Implementation of std::deque

Question

In the prior obsolete implementation of std::deque, I used two std::vectors to contain the elements. But the standard guarantees that pointers and references to elements aren't invalidated when an element at either end is erased or when an element is inserted into either end, which the prior implementation doesn't:

To guarantee that, this new implementation contains every element to its own node:

And here is the implementation of the header deque (rename namespace NDos to namespace std):

#ifndef DEQUE_H_INCLUDED
#define DEQUE_H_INCLUDED
#include <memory>
#include <type_traits>
#include <iterator>
#include <utility>
#include <exception>
#include <initializer_list>
#include <algorithm>
namespace NDos {
    template <class T, class Alloc = std::allocator<T>> class deque {
    public:
        typedef T value_type;
        typedef Alloc allocator_type;
        typedef typename std::allocator_traits<Alloc>::size_type size_type;
        typedef typename std::allocator_traits<Alloc>::difference_type difference_type;
        typedef T &reference;
        typedef const T &const_reference;
        typedef typename std::allocator_traits<Alloc>::pointer pointer;
        typedef typename std::allocator_traits<Alloc>::const_pointer const_pointer;
        class iterator;
        class const_iterator;
        typedef std::reverse_iterator<iterator> reverse_iterator;
        typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
    private:
        typedef std::allocator_traits<Alloc> vatraits;
        Alloc va;
        pointer *ab, *cb, *ae, *ce; // allocated begin, constructed begin, etc.
    public:
        deque() : deque(Alloc()) {}
        explicit deque(const Alloc &alloc) : va(alloc), ab(new pointer[1]), cb(ab), ae(ab + 1), ce(cb) {}
        deque(size_type count, const_reference value, const Alloc &alloc = Alloc()) : va(alloc),
            ab(new pointer[count << 1]), cb(ab + (count >> 1)), ae(ab + (count << 1)), ce(cb + count) {
            std::for_each(cb, ce, [&](pointer &p){
                p = vatraits::allocate(va, 1);
                vatraits::construct(va, p, value);
            });
        }
        explicit deque(size_type count, const Alloc &alloc = Alloc()) : va(alloc),
            ab(new pointer[count << 1]), cb(ab + (count >> 1)), ae(ab + (count << 1)), ce(cb + count) {
            std::for_each(cb, ce, [&](pointer &p){
                p = vatraits::allocate(va, 1);
                vatraits::construct(va, p);
            });
        }
        template <class InputIt> deque(InputIt first,
            std::conditional_t<false, typename std::iterator_traits<InputIt>::value_type, InputIt> last,
            const Alloc &alloc = Alloc()) : deque(alloc) {
            while (first != last)
                push_back(*first++);
        }
        deque(const deque &other) : va(vatraits::select_on_container_copy_construction(other.va)),
            ab(new pointer[other.size() << 1]), cb(ab + (other.size() >> 1)), ae(ab + (other.size() << 1)), ce(cb + other.size()) {
            std::for_each(cb, ce, [&](pointer &p){
                p = vatraits::allocate(va, 1);
                vatraits::construct(va, p, other[&p - cb]);
            });
        }
        deque(const deque &other, const Alloc &alloc) : va(alloc), ab(new pointer[other.size() << 1]),
            cb(ab + (other.size() >> 1)), ae(ab + (other.size() << 1)), ce(cb + other.size()) {
            std::for_each(cb, ce, [&](pointer &p){
                p = vatraits::allocate(va, 1);
                vatraits::construct(va, p, other[&p - cb]);
            });
        }
        deque(deque &&other) : va(std::move(other.va)), ab(other.ab), cb(other.cb), ae(other.ae), ce(other.ce) {}
        deque(deque &&other, const Alloc &alloc) : va(alloc), ab(other.ab), cb(other.cb), ae(other.ae), ce(other.ce) {
            if (va != other.va) {
                ab = new pointer[other.size() << 1];
                cb = ab + (other.size() >> 1);
                ae = ab + (other.size() << 1);
                ce = cb + other.size();
                std::for_each(cb, ce, [&](pointer &p){
                    p = vatraits::allocate(va, 1);
                    vatraits::construct(va, p, std::move(other[&p - cb]));
                });
            }
        }
        deque(std::initializer_list<T> init, const Alloc &alloc = Alloc()) : va(alloc), ab(new pointer[init.size() << 1]),
            cb(ab + (init.size() >> 1)), ae(ab + (init.size() << 1)), ce(cb + init.size()) {
            std::for_each(cb, ce, [&](pointer &p){
                p = vatraits::allocate(va, 1);
                vatraits::construct(va, p, std::move(init.begin()[&p - cb]));
            });
        }
        ~deque() {
            clear();
            delete []ab;
        }
        deque &operator = (const deque &other) {
            if (this == &other)
                return *this;
            if (typename vatraits::propagate_on_container_copy_assignment() && va != other.va) {
                Alloc newva(other.va);
                if (other.size() <= size()) { // ---XXXXXX--- to ---YYYY-----
                    std::for_each(cb, cb + other.size(), [&](pointer &p){
                        vatraits::destroy(va, p);
                        vatraits::deallocate(va, p, 1);
                        p = vatraits::allocate(newva, 1);
                        vatraits::construct(newva, p, other[&p - cb]);
                    });
                    erase(cbegin() + other.size(), cend());
                    ce = cb + other.size();
                } else if (other.size() <= ce - ab) { // ---XXXXXX--- to -YYYYYYYY---
                    std::for_each(ce - other.size(), cb, [&](pointer &p){
                        p = vatraits::allocate(newva, 1);
                        vatraits::construct(newva, p, other[&p - (ce - other.size())]);
                    });
                    std::for_each(cb, ce, [&](pointer &p){
                        vatraits::destroy(va, p);
                        vatraits::deallocate(va, p, 1);
                        p = vatraits::allocate(newva, 1);
                        vatraits::construct(newva, p, other[&p - (ce - other.size())]);
                    });
