A queu that ensures elements are unique before inserting them.

UniqueQueu.h

#include <unordered_set>
#include <queue>
#include <assert.h>
#include <stdexcept>

template <class T>
class UniqueQueu {

    public:

        bool empty() {
            // sanity check. Either both empty or both not empty.
            assert(q.empty() == elems.empty());
            return q.empty();
        }

        T front() {
            return q.front();
        }

        T pop_front() {
            if(q.empty()) std::runtime_error("Error: popping from empty Q\n");
            T victim = q.front();
            elems.erase(victim);
            q.pop();
            return victim;
        }

        void push_back(T arg) {
            // add if not exist.
            if(elems.find(arg) == elems.end()) {
                q.push(arg);
                elems.insert(arg);
            }
        }

    private:
        std::unordered_set<T> elems;
        std::queue<T> q;
};