66 lines
2.6 KiB
Markdown
66 lines
2.6 KiB
Markdown
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---
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**WHAT:**
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This is an example of Bob Nystrom's
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"[Baby's First Garbage Collector](http://journal.stuffwithstuff.com/2013/12/08/babys-first-garbage-collector/),"
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which I've been wanting to implement for a while in order to understand
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it better. To make the problem harder (as I always do), I decided to
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write it in C++, and to make it even more fun, I've implemented it using
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the new Variant container from C++17.
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**WHY:**
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I've never written a garbage collector before. Now I know what it is
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and how it works.
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**DETAILS:**
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The collector Nystrom wrote is a simple bi-color mark-and-sweep
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collector for a singly threaded process with distinct pauses based upon
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a straightforward memory usage heuristic. That heuristic is simply, "Is
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the amount of memory currently in use twice as much as the last time we
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garbage collected?" If the answer is yes, the collector runs and sweeps
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up the stack.
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Nystrom's code is highly readable, and I hope mine is as well. Because
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I used Variant, my Object class has an internal Pair class, and then the
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Variant is just \<int, Pair\>, where "Pair" is a pair of pointers to
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other objects. The entirety of the VM is basically a stack of
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singly-linked lists which either represents integers or collections of
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integers in a Lisp-like structure.
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The allocator creates two kinds of objects, then: pairs, and lists. A
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pair is created by pushing two other objects onto the stack, then
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calling `push()`, which pops them off the stack and replaces them with a
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Pair object. The VM class has two methods, both named `push()`, one of
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which pushes an integer, the other a pair. Since a pair is built from
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objects on the stack, the Pair version takes no arguments, and since
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C++14 and beyond have move semantics that Variant honors,
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Variant\<Pair\> only constructs a single pair. Pretty nice. I was also
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able to use both lambda-style and constructor-style visitors in my
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Variant, which was a fun little bonus.
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**NOTE:**
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I have included the header files for the
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[Mapbox version of Variant](https://github.com/mapbox/variant) since the
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C++17 committee's standards haven't quite reached the general public and
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the Variant implementation is still a subject of some debate. This
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implementation looks straightforward enough and is a header-only
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release. It works with both GCC 4.8.5 and Clang 3.8, and that's good
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enough for me.
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The Mapbox variant is BSD licensed, and a copy of the license is
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included in the Include directory.
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**BUILDING:**
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From the base directory of the project:
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mkdir build
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cd build
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cmake ..
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make
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And you should be able to run the basic tests. It's just one file.
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