Generic weak_ptr implementation.

test.cpp

#include "weak_ptr.h"
#include <cassert>

class T : public base::has_weak_ptr {
};

class NonEmpty {
  int a = 0;
};

class U : public NonEmpty, public base::has_weak_ptr {
};

int main(int argc, char *argv[]) {
  base::weak_ptr<T> test;
  base::weak_ptr<const T> constTest;
  base::weak_ptr<U> nonEmptyTest;
  {
    T object;
    U another;
    test = base::make_weak(&object); // Explicit creation.
    constTest = &object; // Implicit assignment.
    nonEmptyTest = &another; // Not first base class.
    assert(test.get() == &object);
    assert(constTest.get() == &object);
    assert(nonEmptyTest.get() == &another);
  }
  assert(test.get() == nullptr);
  assert(constTest.get() == nullptr);
  assert(nonEmptyTest.get() == nullptr);
  return 0;
}

weak_ptr.h

/*
Copyright (c) 2014-2018 John Preston, https://desktop.telegram.org
Granted to Public Domain
*/
#pragma once

#include <atomic>
#include <memory>

namespace base {

class has_weak_ptr;

namespace details {

struct alive_tracker {
	explicit alive_tracker(const has_weak_ptr *value) : value(value) {
	}

	std::atomic<int> counter = 1;
	std::atomic<const has_weak_ptr*> value;
};

inline alive_tracker *check_and_increment(alive_tracker *tracker) noexcept {
	if (tracker) {
		++tracker->counter;
	}
	return tracker;
}

inline void decrement(alive_tracker *tracker) noexcept {
	if (tracker->counter.fetch_sub(1) == 0) {
		delete tracker;
	}
}

} // namespace details

template <typename T>
class weak_ptr;

class has_weak_ptr {
public:
	has_weak_ptr() = default;
	has_weak_ptr(const has_weak_ptr &other) noexcept {
	}
	has_weak_ptr(has_weak_ptr &&other) noexcept {
	}
	has_weak_ptr &operator=(const has_weak_ptr &other) noexcept {
		return *this;
	}
	has_weak_ptr &operator=(has_weak_ptr &&other) noexcept {
		return *this;
	}

	~has_weak_ptr() {
		if (auto alive = _alive.load()) {
			alive->value.store(nullptr);
			details::decrement(alive);
		}
	}

private:
	template <typename Child>
	friend class weak_ptr;

	details::alive_tracker *incrementAliveTracker() const {
		auto current = _alive.load();
		if (!current) {
			const auto alive = new details::alive_tracker(this);
			if (_alive.compare_exchange_strong(current, alive)) {
				return alive;
			}
			delete alive;
		}
		++current->counter;
		return current;
	}

	mutable std::atomic<details::alive_tracker*> _alive = nullptr;

};

template <typename T>
class weak_ptr {
public:
	weak_ptr() = default;
	weak_ptr(T *value)
	: _alive(value ? value->incrementAliveTracker() : nullptr) {
	}
	weak_ptr(const std::unique_ptr<T> &value)
	: weak_ptr(value.get()) {
	}
	weak_ptr(const std::shared_ptr<T> &value)
	: weak_ptr(value.get()) {
	}
	weak_ptr(const std::weak_ptr<T> &value)
	: weak_ptr(value.lock().get()) {
	}
	weak_ptr(const weak_ptr &other) noexcept
	: _alive(details::check_and_increment(other._alive)) {
	}
	weak_ptr(weak_ptr &&other) noexcept
	: _alive(std::exchange(other._alive, nullptr)) {
	}
	template <
		typename Other,
		typename = std::enable_if_t<
			std::is_base_of_v<T, Other> && !std::is_same_v<T, Other>>>
	weak_ptr(const weak_ptr<Other> &other) noexcept
	: _alive(details::check_and_increment(other._alive)) {
	}
	template <
		typename Other,
		typename = std::enable_if_t<
			std::is_base_of_v<T, Other> && !std::is_same_v<T, Other>>>
	weak_ptr(weak_ptr<Other> &&other) noexcept
	: _alive(std::exchange(other._alive, nullptr)) {
	}

