Reflections on Garbage Collection

By Danny Kalev, ITworld |  How-to

Many C/C++ programmers think of automatic garbage collection as the
Holy Grail of programming languages; one that will free them from every
conceivable bug. This approach is wrong, though. A garbage collector
can't handle every resource that might leak.

A typical garbage collector takes care of recycling unreferenced
memory. In certain programming languages, it can reclaim leaked threads
and even close open files. However, many other platform- and
application-specific resources still need to be released explicitly by
the programmer.

Think of a modem. An application that uses it must release it
explicitly so that other applications can use it; how many garbage
collectors are modem-aware? Take a database lock, for example. Here
again, you can't expect a garbage collector to close an unused database
lock automatically. Indeed, most RDBM systems set a timeout limit on
each transaction but relying on timeouts instead of closing a lock as
soon as possible results in poor performance and constant deadlocks).
Thus, a garbage collector can be useful as long as it takes care of
reclaiming unreferenced memory. When it comes to reclaiming other time-
critical or scarce system resources, you still need to release them
explicitly.

When you think of dynamically allocated objects as an instance of such
resources, you may realize that garbage collection isn't really that
useful after all. Furthermore, using standard C++, you almost never
need to deal with dynamic memory directly. Instead of using char
arrays, bare pointers and dynamic memory allocation, use string
objects, auto_ptr template, and STL containers. These classes manage
their storage automatically. Note also that, in many cases, you can
create objects on the stack rather than allocate them on the heap.

That said, for certain applications (especially ones developed in C),
you could install a free garbage collector such as the Boehm-Demers-
Weiser conservative garbage collector (see
http://www.hpl.hp.com/personal/Hans_Boehm/gc/). Unlike a copying
garbage collector, a conservative garbage collector doesn't move
objects from one memory location to another. It uses constant sized
memory chunks. This way, the heap doesn't become fragmented.

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