Boost C++ Libraries: Ticket #9293: multi_index_container iterator needs base object size

attachment set

anonymous — Wed, 23 Oct 2013 04:25:06 GMT

attachment → test.cpp

use case demonstration

anonymous — Wed, 23 Oct 2013 04:30:40 GMT

I don't have the latest boost installed. If you do, please compile the attached file and let me know if it works. For me, g++ -std=c++0x -c test.c fails, but it works with -D USE_LIST.

status, type changed; resolution set

Joaquín M López Muñoz — Wed, 23 Oct 2013 06:12:21 GMT

status new → closed
type Bugs → Feature Requests
resolution → wontfix

Hi,

The reason your example fails to compile is that multi_index_container does not work on so-called incomplete types, i.e. it needs to know the size of the element type at instantiation time. This is not a bug per se, although the ability to work with incomplete types is of course a desireable one.

Unfortunately I don't expect adding this capability any time soon as it needs some heavy refactoring I don't have currently the time for. Maybe you can replace your code with

struct A;
struct B {
  const A* a_p;
};
struct A {
  const B* b_p;
};

and use iterator_to to retrieve an iterator from a pointer when needed:

http://www.boost.org/libs/multi_index/doc/tutorial/indices.html#iterator_to

Let me know if that works for you.

type changed

anonymous — Wed, 23 Oct 2013 14:38:49 GMT

type Feature Requests → Bugs

I understand, I didn't know about the completeness requirement, I thought I could just blindly replace std::list.

Now I wonder, how is iterator_to implemented? Is there a hidden index, indexed by address?

Joaquín M López Muñoz — Wed, 23 Oct 2013 15:29:44 GMT

I understand, I didn't know about the completeness requirement, I thought I could just blindly replace std::list.

In fact, if my memory serves me well there's no standard requirement that std::list can be instantiated with incomplete types, you happen to be using an stdlib implementation providing this nice-to-have feature.

Now I wonder, how is iterator_to implemented? Is there a hidden index, indexed by address?

There's no black magic here. iterator_to is used in the following context:

iterator it=c.iterator_to(*p);

So, you are providing the function with the node and the container it belongs to, this is all the info required to construct the iterator (not in a general case, but it happens to be so in Boost.MultiIndex.)