How Garbage Collection Works: Let's Build One From Scratch

Introduction Your program keeps creating objects. Every function call, every loop iteration, every parsed JSON response produces new ones. You don't manually delete most of them. You've never written a line of code that says "free this memory now." And yet your application doesn't immediately exhaust all available RAM and crash. So who cleans everything up? The answer is a garbage collector, a piece of the runtime that runs quietly in the background, deciding what your program no longer needs and reclaiming that memory for future use. Most developers interact with it only when something goes wrong: an unexpected pause, a memory leak, or an out-of-memory error that shouldn't be happening. Understanding how it actually works turns those confusing moments into solvable problems. And as a bonus, the core algorithm is simple enough to build yourself. We'll do that by the end of this article. -- 1. The Memory Problem Every time your program creates an object, the runtime allocates a chunk of memory to hold it. A string, a dictionary, a class instance: they all need memory, and that memory has to come from somewhere. The somewhere is a region called the heap , a pool of memory that the program draws from as it runs. When you create an object, the runtime finds a suitable slot in the heap and reserves it. When that object is no longer needed, that slot should be freed so it can be used for something else. In languages like C, you manage this manually. You allocate memory when you need it, and you free it when you're done. This gives you control, but it creates two classic failure modes. Free memory too early and you have a dangling pointer, a reference to memory that's now being used for something else. Forget to free it at all and you have a memory leak: the program slowly consumes more and more memory until it runs out. Automatic memory management exists to eliminate these failure modes. Instead of relying on the programmer to track every allocation and release, the runti