How do I customize hashing and equality with std::queue?

In C++, customizing hashing and equality for `std::queue` can be a bit tricky, as `std::queue` does not provide built-in support for these operations. However, you can achieve this by creating a custom structure that encapsulates the queue and provides definitions for equality and hashing. This can be useful when using containers like `std::unordered_map` or `std::unordered_set` that rely on hashing and equality.

Custom Queue Wrapper

Below is an example of a custom queue wrapper that includes hashing and equality operations:


        #include 
        #include 
        #include 

        // Custom wrapper for std::queue
        template 
        class CustomQueue {
        public:
            void push(const T& value) {
                q.push(value);
            }

            void pop() {
                q.pop();
            }

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

            bool empty() const {
                return q.empty();
            }

            size_t size() const {
                return q.size();
            }

            // Equality operator
            bool operator==(const CustomQueue& other) const {
                return q == other.q;
            }

            // Hash function
            struct HashFunction {
                size_t operator()(const CustomQueue& queue) const {
                    // Example hash combining the queue's elements
                    std::hash hash_fn;
                    size_t hash = 0;
                    for (const auto& elem : queue.elements) { // elements must be made accessible for hashing
                        hash ^= hash_fn(elem) + 0x9e3779b9 + (hash << 6) + (hash >> 2);
                    }
                    return hash;
                }
            };

        private:
            std::queue q;
            // To allow hashing, we need to store elements in a way that's accessible.
            std::vector elements; // This could be used to store elements as they are added.
        };

        int main() {
            CustomQueue cq1;
            cq1.push(1);
            cq1.push(2);
            cq1.push(3);

            CustomQueue cq2;
            cq2.push(1);
            cq2.push(2);
            cq2.push(3);

            std::cout << "Queues are equal: " << (cq1 == cq2) << std::endl;

            // Uncomment for hash example usage (needs adjustment based on actual storage of elements)
            // std::unordered_set, CustomQueue::HashFunction> queueSet;
            // queueSet.insert(cq1);
            // queueSet.insert(cq2);

            return 0;
        }

C++ std::queue hashing equality custom data structures C++ standard library

How do I customize hashing and equality with std::queue?

Custom Queue Wrapper

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How do I customize hashing and equality with std::queue?

Custom Queue Wrapper

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