Re: [igraph] confused about vector pointer memory management

[Szabolcs Horvát]

Thank you for the reponse Tamas.

To explain the practical issue behind the question:

This came up while trying to create a C++ class to be used for RIAA
memory-management of vector_ptr's that contain (pointers to) vectors.

I originally asked about this here:

https://lists.nongnu.org/archive/html/igraph-help/2015-12/msg00027.html

But then I foolishly ignored Gabor's advice, and relied on the item
destructor in my C++ class anyway.  As I remember, the reason I did
this was that I was not sure that a vector_ptr that was handled by
some arbitrary igraph function will not end up with an item destructor
set.  Perhaps some igraph functions set item destructors.  I was
worried that if I destroy the items manually, I may end up with
double-destruction due to an existing item destructor (that I did not
set myself).

What actually happened was the opposite:

https://github.com/igraph/igraph/issues/993

An igraph function ended up double-destroying items because I passed
it a vector_ptr on which I set an item destructor.

Lesson: Never set an item destructor on a vector_ptr that is going to
be passed to a (high level) igraph function!

To fix this, I used this implementation of the class:

https://github.com/szhorvat/IGraphM/blob/master/IGraphM/LibraryResources/Source/IGCommon.h#L168

It is a bit ugly, but I believe it is safe.  It destroys the item
manually, and it uses only those functions which do not themselves
call item destructors.  This means that I could not use
igraph_vector_ptr_clear() at all, and had to re-implement it.  That is
not a big deal, of course, but it does involve messing with the
pointers inside of the igraph_vector_t.

On 10 May 2017 at 17:16, Tamas Nepusz <address@hidden> wrote:
> Hi,
>
>> In particular, I do not understand why igraph_vector_ptr_destroy_all()
>> will both destroy and free all items, but igraph_vector_ptr_clear()
>> will destroy them without freeing.
>
> You are totally right -- igraph_vector_ptr_clear() should probably call the
> element destructors AND free the items. The problem is that when we have
> originally implemented pointer vectors, it did not have any item destructors
> at all; they were added later as an "afterthought", and we did not get this
> part right.
>
> Actually, for me it seems like igraph_vector_ptr_destroy_all() and
> igraph_vector_ptr_free_all() should not have been necessary in the first
> place if we had item destructors from the very beginning. In that case, we
> could have simply said that igraph_vector_ptr_t vectors do not "own" their
> pointers and will not ever free them on their own -- all they could do is to
> call a "destructor" function on them when the items get out of the vector
> somehow. This way the user could have attached igraph_free as a destructor
> function for pointer vectors that claim ownership for any pointer added to
> them, and if the user needed something more complicated than free() (i.e.
> igraph_*_destroy() followed by igraph_free()), then this should have been
> done in a specialized destructor function on a case-by-case basis.
>
> The best way to avoid confusion for the time being is probably not to use
> item destructors at all, or at least ensure that whenever you pass an
> igraph_vector_ptr_t to some part of the code that you don't control, then
> ensure that the vector does not have an item destructor and that the vector
> is empty. This way you won't run into any unexpected side effects.
>
> I am leaning towards trying to fix this by ensuring that the item
> destructors get called when the items are removed from the vector using
> igraph_vector_ptr_clear(), but I'm not sure how this would affect
> third-party code. Personally, I don't use item destructors in the Python
> interface and the C interface also uses them sparingly and only in places
> where the pointer vectors equipped with destructors are not exposed to the
> user, so this shouldn't cause too much problems for the end users, except
> those who have been using the C interface directly and have used item
> destructors before.
>
> T.
>
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