Please review my queue:

template<class T>
class Queue {
public:
    Queue(const std::initializer_list<T>& i) :elem{i} {}
    int size() const {
        return elem.size();
    }
    bool empty() const {
        return elem.empty();
    }
    void enqueue(T&&);
    void dequeue();
    T peek();
    T& operator[](std::size_t);
    typename std::vector <T>::iterator begin() {
        return elem.begin();
    }
    typename std::vector <T>::iterator end() {
        return elem.end();
    }

private:
    std::vector<T> elem;
};

template<class T>
void Queue<T>::enqueue(T&& t) {
    elem.push_back(t);
}

template<class T>
void Queue<T>::dequeue() {
    if (empty()) {
        throw std::out_of_range("underflow");
    }
    elem.erase(elem.begin());
}

template<class T>
T Queue<T>::peek() {
    if (empty()) {
        throw std::out_of_range("underflow");
    }
    return elem.front();
}

template<class T>
T& Queue<T>::operator[](std::size_t i) {
    if (empty() || i < 0 || i >= size()) {
        throw std::out_of_range("underflow");
    }
    return elem[i];
}

How to improve this code?

How can I improve this code?

template<class T>
class Queue {
public:
    Queue(const std::initializer_list<T>& i) :elem{i} {}
    int size() const {
        return elem.size();
    }
    bool empty() const {
        return elem.empty();
    }
    void enqueue(T&&);
    void dequeue();
    T peek();
    T& operator[](std::size_t);
    typename std::vector <T>::iterator begin() {
        return elem.begin();
    }
    typename std::vector <T>::iterator end() {
        return elem.end();
    }

private:
    std::vector<T> elem;
};

template<class T>
void Queue<T>::enqueue(T&& t) {
    elem.push_back(t);
}

template<class T>
void Queue<T>::dequeue() {
    if (empty()) {
        throw std::out_of_range("underflow");
    }
    elem.erase(elem.begin());
}

template<class T>
T Queue<T>::peek() {
    if (empty()) {
        throw std::out_of_range("underflow");
    }
    return elem.front();
}

template<class T>
T& Queue<T>::operator[](std::size_t i) {
    if (empty() || i < 0 || i >= size()) {
        throw std::out_of_range("underflow");
    }
    return elem[i];
}