Re: [Boost-users] Forward declarations and iterator_facade<>

On Fri, Oct 15, 2010 at 11:02 AM, David Abrahams wrote:

At Fri, 15 Oct 2010 17:24:42 +0100, Phil Endecott wrote:

...

Dear Experts,

I have code something like this:

struct S;

class iter:    public boost::iterator_facade { ... };

struct S {    ... };

This used to work, but stopped recently; I believe it may have been after upgrading gcc, not after upgrading Boost itself, but I'm uncertain about that.  The problem is that something inside iterator_facade calls boost::is_POD<S>, which (not unreasonably) doesn't seem to work for the forward-declared S.

I've never been entirely sure what you're allowed to do with forward-declared types like that.  Was I just lucky that it worked before, or is this a regression?

I guess you were just lucky, because we never tested for or intended to provide that capability.

...

Re-ordering the code to remove the circular dependencies will be painful, so if anyone has any other hints I would love to hear them.

I'm afraid there's not going to be any other choice for you unless we are somehow able to add that capability in Boost and you are able to wait for it.

Shouldn't he be able to just specialize boost::is_pod on his forward declared type as true?