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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.
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Use unique_ptr when you want to have single ownership(Exclusive) of the resource. Only one unique_ptr can point to one resource. Since there can be one unique_ptr for single resource its not possible to copy one unique_ptr to another.
An explicit delete for a unique_ptr would be reset() . But do remember that unique_ptr are there so that you don't have to manage directly the memory they hold. That is, you should know that a unique_ptr will safely delete its underlying raw pointer once it goes out of scope.
std::unique_ptr::reset Destroys the object currently managed by the unique_ptr (if any) and takes ownership of p. If p is a null pointer (such as a default-initialized pointer), the unique_ptr becomes empty, managing no object after the call.
unique_ptr objects automatically delete the object they manage (using a deleter) as soon as they themselves are destroyed, or as soon as their value changes either by an assignment operation or by an explicit call to unique_ptr::reset.
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.
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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
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