                    cb = ce - other.size();
                } else if (other.size() <= ae - ab) { // ---XXXXXX--- to YYYYYYYYYY--
                    auto newE = [&](pointer &p){
                        p = vatraits::allocate(newva, 1);
                        vatraits::construct(newva, p, other[&p - ab]);
                    };
                    std::for_each(ab, cb, newE);
                    std::for_each(cb, ce, [&](pointer &p){
                        vatraits::destroy(va, p);
                        vatraits::deallocate(va, p, 1);
                        p = vatraits::allocate(newva, 1);
                        vatraits::construct(newva, p, other[&p - ab]);
                    });
                    std::for_each(ce, ab + other.size(), newE);
                    cb = ab;
                    ce = ab + other.size();
                } else { // reallocation
                    pointer *nab = new pointer[other.size() << 1], *ncb = nab + (other.size() >> 1);
                    std::for_each(ncb, ncb + size(), [&](pointer &p){
                        p = cb[&p - ncb];
                        *p = other[&p - ncb];
                    });
                    std::for_each(ncb + size(), ncb + other.size(), [&](pointer &p){
                        p = vatraits::allocate(newva, 1);
                        vatraits::construct(newva, p, other[&p - ncb]);
                    });
                    cb = ncb;
                    ce = ncb + other.size();
                    delete []ab;
                    ab = nab;
                    ae = nab + (size() << 1);
                }
                va = std::move(newva);
            } else {
                if (other.size() <= size()) {
                    std::copy(other.begin(), other.end(), begin());
                    erase(cbegin() + other.size(), cend());
                    ce = cb + other.size();
                } else if (other.size() <= ce - ab) {
                    std::for_each(ce - other.size(), cb, [&](pointer &p){
                        p = vatraits::allocate(va, 1);
                        vatraits::construct(va, p, other[&p - (ce - other.size())]);
                    });
                    std::copy(other.end() - size(), other.end(), begin());
                    cb = ce - other.size();
                } else if (other.size() <= ae - ab) {
                    auto newE = [&](pointer &p){
                        p = vatraits::allocate(va, 1);
                        vatraits::construct(va, p, other[&p - ab]);
                    };
                    std::for_each(ab, cb, newE);
                    std::copy(other.begin() + (cb - ab), other.begin() + (ce - ab), begin());
                    std::for_each(ce, ab + other.size(), newE);
                    cb = ab;
                    ce = ab + other.size();
                } else {
                    pointer *nab = new pointer[other.size() << 1], *ncb = nab + (other.size() >> 1);
                    std::for_each(ncb, ncb + size(), [&](pointer &p){
                        p = cb[&p - ncb];
                        *p = other[&p - ncb];
                    });
                    std::for_each(ncb + size(), ncb + other.size(), [&](pointer &p){
                        p = vatraits::allocate(va, 1);
                        vatraits::construct(va, p, other[&p - ncb]);
                    });
                    cb = ncb;
                    ce = ncb + other.size();
                    delete []ab;
                    ab = nab;
                    ae = nab + (size() << 1);
                }
            }
            return *this;
        }
        deque &operator = (deque &&other) noexcept(typename vatraits::is_always_equal()) {
            if (this == &other)
                return *this;
            if (va == other.va) {
                clear();
                delete []ab;
                ab = other.ab;
                ae = other.ae;
                cb = other.cb;
                ce = other.ce;
                other.ab = nullptr;
            } else if (typename vatraits::propagate_on_container_move_assignment()) {
                if (other.size() <= size()) {
                    std::for_each(cb, cb + other.size(), [&](pointer &p){
                        vatraits::destroy(va, p);
                        vatraits::deallocate(va, p, 1);
                        p = vatraits::allocate(other.va, 1);
                        vatraits::construct(other.va, p, std::move(other[&p - cb]));
                    });
                    erase(cbegin() + other.size(), cend());
                    ce = cb + other.size();
                } else if (other.size() <= ce - ab) {
                    std::for_each(ce - other.size(), cb, [&](pointer &p){
                        p = vatraits::allocate(other.va, 1);
                        vatraits::construct(other.va, p, std::move(other[&p - (ce - other.size())]));
                    });
                    std::for_each(cb, ce, [&](pointer &p){
                        vatraits::destroy(va, p);
                        vatraits::deallocate(va, p, 1);
                        p = vatraits::allocate(other.va, 1);
                        vatraits::construct(other.va, p, std::move(other[&p - (ce - other.size())]));
                    });
                    cb = ce - other.size();
                } else if (other.size() <= ae - ab) {
                    auto newE = [&](pointer &p){
                        p = vatraits::allocate(other.va, 1);
                        vatraits::construct(other.va, p, std::move(other[&p - ab]));
                    };
                    std::for_each(ab, cb, newE);
                    std::for_each(cb, ce, [&](pointer &p){
                        vatraits::destroy(va, p);
                        vatraits::deallocate(va, p, 1);
                        p = vatraits::allocate(other.va, 1);
                        vatraits::construct(other.va, p, std::move(other[&p - ab]));
                    });
                    std::for_each(ce, ab + other.size(), newE);
                    cb = ab;
                    ce = ab + other.size();
                } else {
                    pointer *nab = new pointer[other.size() << 1], *ncb = nab + (other.size() >> 1);
