Garbage Collection – Need for Garbage Collection in a Stack-Based Language

So, what's the point of using garbage collection?

I'm assuming you mean reference counted smart pointers and I'll note that they are a (rudimentary) form of garbage collection so I'll answer the question "what are the advantages of other forms of garbage collection over reference counted smart pointers" instead.

• Accuracy. Reference counting alone leaks cycles so reference counted smart pointers will leak memory in general unless other techniques are added to catch cycles. Once those techniques are added, reference counting's benefit of simplicity has vanished. Also, note that scope-based reference counting and tracing GCs collect values at different times, sometimes reference counting collects earlier and sometimes tracing GCs collect earlier.

• Throughput. Smart pointers are one of the least efficient forms of garbage collection, particularly in the context of multi-threaded applications when reference counts are bumped atomically. There are advanced reference counting techniques designed to alleviate this but tracing GCs are still the algorithm of choice in production environments.

• Latency. Typical smart pointer implementations allow destructors to avalanche, resulting in unbounded pause times. Other forms of garbage collection are much more incremental and can even be real time, e.g. Baker's treadmill.

The garbage collector does scan the stack -- to see what things in the heap are currently being used (pointed to) by things on the stack.

It makes no sense for the garbage collector to consider collecting stack memory because the stack is not managed that way: Everything on the stack is considered to be "in use." And memory used by the stack is automatically reclaimed when you return from method calls. Memory management of stack space is so simple, cheap and easy that you wouldn't want garbage collection to be involved.

(There are systems, such as smalltalk, where stack frames are first-class objects stored in the heap and garbage collected like all other objects. But that's not the popular approach these days. Java's JVM and Microsoft's CLR use the hardware stack and contiguous memory.)