Re: [Boost-users] boost shared_ptr and parent child relation ships

From: Nat Goodspeed (nat_at_[hidden])
Date: 2008-06-20 17:10:15

gast128 wrote:

> you can not make a cycle in shared_ptrs between 
> parent -> child and child -> parent. Thus now we use a weak_ptr for child -> 
> parent, but actually the child can not exist without parent. So if one shares 
> a child  through a shared_ptr, a hypothetical situation could exist that its 
> parent get destroyed, but a child lifes on (in an invalid state due to missing 
> parent) because a client has requested it.

Are consumers of this structure more likely to hold pointers to the 
parent or to a child?

It sounds to me as though you want more direct control over the 
lifespans of parent and child. What if the parent's destructor were to 
delete all children outright? Then you'd never have orphans.

You could hand out shared_ptrs to the parent object, so that it would 
persist as long as it had any consumers. As soon as the last consumer 
forgot about it, it would clean up all its own children.

With that arrangement, you'd probably want direct consumers of the 
children to get weak_ptrs rather than shared_ptrs. That way a consumer 
could tell if the child had vanished.

The parent should then hold the only shared_ptr to each child. (I'd 
suggest that the parent track its children with dumb child* pointers, 
but you need a shared_ptr on which to base your weak_ptrs.)

And -- since you now know that the existence of a child implies the 
existence of its parent -- you could use dumb parent* pointers for the 
back-references from child to parent. That eliminates the need for the 
parent to obtain shared_from_this at constructor time.

Does this address your constraints?