                    std::for_each(ncb, ncb + size(), [&](pointer &p){
                        p = cb[&p - ncb];
                        *p = std::move(other[&p - ncb]);
                    });
                    std::for_each(ncb + size(), ncb + other.size(), [&](pointer &p){
                        p = vatraits::allocate(other.va, 1);
                        vatraits::construct(other.va, p, std::move(other[&p - ncb]));
                    });
                    cb = ncb;
                    ce = ncb + other.size();
                    delete []ab;
                    ab = nab;
                    ae = nab + (size() << 1);
                }
                va = std::move(other.va);
            } else {
                if (other.size() <= size()) {
                    std::move(other.begin(), other.end(), begin());
                    erase(cbegin() + other.size(), cend());
                    ce = cb + other.size();
                } else if (other.size() <= ce - ab) {
                    std::for_each(ce - other.size(), cb, [&](pointer &p){
                        p = vatraits::allocate(va, 1);
                        vatraits::construct(va, p, std::move(other[&p - (ce - other.size())]));
                    });
                    std::move(other.end() - size(), other.end(), begin());
                    cb = ce - other.size();
                } else if (other.size() <= ae - ab) {
                    auto newE = [&](pointer &p){
                        p = vatraits::allocate(va, 1);
                        vatraits::construct(va, p, std::move(other[&p - ab]));
                    };
                    std::for_each(ab, cb, newE);
                    std::move(other.begin() + (cb - ab), other.begin() + (ce - ab), begin());
                    std::for_each(ce, ab + other.size(), newE);
                    cb = ab;
                    ce = ab + other.size();
                } else {
                    pointer *nab = new pointer[other.size() << 1], *ncb = nab + (other.size() >> 1);
                    std::for_each(ncb, ncb + size(), [&](pointer &p){
                        p = cb[&p - ncb];
                        *p = std::move(other[&p - ncb]);
                    });
                    std::for_each(ncb + size(), ncb + other.size(), [&](pointer &p){
                        p = vatraits::allocate(va, 1);
                        vatraits::construct(va, p, std::move(other[&p - ncb]));
                    });
                    cb = ncb;
                    ce = ncb + other.size();
                    delete []ab;
                    ab = nab;
                    ae = nab + (size() << 1);
                }
            }
            return *this;
        }
        deque &operator = (std::initializer_list<T> other) {
            if (other.size() <= size()) {
                std::move(other.begin(), other.end(), begin());
                erase(cbegin() + other.size(), cend());
                ce = cb + other.size();
            } else if (other.size() <= ce - ab) {
                std::for_each(ce - other.size(), cb, [&](pointer &p){
                    p = vatraits::allocate(va, 1);
                    vatraits::construct(va, p, std::move(other.begin()[&p - (ce - other.size())]));
                });
                std::move(other.end() - size(), other.end(), begin());
                cb = ce - other.size();
            } else if (other.size() <= ae - ab) {
                auto newE = [&](pointer &p){
                    p = vatraits::allocate(va, 1);
                    vatraits::construct(va, p, std::move(other.begin()[&p - ab]));
                };
                std::for_each(ab, cb, newE);
                std::move(other.begin() + (cb - ab), other.begin() + (ce - ab), begin());
                std::for_each(ce, ab + other.size(), newE);
                cb = ab;
                ce = ab + other.size();
            } else {
                pointer *nab = new pointer[other.size() << 1], *ncb = nab + (other.size() >> 1);
                std::for_each(ncb, ncb + size(), [&](pointer &p){
                    p = cb[&p - ncb];
                    *p = std::move(other.begin()[&p - ncb]);
                });
                std::for_each(ncb + size(), ncb + other.size(), [&](pointer &p){
                    p = vatraits::allocate(va, 1);
                    vatraits::construct(va, p, std::move(other.begin()[&p - ncb]));
                });
                cb = ncb;
                ce = ncb + other.size();
                delete []ab;
                ab = nab;
                ae = nab + (size() << 1);
            }
            return *this;
        }
        void assign(size_type count, const_reference value) {
            if (count <= size()) {
                std::fill_n(begin(), count, value);
                erase(cbegin() + count, cend());
                ce = cb + count;
            } else if (count <= ce - ab) {
                std::for_each(ce - count, cb, [&](pointer &p){
                    p = vatraits::allocate(va, 1);
                    vatraits::construct(va, p, value);
                });
                std::fill(begin(), end(), value);
                cb = ce - count;
            } else if (count <= ae - ab) {
                auto newE = [&](pointer &p){
                    p = vatraits::allocate(va, 1);
                    vatraits::construct(va, p, value);
                };
                std::for_each(ab, cb, newE);
                std::fill(begin(), end(), value);
                std::for_each(ce, ab + count, newE);
                cb = ab;
                ce = ab + count;
            } else {
                pointer *nab = new pointer[count << 1], *ncb = nab + (count >> 1);
                std::for_each(ncb, ncb + size(), [&](pointer &p){
                    p = cb[&p - ncb];
                    *p = value;
                });
                std::for_each(ncb + size(), ncb + count, [&](pointer &p){
                    p = vatraits::allocate(va, 1);
                    vatraits::construct(va, p, value);
                });
                cb = ncb;
                ce = ncb + count;
                delete []ab;
                ab = nab;
                ae = nab + (size() << 1);
            }
        }
