Sunday, August 16, 2009

objects confined to call stack

Note on c++ -- unlike java, class objects can live on stack, and are automatically confined
--
Doug Lea's chapter on Confinement (within the Exclusion section) briefly explains the important type of object confined to a call stack. These are as thread-safe as an object instantiated as a local variable and never leaked outside the method.

Note (static or instance) fields constitute the other variables in java -- not local, so never confined to any call stack

Q: Take any snapshot of the object reference graph (a directed graph) as a stop-the-world garbage collector uses, will this object be reachable from outside this /method/?

If you *analyze* the source code and reach a answer of "No", then it's thread-safe. Now substitute /..../ with "call stack". The analysis becomes less obvious. It's possible for an object to be created in one method, passed around on the call stack, and stay confined to the call stack. See Doug Lea's 2.3.1.

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