gistfile1.cpp

std::mutex weakReferencesMutex;
std::unordered_multimap<Value*, std::atomic<Value*>> weakReferences;

class Value {
public:
    virtual ~Value() {
        std::lock_guard<decltype(weakReferencesMutex)> lock(weakReferencesMutex);
        for (auto range = weakReferences.equal_range(this); range.first != range.second; ++range.first) {
            range.first->second = nullptr;
        }
        weakReferences.erase(this);
    }

private:
    mutable std::atomic<long> referenceCount;

    friend void retain(Value const&);
    friend void release(Value const&);
};

std::atomic<Value*> const& obtainWeakReference(Value const& value) {
    std::lock_guard<decltype(weakReferencesMutex)> lock(weakReferencesMutex);
    return *weakReferences.emplace(&value, std::atomic<Value*>(&value));
}

void retain(Value const& value) {
    ++value.referenceCount;
}

void release(Value const& value) {
    if (--value.referenceCount == 0) {
        delete &value;
    }
}