        template <class InputIt> std::conditional_t<false, typename std::iterator_traits<InputIt>::value_type, void>
            assign(InputIt first, InputIt last) {
            clear();
            while (first != last)
                push_back(*first++);
        }
        void assign(std::initializer_list<T> other) {
            *this = other;
        }
        Alloc get_allocator() const {
            return va;
        }
        reference at(size_type pos) {
            return (pos < size()) ? *cb[pos] : throw std::out_of_range("Out of range from std::deque::at!");
        }
        const_reference at(size_type pos) const {
            return (pos < size()) ? *cb[pos] : throw std::out_of_range("Out of range from std::deque::at!");
        }
        reference operator [] (size_type pos) {
            return *cb[pos];
        }
        const_reference operator [] (size_type pos) const {
            return *cb[pos];
        }
        reference front() {
            return **cb;
        }
        const_reference front() const {
            return **cb;
        }
        reference back() {
            return *ce[-1];
        }
        const_reference back() const {
            return *ce[-1];
        }
        iterator begin() noexcept {
            return iterator(cb);
        }
        const_iterator begin() const noexcept {
            return const_iterator(cb);
        }
        const_iterator cbegin() const noexcept {
            return const_iterator(cb);
        }
        iterator end() noexcept {
            return iterator(ce);
        }
        const_iterator end() const noexcept {
            return const_iterator(ce);
        }
        const_iterator cend() const noexcept {
            return const_iterator(ce);
        }
        reverse_iterator rbegin() noexcept {
            return make_reverse_iterator(begin());
        }
        const_reverse_iterator rbegin() const noexcept {
            return make_reverse_iterator(begin());
        }
        const_reverse_iterator crbegin() const noexcept {
            return make_reverse_iterator(crbegin());
        }
        reverse_iterator rend() noexcept {
            return make_reverse_iterator(end());
        }
        const_reverse_iterator rend() const noexcept {
            return make_reverse_iterator(end());
        }
        const_reverse_iterator crend() const noexcept {
            return make_reverse_iterator(cend());
        }
        bool empty() const noexcept {
            return cb == ce;
        }
        size_type size() const noexcept {
            return ce - cb;
        }
        size_type max_size() const noexcept {
            return vatraits::max_size(va);
        }
        void shrink_to_fit() {
            pointer *nab = new pointer[size()];
            ce = ae = std::move(cb, ce, nab);
            delete []ab;
            cb = ab = nab;
        }
        void clear() noexcept {
            std::for_each(cb, ce, [&](pointer &p){
                vatraits::destroy(va, p);
                vatraits::deallocate(va, p, 1);
            });
            ce = cb;
        }
        iterator insert(const_iterator pos, const_reference value) {
            return emplace(pos, value);
        }
        iterator insert(const_iterator pos, T &&value) {
            return emplace(pos, std::move(value));
        }
        iterator insert(const_iterator pos, size_type count, const_reference value) {
            if (0 == count) return iterator(pos.refer);
            pointer *i;
            if (count <= cb - ab) { // ---XXXXXX--- to -XYYXXXXX---
                i = std::move(cb, pos.refer, cb - count);
                cb -= count;
            } else if (size() + count <= ae - ab) { // ---XXXXXX--- to XXYYYYXXXX--
                i = std::move(cb, pos.refer, ab);
                pointer *j = std::move_backward(pos.refer, ce, ab + (count + size()));
                ce = ab + (count + size());
                cb = ab;
            } else { // reallocation required
                pointer *nab = new pointer[size() + count << 1], *ncb = nab + (size() + count >> 1);
                i = std::move(cb, pos.refer, ncb);
                ce = std::move(pos.refer, ce, i + count);
                cb = ncb;
                delete []ab;
                ab = nab;
                ae = nab + (size() + count << 1);
            }
            std::for_each(i, i + count, [&](pointer &p){
                p = vatraits::allocate(va, 1);
                vatraits::construct(va, p, value);
            });
            return iterator(i);
        }
        template <class InputIt> std::conditional_t<false, typename std::iterator_traits<InputIt>::value_type, iterator>
            insert(const_iterator pos, InputIt first, InputIt last) {
            if (first == last) return pos;
            iterator result(pos.refer - 1);
            while (first != last)
                insert(pos, *first++);
            return result;
        }
        iterator insert(const_iterator pos, std::initializer_list<T> ilist) {
            if (0 == ilist.size()) return iterator(pos.refer);
            pointer *i;
            if (ilist.size() <= cb - ab) {
                i = std::move(cb, pos.refer, cb - ilist.size());
                cb -= ilist.size();
            } else if (size() + ilist.size() <= ae - ab) {
                i = std::move(cb, pos.refer, ab);
                std::move_backward(pos.refer, ce, ab + (ilist.size() + size()));
                ce = ab + ilist.size() + size();
                cb = ab;
            } else {
                pointer *nab = new pointer[size() + ilist.size() << 1], *ncb = nab + (size() + ilist.size() >> 1);
                i = std::move(cb, pos.refer, ncb);
                ce = std::move(pos.refer, ce, i + ilist.size());
                cb = ncb;
                delete []ab;
                ab = nab;
                ae = nab + (size() + ilist.size() << 1);
            }
            pointer *j = i;
            for (auto &v : ilist) {
                *j = vatraits::allocate(va, 1);
                vatraits::construct(va, *j++, std::move(v));
            }
            return iterator(i);
        }
        template <class ...Args> iterator emplace(const_iterator pos, Args &&...args) {
            pointer *i;
