[Condor-devel] shared_ptr in classads

[Dan Bradley <dan@xxxxxxxxxxxx>]

Hi all,

In order to fix the following memory leak in classads

https://condor-wiki.cs.wisc.edu/index.cgi/tktview?tn=3082

I would have liked to use shared_ptr for pointers to ExprList and 
ClassAd objects in the classad Value object.  This would allow us to 
have ClassAd functions that return dynamically generated lists and ads.

Both ExprList and ClassAd derive from ExprTree and are frequently 
pointed to via a pointer to the base class, so one approach would be to 
use shared_ptr for all places where ExprTree objects are owned in the 
ClassAd library.  I have several concerns with this:

1. extra heap allocation for the reference counter for every ExprTree 
(i.e. every little piece of every classad expression)

2. impact on public interfaces

I am concerned that if we change all the public interfaces, we will end 
up having to change ClassAd* to shared_ptr<ClassAd> throughout much of 
condor.  I think this is reasonably straightforward and probably the 
"right" thing to do, but touching that much code isn't ideal.  I would 
want to know that everyone is wildly enthusiastic first.

One alternative to the big conversion to shared_ptr throughout condor is 
to just add another type within the Value object: an "owned" list.  When 
you want to put a list into a Value, you would have to choose between 
giving the Value object ownership or not. It's not as nice of an 
interface, but we don't often write new classad functions that create 
dynamic lists.

After playing with the shared_ptr approach, I have concluded that unless 
I hear that people are gung-ho about converting to shared_ptr throughout 
Condor, the minimal approach is better for now.

Thoughts?

--Dan