Returning pointers in a thread-safe way.

Question

Assume I have a thread-safe collection of Things (call it a ThingList), and I want to add the following function.

Thing * ThingList::findByName(string name)
{
  return &item[name]; // or something similar..
}

But by doing this, I've delegated the responsibility for thread safety to the calling code, which would have to do something like this:

try 
{
  list.lock(); // NEEDED FOR THREAD SAFETY
  Thing *foo = list.findByName("wibble");
  foo->Bar = 123;
  list.unlock();  
}
catch (...) 
{
  list.unlock();
  throw;
} 

Obviously a RAII lock/unlock object would simplify/remove the try/catch/unlocks, but it's still easy for the caller to forget.

There are a few alternatives I've looked at:

• Return Thing by value, instead of a pointer - fine unless you need to modify the Thing
• Add function ThingList::setItemBar(string name, int value) - fine, but these tend to proliferate
• Return a pointerlike object which locks the list on creation and unlocks it again on destruction. Not sure if this is good/bad practice...

What's the right approach to dealing with this?

Answer 1

There is no one "right approach"; it depends on the needs of your application.

If at all possible, return things by value, or return a copy that the caller can do whatever they want with.

A variant of the above is to return a modifiable copy, and then provide a way to atomically merge a modified object back into the list. Something like:

Thing t = myThingList.getThing(key);
t.setFoo(f);
t.setBar(b);
myThingList.merge(t);     // ThingList atomically updates the appropriate element

However, that can cause problems if multiple threads try to update the same object.

The "pointerlike object" idea sounds cool, but I suspect it would lead to hard-to-find bugs when some lock isn't getting released somewhere.

I'd try to keep all the locking/unlocking code within ThingList, so the ThingList::set... functions are probably what I would do.

Answer 2

store and return boost::shared_ptr s

you have to lock during access but you are safe after the unlock