            if ((pos.refer - cb <= ce - pos.refer || ce == ae) && ab < cb) {
                // more efficient to move elements before pos, or no space at back
                i = std::move(cb, pos.refer, cb - 1);
                --cb;
                *i = vatraits::allocate(va, 1);
                vatraits::construct(va, *i, std::forward<Args>(args)...);
            } else if ((pos.refer - cb > ce - pos.refer || ab == cb) && ce < ae) {
                // more efficient to move elements after pos, or no space at front
                i = std::move_backward(pos.refer, ce, ce + 1) - 1;
                ++ce;
                *i = vatraits::allocate(va, 1);
                vatraits::construct(va, *i, std::forward<Args>(args)...);
            } else { // reallocation required
                pointer *nab = new pointer[size() << 1], *ncb = nab + (size() >> 1);
                i = std::move(cb, pos.refer, ncb);
                *i = vatraits::allocate(va, 1);
                vatraits::construct(va, *i, std::forward<Args>(args)...);
                ce = std::move(pos.refer, ce, i + 1);
                    cb = ncb;
                delete []ab;
                ab = nab;
                ae = nab + (size() << 1);
            }
            return iterator(i);
        }
        void push_back(const_reference value) {
            insert(cend(), value);
        }
        void push_back(T &&value) {
            insert(cend(), std::move(value));
        }
        template <class ...Args> void emplace_back(Args &&...args) {
            emplace(cend(), std::forward<Args>(args)...);
        }
        void push_front(const_reference value) {
            insert(cbegin(), value);
        }
        void push_front(T &&value) {
            insert(cbegin(), std::move(value));
        }
        template <class ...Args> void emplace_front(Args &&...args) {
            emplace(cbegin(), std::forward<Args>(args)...);
        }
        iterator erase(const_iterator pos) {
            vatraits::destroy(va, *pos.refer);
            vatraits::deallocate(va, *pos.refer, 1);
            if (ce - pos.refer <= pos.refer - cb) // more efficient to move elements before pos
                std::move(pos.refer + 1, ce--, pos.refer);
            else // more efficient to move elements after pos
                std::move_backward(cb++, pos.refer, pos.refer + 1);
            return iterator(pos.refer);
        }
        iterator erase(const_iterator first, const_iterator last) {
            std::for_each(first.refer, last.refer, [&](pointer &p){
                vatraits::destroy(va, p);
                vatraits::deallocate(va, p, 1);
            });
            if (ce - last.refer <= first.refer - cb) {
                std::move(last.refer, ce, first.refer);
                ce -= last - first;
            } else {
                std::move_backward(cb, first.refer, last.refer);
                cb += last - first;
            }
            return iterator(first.refer);
        }
        void pop_back() {
            erase(cend() - 1);
        }
        void pop_front() {
            erase(cbegin());
        }
        void resize(size_type count) {
            auto newE = [&](pointer &p){
                p = vatraits::allocate(va, 1);
                vatraits::construct(va, p);
            };
            if (count <= size()) // ---XXXXXX--- to ---XXXX-----
                erase(cbegin() + count, cend());
            else if (count <= ae - cb) { // ---XXXXXX--- to ---XXXXXXYY-
                std::for_each(ce, cb + count, newE);
                ce = cb + count;
            } else if (count <= ae - ab) { // ---XXXXXX--- to XXXXXXYYYY--
                pointer *i = std::move(cb, ce, ab);
                std::for_each(i, ab + count, newE);
            } else { // reallocation
                pointer *nab = new pointer[count << 1], *ncb = nab + (count >> 1), *nce = ncb + count;
                std::for_each(ncb, nce, newE);
                cb = ncb;
                ce = nce;
            }
        }
        void resize(size_type count, const_reference value) {
            auto newE = [&](pointer &p){
                p = vatraits::allocate(va, 1);
                vatraits::construct(va, p, value);
            };
            if (count <= size())
                erase(cbegin() + count, cend());
            else if (count <= ae - cb) {
                std::for_each(ce, cb + count, newE);
                ce = cb + count;
            } else if (count <= ae - ab) {
                pointer *i = std::move(cb, ce, ab);
                std::for_each(i, ab + count, newE);
            } else {
                pointer *nab = new pointer[count << 1], *ncb = nab + (count >> 1);
                std::move(cb, ce, ncb);
                std::for_each(ncb + size(), ncb + count, newE);
                cb = ncb;
                ce = ncb + count;
            }
        }
        void swap(deque &other) noexcept(typename vatraits::is_always_equal()) {
            std::swap(ab, other.ab);
            std::swap(cb, other.cb);
            std::swap(ce, other.ce);
            std::swap(ae, other.ae);
            if (typename vatraits::propagate_on_container_swap())
                swap(va, other.va);
        }
    };
    template <class T, class Alloc> class deque<T, Alloc>::iterator {
        friend deque<T, Alloc>;
    public:
        typedef typename deque<T, Alloc>::difference_type difference_type;
        typedef T value_type;
        typedef typename deque<T, Alloc>::pointer pointer;
        typedef T &reference;
        typedef std::random_access_iterator_tag iterator_category;
    protected:
        pointer *refer;
        iterator(pointer *r) : refer(r) {}
    public:
        iterator(const iterator &other) : refer(other.refer) {}
        iterator() : refer(nullptr) {}
        iterator &operator = (const iterator &other) {
            refer = other.refer;
            return *this;
        }
        // destructor : default
        reference operator * () const {
            return **refer;
        }
        pointer operator -> () const {
            return *refer;
        }
        reference operator [] (difference_type d) const {
            return *refer[d];
        }
        iterator &operator ++ () {
