Returning pointers in a thread-safe way
Assume I have a thread-safe collection of Things (call it a ThingList), and I want to add the following function.
开发者_开发问答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?
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.
store and return boost::shared_ptr s
you have to lock during access but you are safe after the unlock
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