Why is there no std::allocate_unique function in C++14?

Question

Why does shared_ptr have allocate_shared while unique_ptr does not have allocate_unique?
I would like to make a unique_ptr using my own allocator: do I have to allocate the buffer myself and then assign it to a unique_ptr?
This seems like an obvious idiom.

Answer 1

Why does shared_ptr have allocate_shared while unique_ptr does not have allocate_unique?

shared_ptr needs it so that it can allocate its internal shared state (the reference count and deleter), as well as the shared object, using the allocator. unique_ptr only manages the object; so there is no need to provide an allocator to the unique_ptr itself, and less need for an allocate function.

_{(There is also less need for make_unique for the same reason, which is probably why it didn't feature in C++11, but was added to C++14 by popular demand for the sake of consistency. Maybe the same demand will add allocate_unique to a future standard.)}

do I have to allocate the buffer myself and then assign it to a unique_ptr?

Yes. Or you could write your own allocate_unique; unlike allocate_shared, it's possible, and reasonably straightforward, to implement it separately from unique_ptr itself. (As mentioned in the comments, you'd have to make sure it used an appropriate deleter for the allocator; the default deleter would use delete and go horribly wrong).

This seems like an obvious idiom.

Indeed. But so are many other idioms, and not everything can (or should) be standardised.

For a more official justification of the current lack of allocate_unique, see the proposal for make_unique, specifically section 4 (Custom Deleters).

Answer 2

do I have to allocate the buffer myself and then assign it to a unique_ptr?

Not just a buffer, a pointer to an object. But the object might need to be destroyed by the allocator, and the memory definitely needs to deallocated by the allocator, so you also need to pass the allocator the the unique_ptr. It doesn't know how to use an allocator, so you need to wrap it in a custom deleter, and that becomes part of the unique_ptr's type.

I think a generic solution would look something like this:

#include <memory>

template<typename Alloc>
struct alloc_deleter
{
  alloc_deleter(const Alloc& a) : a(a) { }

  typedef typename std::allocator_traits<Alloc>::pointer pointer;

  void operator()(pointer p) const
  {
    Alloc aa(a);
    std::allocator_traits<Alloc>::destroy(aa, std::addressof(*p));
    std::allocator_traits<Alloc>::deallocate(aa, p, 1);
  }

private:
  Alloc a;
};

template<typename T, typename Alloc, typename... Args>
auto
allocate_unique(const Alloc& alloc, Args&&... args)
{
  using AT = std::allocator_traits<Alloc>;
  static_assert(std::is_same<typename AT::value_type, std::remove_cv_t<T>>{}(),
                "Allocator has the wrong value_type");

  Alloc a(alloc);
  auto p = AT::allocate(a, 1);
  try {
    AT::construct(a, std::addressof(*p), std::forward<Args>(args)...);
    using D = alloc_deleter<Alloc>;
    return std::unique_ptr<T, D>(p, D(a));
  }
  catch (...)
  {
    AT::deallocate(a, p, 1);
    throw;
  }
}

int main()
{
  std::allocator<int> a;
  auto p = allocate_unique<int>(a, 0);
  return *p;
}