            ++refer;
            return *this;
        }
        iterator operator ++ (int) {
            iterator i(*this);
            ++*this;
            return i;
        }
        iterator &operator -- () {
            --refer;
            return *this;
        }
        iterator operator -- (int) {
            iterator i(*this);
            --*this;
            return i;
        }
        friend bool operator == (const iterator &a, const iterator &b) {
            return a.refer == b.refer;
        }
        friend bool operator != (const iterator &a, const iterator &b) {
            return a.refer != b.refer;
        }
        friend bool operator < (const iterator &a, const iterator &b) {
            return a.refer < b.refer;
        }
        friend bool operator > (const iterator &a, const iterator &b) {
            return a.refer > b.refer;
        }
        friend bool operator <= (const iterator &a, const iterator &b) {
            return a.refer <= b.refer;
        }
        friend bool operator >= (const iterator &a, const iterator &b) {
            return a.refer >= b.refer;
        }
        friend iterator operator + (const iterator &i, difference_type d) {
            return iterator(i.refer + d);
        }
        friend iterator operator + (difference_type d, const iterator &i) {
            return iterator(i.refer + d);
        }
        friend iterator operator - (const iterator &i, difference_type d) {
            return iterator(i.refer - d);
        }
        iterator &operator += (difference_type d) {
            refer += d;
            return *this;
        }
        iterator &operator -= (difference_type d) {
            refer -= d;
            return *this;
        }
        friend difference_type operator - (const iterator &a, const iterator &b) {
            return a.refer - b.refer;
        }
    };
    template <class T, class Alloc> class deque<T, Alloc>::const_iterator {
        friend deque<T, Alloc>;
    public:
        typedef typename deque<std::add_const_t<T>, Alloc>::difference_type difference_type;
        typedef std::add_const_t<T> value_type;
        typedef typename deque<std::add_const_t<T>, Alloc>::pointer pointer;
        typedef std::add_const_t<T> &reference;
        typedef std::random_access_iterator_tag iterator_category;
    private:
        typename deque<T, Alloc>::pointer *refer;
        const_iterator(decltype(refer) r) : refer(r) {}
    public:
        const_iterator(const const_iterator &other) : refer(other.refer) {}
        const_iterator() : refer(nullptr) {}
        const_iterator &operator = (const const_iterator &other) {
            refer = other.refer;
            return *this;
        }
        // destructor : default
        reference operator * () const {
            return **refer;
        }
        pointer operator -> () const {
            return *refer;
        }
        reference operator [] (difference_type d) const {
            return *refer[d];
        }
        const_iterator &operator ++ () {
            ++refer;
            return *this;
        }
        const_iterator operator ++ (int) {
            const_iterator i(*this);
            ++*this;
            return i;
        }
        const_iterator &operator -- () {
            --refer;
            return *this;
        }
        const_iterator operator -- (int) {
            const_iterator i(*this);
            --*this;
            return i;
        }
        friend bool operator == (const const_iterator &a, const const_iterator &b) {
            return a.refer == b.refer;
        }
        friend bool operator != (const const_iterator &a, const const_iterator &b) {
            return a.refer != b.refer;
        }
        friend bool operator < (const const_iterator &a, const const_iterator &b) {
            return a.refer < b.refer;
        }
        friend bool operator > (const const_iterator &a, const const_iterator &b) {
            return a.refer > b.refer;
        }
        friend bool operator <= (const const_iterator &a, const const_iterator &b) {
            return a.refer <= b.refer;
        }
        friend bool operator >= (const const_iterator &a, const const_iterator &b) {
            return a.refer >= b.refer;
        }
        friend const_iterator operator + (const const_iterator &i, difference_type d) {
            return const_iterator(i.refer + d);
        }
        friend const_iterator operator + (difference_type d, const const_iterator &i) {
            return const_iterator(i.refer + d);
        }
        friend const_iterator operator - (const const_iterator &i, difference_type d) {
            return const_iterator(i.refer - d);
        }
        const_iterator &operator += (difference_type d) {
            refer += d;
            return *this;
        }
        const_iterator &operator -= (difference_type d) {
            refer -= d;
            return *this;
        }
        friend difference_type operator - (const const_iterator &a, const const_iterator &b) {
            return a.refer - b.refer;
        }
    };
    template <class T, class Alloc> bool operator == (const deque<T,Alloc> &lhs, const deque<T,Alloc> &rhs) {
        return std::equal(lhs.cbegin(), lhs.cend(), rhs.cbegin(), rhs.cend());
    }
    template <class T, class Alloc> bool operator != (const deque<T,Alloc> &lhs, const deque<T,Alloc> &rhs) {
        return !(lhs == rhs);
    }
    template <class T, class Alloc> bool operator < (const deque<T,Alloc> &lhs, const deque<T,Alloc> &rhs) {
        return std::lexicographical_compare(lhs.cbegin(), lhs.cend(), rhs.cbegin(), rhs.cend());
    }
    template <class T, class Alloc> bool operator > (const deque<T,Alloc> &lhs, const deque<T,Alloc> &rhs) {
        return rhs < lhs;
    }
    template <class T, class Alloc> bool operator <= (const deque<T,Alloc> &lhs, const deque<T,Alloc> &rhs) {
        return !(rhs < lhs);
    }
    template <class T, class Alloc> bool operator >= (const deque<T,Alloc> &lhs, const deque<T,Alloc> &rhs) {
        return !(lhs < rhs);
    }
    template <class T, class Alloc> void swap(deque<T, Alloc> &lhs, deque<T, Alloc> &rhs) noexcept(noexcept(lhs.swap(rhs))) {
        lhs.swap(rhs);
    }
}
#endif // DEQUE_H_INCLUDED