	weak_ptr &operator=(T *value) {
		reset(value);
		return *this;
	}
	weak_ptr &operator=(const std::unique_ptr<T> &value) {
		reset(value.get());
		return *this;
	}
	weak_ptr &operator=(const std::shared_ptr<T> &value) {
		reset(value.get());
		return *this;
	}
	weak_ptr &operator=(const std::weak_ptr<T> &value) {
		reset(value.lock().get());
		return *this;
	}
	weak_ptr &operator=(const weak_ptr &other) noexcept {
		if (_alive != other._alive) {
			destroy();
			_alive = details::check_and_increment(other._alive);
		}
		return *this;
	}
	weak_ptr &operator=(weak_ptr &&other) noexcept {
		if (_alive != other._alive) {
			destroy();
			_alive = std::exchange(other._alive, nullptr);
		}
		return *this;
	}
	template <
		typename Other,
		typename = std::enable_if_t<
			std::is_base_of_v<T, Other> && !std::is_same_v<T, Other>>>
	weak_ptr &operator=(const weak_ptr<Other> &other) noexcept {
		if (_alive != other._alive) {
			destroy();
			_alive = details::check_and_increment(other._alive);
		}
		return *this;
	}
	template <
		typename Other,
		typename = std::enable_if_t<
			std::is_base_of_v<T, Other> && !std::is_same_v<T, Other>>>
	weak_ptr &operator=(weak_ptr<Other> &&other) noexcept {
		if (_alive != other._alive) {
			destroy();
			_alive = std::exchange(other._alive, nullptr);
		}
		return *this;
	}

	~weak_ptr() {
		destroy();
	}

	T *get() const noexcept {
		const auto strong = _alive ? _alive->value.load() : nullptr;
		if constexpr (std::is_const_v<T>) {
			return static_cast<T*>(strong);
		} else {
			return const_cast<T*>(static_cast<const T*>(strong));
		}
	}
	explicit operator bool() const noexcept {
		return (_alive && _alive->value);
	}
	T &operator*() const noexcept {
		return *get();
	}
	T *operator->() const noexcept {
		return get();
	}

	void reset(T *value = nullptr) {
		if (get() != value) {
			destroy();
			_alive = value ? value->incrementAliveTracker() : nullptr;
		}
	}

private:
	void destroy() noexcept {
		if (_alive) {
			details::decrement(_alive);
		}
	}

	details::alive_tracker *_alive = nullptr;

	template <typename Other>
	friend class weak_ptr;

};

template <typename T>
inline bool operator==(const weak_ptr<T> &pointer, std::nullptr_t) noexcept {
	return (pointer.get() == nullptr);
}

template <typename T>
inline bool operator==(std::nullptr_t, const weak_ptr<T> &pointer) noexcept {
	return (pointer == nullptr);
}

template <typename T>
inline bool operator!=(const weak_ptr<T> &pointer, std::nullptr_t) noexcept {
	return !(pointer == nullptr);
}

template <typename T>
inline bool operator!=(std::nullptr_t, const weak_ptr<T> &pointer) noexcept {
	return !(pointer == nullptr);
}

template <
	typename T,
	typename = std::enable_if_t<std::is_base_of_v<has_weak_ptr, T>>>
weak_ptr<T> make_weak(T *value) {
	return value;
}

template <
	typename T,
	typename = std::enable_if_t<std::is_base_of_v<has_weak_ptr, T>>>
weak_ptr<T> make_weak(const std::unique_ptr<T> &value) {
	return value;
}

template <
	typename T,
	typename = std::enable_if_t<std::is_base_of_v<has_weak_ptr, T>>>
weak_ptr<T> make_weak(const std::shared_ptr<T> &value) {
	return value;
}

template <
	typename T,
	typename = std::enable_if_t<std::is_base_of_v<has_weak_ptr, T>>>
weak_ptr<T> make_weak(const std::weak_ptr<T> &value) {
	return value;
}

} // namespace base