Question

I want to use multiset to count some custom defined keys. The keys are not comparable numerically, comparing two keys does not mean anything, but their equality can be checked.

I see that multiset template wants a Compare to order the multiset. The order is not important to me, only the counts are important. If I omit Compare completely what happens? Does multiset work without any problems for my custom keys? If I cannot use std::multiset what are my alternatives?

Answer 1

You cannot use std::multiset if you don’t have a strict weak ordering. Your options are:

1. Impose a strict-weak ordering on your data. If your key is a “linear” data structure, it is usually a good idea to compare it lexicographically.

2. Use an unordered container equivalent, e.g., boost::unordered_multiset. For that, you will need to make your custom data-type hash-able, which is often-times easier than imposing some kind of order.