Return the old destructor pointer in mem_destructor()

[binarno]

mem_destructor() allows to set a referenced memory destructor.
However, a new destructor may still want to call the previously set destructor.

For instance, a new destructor may just want to perform some other action before continuing the destruction process as before (notifying some other object, logging, etc).

For this reason I propose to add a return value to mem_destructor so that the new destructor can simply continue the previous destruction process:

/**
 * Set or unset a destructor for a memory object
 *
 * @param data Memory object
 * @param dh   called when destroyed, NULL for remove
 * 
 * @return A pointer to the previously set destructor
 */
mem_destroy_h *mem_destructor(void *data, mem_destroy_h *dh)
{
    struct mem *m;

    if (!data)
        return NULL;

    m = get_mem(data);

    MAGIC_CHECK(m);

    mem_destroy_h *old_dh = m->dh;

    m->dh = dh;

    return old_dh;
}

[sreimers]

I'm not against this, but I wonder how helpful it really is. mem_destructor was recently introduced to add a destructor (independent of mem_alloc) at a later stage or reset, not really to replace a existing with a new one. You can always mem_ref within a destructor to avoid destructing (free) if you need multiple stages of destructing. Do you have a real use case for this? And can you show it?

[binarno]

In my case I need to extend the destruction process of a call structure with a method that perform some other cleanup and logging.
The library is being use to create a softphone in Qt (C++), and there are some C++ wrappers for the C structures.

Because the call's destructor already knows how to destroy the call structure and ideally is hidden from the client, I would not copy the default call destructor code into the new one, but I would have the new one perform its own things and then call the old destructor so it can destroy the call object.

By modifying mem_destructor to return a pointer to the previous destructor, the new one can do its own things and then simply call the previous destructor to take care of the complete removal of the structure.