Home/ Questions/Q 6599111
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-25T18:26:52+00:00

I’d like use boost::unordered_map<key,value> , where key is a std::set<int> . Since a set

  • 0

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.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    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!

    • 0