allocator_traits rebind_alloc example

allocator_example.cpp

#include <memory>
#include <string>

// Example allocator, doesn't do anything but implements std::allocator_traits
template<typename T>
struct null_allocator {
  using value_type = T;

  null_allocator() {}

  template<typename U>
  null_allocator(const null_allocator<U>&) {}

  T* allocate(size_t size) {
    (void) size;
    return NULL;
  }

  void deallocate(T* ptr, size_t size) {
    (void) ptr;
    (void) size;
  }
};

template<typename T, typename U>
constexpr bool operator== (const null_allocator<T>&, const null_allocator<U>&) noexcept {
  return true;
}

template<typename T, typename U>
constexpr bool operator!= (const null_allocator<T>&, const null_allocator<U>&) noexcept {
  return false;
}

template<typename T>
using RebindAlloc = typename std::allocator_traits<null_allocator<void>>::template rebind_alloc<T>;

using MyString = std::basic_string<char, std::char_traits<char>, RebindAlloc<char>>;

int main(int /*argc*/, char ** /*argv*/)
{
  // check that null_allocator follows allocator_traits
  test::null_allocator<void> alloc;
  void * ptr = std::allocator_traits<null_allocator<void>>::allocate(alloc, 1);

  MyString string;
}

Can you tell me how to compile the example? It reported error when I compiled it in gcc 4.9.