Memory Leaks Outlast RAII, GC, and Rust Because Runtimes Can't Judge Intent

Why Programmers Still Can't Completely Solve Memory Leaks

Good morning, programming enthusiasts!

I've been thinking about a very interesting question recently.

We already have RAII in C++, GC in Java, Rust's ownership mechanism, and even all kinds of memory analysis tools. Why, after all these decades, can programmers still not completely solve memory leaks?

I think there is a question worth thinking about here:

Is a memory leak not only a matter of memory allocation and release, but also a matter of whether the program can accurately determine whether a piece of memory is still useful?

Take a moment to think about it, and let's continue.

What Is a Memory Leak

Let's look at two examples.

C++: Forgetting to free memory

void test() {
    int* p = new int(100);
}

When the function finishes executing, p disappears, but the memory on the heap is not freed.

This is the most classic memory leak: the program loses the way to access this memory, and there is no way to release it either.

This kind of problem already has quite mature solutions.

For example, Rust:

fn test() {
    let p = Box::new(100);
}

When the function returns normally, p is destroyed and the heap memory is automatically freed.

It looks like the problem is solved, right?

But let's look at the second situation.

Java: Memory remains referenced

import java.util.ArrayList;
import java.util.List;

class Cache {
    static List<byte[]> data = new ArrayList<>();

    static void add() {
        data.add(new byte[1024 * 1024]);
    }
}

Every call to add() adds a new 1 MiB (1024 × 1024 bytes) array, plus overhead for the objects and list management.

Even with a garbage collector, memory can keep growing.

Why?

Because these arrays can always be accessed through the static field data.

Of course, memory growth itself does not equal a leak. If this data is no longer needed but is still retained, that is what is usually called a logical memory leak.

A GC can tell whether an object is reachable, but it cannot tell whether the object is still useful to the business.

This brings us to the truly difficult part of memory leaks.

Memory Management Mechanisms Cannot Understand the Programmer's Intent

Kotlin: Suppose we have a cache

object Cache {
    val images = mutableMapOf<String, Bitmap>()
}

As the program runs, the images grow in number.

Now the question is:

When should a particular image be deleted?

It hasn't been accessed for 10 seconds?
Memory usage exceeds 100 MB?
The user closes the current page?
Or will this image never be accessed again?

These judgments all depend on the specific business.

A compiler, a GC, or an operating system can hardly know the answer automatically.

From a computer's perspective, as long as an object is still reachable, it may be accessed in the future.

But from a programmer's perspective, this object may have had no value for a long time.

Therefore:

The lifecycle of memory and the effective lifecycle of a business object are not always the same thing.

Rust Hasn't Solved This Problem Either

Rust is very good at solving problems where resource ownership is unclear.

But Rust does not guarantee that a program will never have memory leaks.

For example:

use std::rc::Rc;
use std::cell::RefCell;

struct Node {
    next: RefCell<Option<Rc<Node>>>,
}

fn main() {
    let a = Rc::new(Node {
        next: RefCell::new(None),
    });

    let b = Rc::new(Node {
        next: RefCell::new(None),
    });

    *a.next.borrow_mut() = Some(Rc::clone(&b));
    *b.next.borrow_mut() = Some(Rc::clone(&a));
}

This creates a reference cycle.

Even after main() returns and the local variables are destroyed, the two nodes still hold each other through Rc.

As a result, their strong reference counts cannot drop to zero, the nodes' destructors never run, and the heap memory is not freed through the normal destruction process. After the process exits, the operating system still reclaims the memory it occupied.

Rust provides Weak to break such cycles, but only if the programmer knows which reference relationship should not own the object.

This shows again that a language can help manage ownership, but it cannot necessarily decide for the programmer what a reasonable ownership relationship is.

In addition, Rust allows using std::mem::forget to deliberately prevent destruction.

By the way, memory leaks are not among the memory safety errors Rust promises to eliminate.

Why a Computer Can't Judge This Either

This problem is actually even more complex.

Kotlin: Suppose there is an object in the program

val data = createLargeObject()

while (someCondition()) {
    doSomething()
}

use(data)

Assume someCondition() and doSomething() never access data and never exit early through exceptions or other means.

During loop execution, will data be used in the future? That depends on whether the loop can eventually end.

If the loop never ends, data will never be used.

If the loop eventually ends, the data it needs cannot be casually discarded before the later use(data) executes.

Here is the problem: for an arbitrary program, we cannot design a general algorithm that accurately determines whether it will eventually terminate.

This is related to the famous halting problem in computer science.

Note, however, that the halting problem does not mean all memory leaks are undetectable. Many leaks can be found through static analysis, runtime analysis, or specific language constraints.

What it says is: if we demand a general algorithm that, for arbitrary programs and arbitrary execution conditions, accurately understands future behavior and whether an object is still useful, we run into a theoretical limit.

Can This Be Completely Solved in the Future?

On this question, I think several goals need to be distinguished:

Goal	Current situation
Avoid forgetting to call `free`	RAII, GC, and Rust have largely solved this
Avoid dangling pointers and double frees	Languages like Rust can provide strong safety guarantees
Automatically reclaim unreachable objects	Tracing GC can already do this
Automatically identify all useless but still reachable objects	No general, fully accurate solution exists
Guarantee that program memory never grows abnormally	Also involves cache strategy, business requirements, and resource budgets

So current memory management technology has actually solved a large number of mechanical problems.

But what has not been completely solved is: how long should an object actually live?

This decision often belongs to program design, not to the memory allocator.

One More Thing

This question can be pushed one step further.

GC and Rust's ownership mechanism are essentially answering two different questions:

GC: Can this object still be accessed?
Rust: Who owns this resource, and when should destruction run?

But there is a third question:

Should this object still exist?

The first two questions can be handled by relatively clear rules, while the third is closely tied to business semantics.

For that very reason, even if a stricter language than Rust appears in the future, or a garbage collector more advanced than current GCs, it does not mean memory leaks will completely disappear.

I even think programmers have not completely solved memory leaks not because we have not yet invented smart enough memory management algorithms, but because memory management mechanisms can understand the relationships between objects, but not necessarily the meaning of those objects' existence.