Previously, we explored a basic implementation of unique_ptr in "Understanding the Inner Workings of C++ Smart Pointers - The unique_ptr." Now, let's enhance that model by incorporating a custom deleter, similar to what the Standard Library provides.

Understanding the Inner Workings of C++ Smart Pointers - The Unique_ptr with Custom Deleter

by Andreas Fertig

From the article:

Let's first establish why somebody would want a custom deleter.

One example is that the object was allocated via a local heap, and such must be returned by calling the corresponding deallocation function.

Another example is fopen. This function returns a FILE* object that you are supposed to delete by calling fclose. A classic job for a unique pointer. But you cannot call delete on the FILE pointer.

Here are two examples of using a unique_ptr with a custom deleter.

void MyDeleter(Object* ptr)
{
  delete ptr;
}

unique_ptr<Object> alfred{new Object{}};
static_assert(sizeof(alfred) == sizeof(void*));

unique_ptr<Object, decltype(MyDeleter)> robin{new Object{}, &MyDeleter};
static_assert(sizeof(robin) == sizeof(void*) * 2);

Oh yes, the first object, alfred, doesn't provide a custom deleter. Only robin does. Behind the curtains, both do. Let's look at a modified unique_ptr implementation that handles the custom deleter case.