Discontiguous Allocator

#include <cstddef>
#include <iostream>
#include <iterator>
#include <limits>
#include <memory>

template <typename T>
class ScaledPointer : public std::iterator<std::random_access_iterator_tag, T> {
  T* ptr;
public:
  ScaledPointer() = default;
  ScaledPointer(T* ptr) : ptr(ptr) {}
  template <typename U>
  explicit ScaledPointer(U* ptr) : ptr(static_cast<T*>(ptr)) {}
  template <typename U>
  explicit ScaledPointer(const ScaledPointer<U>& other) :
    ptr(static_cast<T*>(other.ptr)) {}

  explicit operator bool () const { return bool{ptr}; }

  T& operator * () const {
    return *ptr;
  }
  T* operator -> () const {
    return ptr;
  }

  T& operator [] (std::ptrdiff_t n) const {
    return ptr[2 * n];
  }

  ScaledPointer& operator ++ () {
    ptr += 2;
    return *this;
  }
  ScaledPointer operator ++ (int) {
    ScaledPointer tmp(*this);
    ++*this;
    return tmp;
  }

  ScaledPointer& operator -- () {
    ptr -= 2;
    return *this;
  }
  ScaledPointer operator -- (int) {
    ScaledPointer tmp(*this);
    --*this;
    return tmp;
  }

  template <typename U, typename V>
  friend bool operator == (const ScaledPointer<U>& u, const ScaledPointer<V>& v) {
    return u.ptr == v.ptr;
  }
  template <typename U, typename V>
  friend bool operator != (const ScaledPointer<U>& u, const ScaledPointer<V>& v) {
    return !(u == v);
  }

  template <typename U, typename V>
  friend bool operator < (const ScaledPointer<U>& u, const ScaledPointer<V>& v) {
    return u.ptr < v.ptr;
  }
  template <typename U, typename V>
  friend bool operator > (const ScaledPointer<U>& u, const ScaledPointer<V>& v) {
    return v < u;
  }
  template <typename U, typename V>
  friend bool operator <= (const ScaledPointer<U>& u, const ScaledPointer<V>& v) {
    return !(v < u);
  }
  template <typename U, typename V>
  friend bool operator >= (const ScaledPointer<U>& u, const ScaledPointer<V>& v) {
    return !(u < v);
  }

  ScaledPointer& operator += (std::ptrdiff_t n) {
    ptr += 2 * n;
    return *this;
  }
  friend ScaledPointer operator + (const ScaledPointer& u, std::ptrdiff_t n) {
    ScaledPointer tmp = u;
    tmp += n;
    return tmp;
  }


  ScaledPointer& operator -= (std::ptrdiff_t n) {
    ptr -= 2 * n;
    return *this;
  }
  friend ScaledPointer operator - (const ScaledPointer& u, std::ptrdiff_t n) {
    ScaledPointer tmp = u;
    tmp -= n;
    return tmp;
  }

  friend std::ptrdiff_t operator - (const ScaledPointer& a, const ScaledPointer& b) {
    return (a.ptr - b.ptr) / 2;
  }
};

template <typename T>
class ScaledAllocator {
public:
  typedef ScaledPointer<T> pointer;
  typedef T value_type;
  typedef std::size_t size_type;

  pointer allocate(size_type n) {
    const std::size_t size = (n * (2 * sizeof(T)));
    void* p = ::operator new(size);
    std::cout << __FUNCTION__ << '(' << n << ") = " << p << std::endl;
    std::fill_n((unsigned*)p, size / sizeof(unsigned), 0xFEEDFACEU);
    return pointer{p};
  }

  void deallocate(pointer p, size_type n) {
    std::cout << __FUNCTION__ << '(' << &*p << ", " << n << ')' << std::endl;
    ::operator delete(&*p);
  }

  static size_type max_size() {
    return std::numeric_limits<size_type>::max() / 2;
  }

  template <typename U, typename V>
  friend bool operator == (const ScaledAllocator<U>&, const ScaledAllocator<V>&) {
    return true;
  }
  template <typename U, typename V>
  friend bool operator != (const ScaledAllocator<U>&, const ScaledAllocator<U>&) {
    return false;
  }
};

#include <algorithm>
#include <vector>

int main() {
  using namespace std;
  cout << hex << showbase;

  vector<unsigned, ScaledAllocator<unsigned>> vec = {0,1,2,3,4};
  for_each(begin(vec), end(vec), [](unsigned i){ cout << i << ' '; });
  cout << endl;

  auto p = vec.data();
  for(auto i = decltype(vec.size()){0}, n = vec.size(); i < n; ++i)
    cout << p[i] << ' ';
  cout << endl;
}