I have few question:

1. Though the standard requires the complexity of std::push_back and std::push_front to be amortised O(1), my implementation has O(n).
   How can I get O(1)?

2. The standard makes strong exception guarantee for std::insert, std::emplace, etc.
   How can I implement that?

Answer 1

From what I understood you're storing a dynamic array of pointers to overcome the issue of invalidating iterators.

Complicated implementation:

The thing is that the implementation, quite frankly, overly complicated. I guess you felt it yourself. You could do it in a much easier way by just using std::vector<std::unique_ptr<>>, or even better, std::list<> (I never thought that I will suggest to use list, but whatever). Vector of pointers is a complicated synonym of list. It might have some beneficial features, but list is used in these cases. You'll have cache misses anyway. When vector resizes there is no danger to lose the data, because move constructor of std::unique_ptr<> is noexcept. I believe this way you can cut down the code by multiple tens of times. Also it will help you with question #1, since vector has amortization, and list is always constant time insertion at back (well, at least in practice).

For question #2, try to construct on heap. If throws, hell with it, let it propagate (container is not modified yet). If not, keep the pointer. Move all the pointers from that point to the right, and put the kept value into it's place, done. Also, checkout copy and swap idiom. It is the same way std::vector<> provides strong exception guarantee. Always learn idioms. Their feature is that their power rise with the power of the programmer. Use them wisely, and don't abuse them.

