What is unique_ptr::deleter_type::pointer for?

Question

std::unique_ptr<T,D> is specified to store not a T* as you might expect, but an object of type std::unique_ptr<T,D>::pointer . This is defined to be basically D::pointer if such a type exists, and T* otherwise. Thus, you can customize the underlying raw pointer type by customizing your deleter appropriately.

When is it a good idea to do this? What is it used for? The only discussion I've been able to find is this note , which alludes to "better support[ing] containers and smart pointers in shared memory contexts", but that doesn't exactly shed a lot of light.

Answer 1

It is used when the deleter does not operate on T* values, obviously. That is why the deleter can specify a different data type than T* . A common use case is Win32 handles:

Using std::unique_ptr for Windows HANDLEs

Answer 2

The original motivation was to enable the use of boost::offset_ptr as the representation under unique_ptr , which would enable the use of unique_ptr in process-shared memory. Structures in process shared-memory should not contain pointers or references, only offsets.

I'm pleased to learn that the same feature can be useful in the Windows API.