specific of your particular use case of set, I doubt there is any point in providing your own hash function.