Yet there is a better idea:

I think now you might understand where I'm pointing to. Yes, implementation of g++ is quite good. Currently, your implementation will have one pointer per object memory overhead. When you store them in small contiguous arrays you'll get less memory overhead plus better cache consistency as a free bonus. This way they can guarantee that insertion/deletion from ends will not invalidate iterators. In fact, even cppreference page about deque says that they use list of fixed size arrays.

Since we got the biggest problem out of the way, lets have a look at smaller things:

• As long as you don't step into dark side no more than 50 lines for a function. You might want to slightly raise the limit. The move constructor can cause heart attack if someone is told to maintain it.

• Self defining friend functions inside of the class definitions make VC++ dizzy. Be careful.

• Storing them individually introduces more risk for cache misses. There is no point to use C++ if you don't take advantage of memory layout manipulation. Java/C# are great languages if you don't care much about performance.

• for SFINAE, you should use std::enable_if<>. It is much easier to use. std::conditiona_t<false, ...> is very strange control of compilation.

• ce[-1] makes me feel very uncomfortable.

Overall, your implementation seem to have all the needed features, just not implemented in the best way. Nevertheless, good job. Don't get stuck on this, you can do better.

Answer 2

I am not a decent C++ programmer myself, so I cannot help you with the question number 2. However, what comes to the first question, the answer is yes: you can (easily) guarantee amortized constant time for all pushes and pops from both the ends, and that is how:

1. Have an array or std::vector in your deque.
2. Have an explicit size integer. This will simplify a bit.
3. Have an index called, say, head_index. It is supposed to point to the front of your deque.
4. Have an index called, say, tail_index. It is supposed to point to the tail of your deque, or, also, it points into the location at which the next element will be appended.

Now suppose you deque is 5 elements long and contains 3 actual items: _ 1 2 3 _. After the next push_back(4) you get _ 1 2 3 4. After the next push_back(5), you "wrap around" and set it at beginning of the storage array: 5 1 2 3 4.

In other words, all you need is some modulo arithmetics.

All in all, I had this in mind:

#include <iostream>
#include <deque>
#include <stdexcept>

// We need this since the actual implementation of the operator % depends on the
// implementation.
int mod(int x, int m) {
    int r = x % m;
    return r < 0 ? r + m : r;
}

template<typename T>
class efficient_deque
{
public:
    efficient_deque()
    :
    storage{new T[4]},
    size{0},
    capacity{4},
    head_index{0},
    tail_index{0}
    {}

    ~efficient_deque()
    {
        delete[] storage;
    }

    void push_front(T elem)
    {
        ensure_capacity();
        storage[head_index = mod(head_index - 1, capacity)] = elem;
        size++;
    }

    void push_back(T elem)
    {
        ensure_capacity();
        storage[tail_index] = elem;
        tail_index = mod(tail_index + 1, capacity);
        size++;
    }

    T pop_front()
    {
        check_not_empty();
        T ret = storage[head_index];
        head_index = mod(head_index + 1, capacity);
        --size;
        return ret;
    }

    T pop_back()
    {
        check_not_empty();
        tail_index = mod(tail_index - 1, capacity);
        --size;
        return storage[tail_index];
    }

    T operator[](int i)
    {
        return storage[mod(i + head_index, capacity)];
    }

    size_t length()
    {
        return size;
    }

private:
    T* storage;
    size_t size;
    size_t capacity;
    size_t head_index;
    size_t tail_index;

    void ensure_capacity()
    {
        if (size == capacity)
        {
            T* new_storage = new T[2 * capacity];

            for (size_t i = 0; i < size; ++i)
            {
                new_storage[i] = storage[(head_index + i) % size];
            }

            delete[] storage;
            storage = new_storage;
            capacity *= 2;
            head_index = 0;
            tail_index = size;
        }
    }

    void check_not_empty()
    {
        if (size == 0)
        {
            throw std::runtime_error{"Popping an empty deque."};
        }
    }
};

bool equals(std::deque<int>& d, efficient_deque<int>& dd)
{
    if (d.size() != dd.length())
    {
        return false;
    }

    for (int i = 0; i != d.size(); ++i)
    {
        if (d.at(i) != dd[i])
        {
            return false;
        }
    }

    return true;
}

// Unit "test":
int main(int argc, const char * argv[]) {
    efficient_deque<int> d;
    std::deque<int> d2;

    srand(time(NULL));

    for (int i = 0; i < 10000; ++i)
    {
        int coin = rand() % 10;

        switch (coin) {
            case 0:
                if (d2.size() > 0)
                {
                    d2.pop_front();
                    d.pop_front();
                }

                break;

            case 1:
                if (d2.size() > 0)
                {
                    d2.pop_back();
                    d.pop_back();
                }

                break;

            case 2:
            case 3:
            case 4:
            case 5:

                d2.push_front(coin);
                d.push_front(coin);
                break;

            default:

                d2.push_back(coin);
                d.push_back(coin);
        }

        if (equals(d2, d) == false)
        {
            exit(10);
        }
    }

    std::cout << "done!" << std::endl;
    return 0;
}

Hope that helps.