C++ boost unordered_set compile error with is_always_equal

If you are facing a compilation error with boost::unordered_set and is_always_equal, it is likely due to the fact that is_always_equal is not a supported type for boost::unordered_set.

is_always_equal is a type trait provided by the C++ standard library to determine if a hash function object is always equal to another hash function object. However, boost::unordered_set requires the hash function object to be copy constructible and assignable, which is_always_equal is not.

To fix the compilation error, you should use a hash function object that satisfies the requirements of boost::unordered_set. You can either implement your own hash function object or use one of the predefined ones provided by Boost, such as boost::hash.

Here's an example of using boost::hash with boost::unordered_set:

#include <boost/unordered_set.hpp>
#include <boost/functional/hash.hpp>

int main() {
    boost::unordered_set<int, boost::hash<int>> mySet;
    mySet.insert(1);
    mySet.insert(2);
    mySet.insert(3);

    for (const auto& element : mySet) {
        std::cout << element << " ";
    }

    return 0;
}

In this example, we use boost::hash<int> as the hash function object for boost::unordered_set. This ensures that the requirements of boost::unordered_set are satisfied, and the compilation error should be resolved