How to Implement a Garbage Collector in C++

Implementing a garbage collector (GC) in C++ is a complex but educational task that provides deep insights into memory management, compilers, and low-level systems. Since C++ doesn’t have a built-in garbage collector like Java or Python, any GC mechanism must be manually integrated. The most common approaches include reference counting, tracing (mark-and-sweep), or even integrating a third-party GC library.

1. Understanding the Need for a Garbage Collector

C++ uses manual memory management through new and delete. While this provides fine-grained control, it increases the risk of memory leaks, dangling pointers, and double deletions. A GC automates memory cleanup by identifying and reclaiming unused memory, reducing the programmer’s burden and making applications more robust.

2. Core Components of a Garbage Collector

A basic garbage collector generally includes the following components:

• Memory allocation mechanism to track all allocated objects.

• Root set identification to find starting points of reachable objects.

• Object graph traversal to mark reachable memory.

• Memory reclamation to clean up unreachable memory.

3. Choosing the GC Strategy

Reference Counting

Each object keeps a count of how many references point to it. When the count drops to zero, the object is deleted.

Pros:

• Immediate cleanup.

• Simple to implement.

Cons:

• Fails to collect cyclic references.

• Overhead of maintaining counters.

Tracing Garbage Collection

Includes algorithms like Mark-and-Sweep or Mark-and-Compact.

Pros:

• Handles cycles.

• Full control over collection timing.

Cons:

• More complex to implement.

• Requires stopping the world or multithreaded handling.

4. Implementing Reference Counting in C++

cpp
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class RefCounted {
private:
    int refCount;

public:
    RefCounted() : refCount(0) {}

    void addRef() {
        ++refCount;
    }

    void releaseRef() {
        if (--refCount == 0) {
            delete this;
        }
    }

    virtual ~RefCounted() {}
};

template<typename T>
class SmartPointer {
private:
    T* ptr;

public:
    SmartPointer(T* p = nullptr) : ptr(p) {
        if (ptr) ptr->addRef();
    }

    SmartPointer(const SmartPointer<T>& other) : ptr(other.ptr) {
        if (ptr) ptr->addRef();
    }

    ~SmartPointer() {
        if (ptr) ptr->releaseRef();
    }

    T& operator*() { return *ptr; }
    T* operator->() { return ptr; }
};

This is a basic implementation and lacks thread safety or weak pointer support.

5. Implementing a Mark-and-Sweep Collector

A minimal mark-and-sweep collector requires:

• A memory allocator.

• A list of allocated objects.

• A method to find all root references.

• Marking reachable objects.

• Sweeping unreachable ones.

Step 1: Object Base Class

cpp
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class GCObject {
public:
    bool marked = false;
    virtual void trace(std::function<void(GCObject*)>) = 0;
    virtual ~GCObject() = default;
};

Step 2: GC Manager

cpp
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class GCManager {
private:
    std::unordered_set<GCObject*> objects;
    std::unordered_set<GCObject*> roots;

public:
    void registerObject(GCObject* obj) {
        objects.insert(obj);
    }

    void addRoot(GCObject* root) {
        roots.insert(root);
    }

    void removeRoot(GCObject* root) {
        roots.erase(root);
    }

    void collect() {
        // Mark phase
        for (GCObject* root : roots) {
            mark(root);
        }

        // Sweep phase
        for (auto it = objects.begin(); it != objects.end();) {
            if (!(*it)->marked) {
                delete *it;
                it = objects.erase(it);
            } else {
                (*it)->marked = false;
                ++it;
            }
        }
    }

private:
    void mark(GCObject* obj) {
        if (obj->marked) return;
        obj->marked = true;
        obj->trace([this](GCObject* child) { mark(child); });
    }
};

Step 3: Example of Traced Object

cpp
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class MyObject : public GCObject {
public:
    GCObject* child = nullptr;

    void trace(std::function<void(GCObject*)> visitor) override {
        if (child) visitor(child);
    }
};

You must call gc.registerObject(new MyObject()) and use gc.addRoot(obj) where appropriate.

6. Integrating Allocation with the GC

To tie object creation with garbage collection tracking:

cpp
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template <typename T, typename... Args>
T* gcNew(GCManager& gc, Args&&... args) {
    T* obj = new T(std::forward<Args>(args)...);
    gc.registerObject(obj);
    return obj;
}

Use MyObject* obj = gcNew<MyObject>(gcManager); for allocation.

7. Handling Cycles with Tracing GC

Unlike reference counting, mark-and-sweep detects cycles by design. This makes it suitable for applications where object graphs are complex and interconnected.

8. Performance Considerations

• Pause time: GC often requires a stop-the-world approach which can affect real-time systems.

• Incremental GC: Breaks up the GC process to reduce pause time.

• Generational GC: Divides objects into generations, collecting young objects more frequently.

These advanced techniques require more complex object tracking and heuristics.

9. Alternative: Use Existing GC Libraries

Instead of building from scratch, you can integrate with existing libraries like:

• Boehm-Demers-Weiser GC: Conservative GC for C/C++.

• Microsoft’s C++/CLI GC: Available only on .NET.

Example with Boehm GC:

cpp
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#include <gc/gc.h>

int main() {
    GC_INIT();
    int* arr = (int*)GC_MALLOC(sizeof(int) * 100);
    arr[0] = 42;
}

This requires linking with the Boehm GC library.

10. Conclusion

Creating a garbage collector in C++ teaches deep insights into memory management. Starting with reference counting provides immediate benefits, while tracing GCs offer robust long-term solutions at the cost of complexity. For production use, combining smart pointers (std::shared_ptr, std::unique_ptr) with third-party GC libraries can balance safety and performance.