C++20协程解糖

std::future和promise在C++20里面没法直接用的唯一原因就是不支持then，虽然MSVC有一个弱智版开线程阻塞实现的future.then，能then了但不保序，而且libstdc++也用不了。folly之类的库有靠谱的实现，但是功能太齐全太复杂，不适合新手学习。因此我们先从弱智版future promise schedular开始，从源头讲解如何实现协程相关设施。

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基本结构

我们要实现的功能很简单：

1. 单线程模型
2. promise是入口，future是出口
3. promise支持set_result, get_future
4. future支持add_finish_callback，在promise set_result之后按序调用
5. callback在下次调度时调用而不是立即调用

结构大概是这样

状态都存在shared_state里面，future和promise实际上只是个空壳

先搭框架

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首先是shared_state，最直观的，shared_state需要存储最终设置的结果T，以及记录结果有没有设置。这里要求T必须支持默认构造，省事。

---

template<class T>
class SharedState {
    friend class Future<T>;
    friend class Promise<T>;
public:
    SharedState()
    {}
    SharedState(const SharedState&) = delete;
    SharedState(SharedState&&) = delete;
    SharedState& operator=(const SharedState&) = delete;
    SharedState& operator=(SharedState&&) = delete;

private:
    template<class U>
    void set(U&& v) {
        if (settled) {
            return;
        }
        settled = true;
        value = std::forward<U>(v);
    }

    bool settled = false;
    T value;
};

---

然后是引用了shared_state的promise和future

---

template<class T>
class Promise {
public:
    Promise()
        : _state(std::make_shared<SharedState<T>>())
    {}

    Future<T> get_future();

    template<class U>
    void set_result(U&& value) {
        if (_state->settled) {
            throw std::invalid_argument("already set result");
        }
        _state->set(std::forward<U>(value));
    }
private:
    std::shared_ptr<SharedState<T>> _state;
};

template<class T>
class Future {
    friend class Promise<T>;
private:
    Future(std::shared_ptr<SharedState<T>> state)
        : _state(std::move(state))
    {
    }
private:
    std::shared_ptr<SharedState<T>> _state;
};

template<class T>
Future<T> Promise<T>::get_future() {
    return Future<T>(_state);
}

---

先把调度器的壳写上，以后会用到

---

class Schedular {
    template<class T>
    friend class SharedState;
public:
    Schedular() = default;
    Schedular(Schedular&&) = delete;
    Schedular(const Schedular&) = delete;
    Schedular& operator=(Schedular&&) = delete;
    Schedular& operator=(const Schedular&) = delete;

    void poll() {
        // TODO
    }
};

---

再补功能

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首先future要支持add_finish_callback

---

template<class T>
class Future {
    // ...
    // public
    void add_finish_callback(std::function<void(T&)> callback) {
        _state->add_finish_callback(std::move(callback));
    }
};

---

为什么要把callback实际加到_state里面去呢，因为之后需要post所有callback给schedular，而schedular要接受各种Future<T>的callback，要做类型擦除太麻烦了，所以索性，把callback存入_state，_state正好是堆对象，把他的类型擦除了，丢给schedular去post，还是因为简单

既然callback实际加给了shared_state，那SharedState也得补充对应的功能

---

template<class T>
class SharedState {
    // ...
    // private
    void add_finish_callback(std::function<void(T&)> callback) {
        finish_callbacks.push_back(std::move(callback));
        // TODO
    }

    std::vector<std::function<void(T&)>> finish_callbacks;
};

---

然后，就是在shared_state set的时候，或者shared_state已有结果，但刚刚新增了callback的时候，把shared_state自己发送给Schedular，等待下一帧被调度到时调用callback

为此，SharedState本身要存储一个Schedular的指针，那么Promise就得接受Schedular作为构造函数参数，SharedState还要记录自己是否已经被post给Schedular，不需要重复post

---

template<class T>
class SharedState {
    // ...
    // public
    // 构造函数增加一个参数
    SharedState(Schedular& schedular)
        : schedular(&schedular)
    {}

    // ...
    // private
    // set增加内容
    template<class U>
    void set(U&& v) {
        if (settled) {
            return;
        }
        settled = true;
        value = std::forward<U>(v);
        post_all_callbacks();
    }

    void add_finish_callback(std::function<void(T&)> callback) {
        finish_callbacks.push_back(std::move(callback));
        post_all_callbacks();
    }

    void post_all_callbacks();

    bool settled = false;
    bool callback_posted = false;
    Schedular* schedular = nullptr;
    T value;
    std::vector<std::function<void(T&)>> finish_callbacks;
};

template<class T>
class Promise {
    // ...
	// public
	// 构造函数增加参数
    Promise(Schedular& schedular)
        : _schedular(&schedular)
        , _state(std::make_shared<SharedState<T>>(*_schedular))
    {}
};

// ...
// 在Schedular定义的后面
template<class T>
void SharedState<T>::post_all_callbacks() {
    if (callback_posted) {
        return;
    }
    callback_posted = true;
    schedular->post_call_state(shared_from_this());
}

---

可以发现，在post_all_callback时，SharedState把自己shared_from_this()后发送给了schedular，显然这里既要enable_shared_from_this，又要类型擦除，于是

---

class SharedStateBase : public std::enable_shared_from_this<SharedStateBase> {
    friend class Schedular;
public:
    virtual ~SharedStateBase() = default;
};

template<class T>
class SharedState : public SharedStateBase {
    // ...
};

---

Schedular里面也得补充存储shared_ptr<SharedStateBase>的东西

---

class Schedular {
    // ...
    // private
    void post_call_state(std::shared_ptr<SharedStateBase> state) {
        pending_states.push_back(std::move(state));
    }

    std::vector<std::shared_ptr<SharedStateBase>> pending_states;
};

---

下面，就轮到Schedular的poll函数在每帧调用被post过来的SharedStateBase了

---

class Schedular {
    // ...
    // public
    void poll() {
        size_t sz = pending_states.size();
        for (size_t i = 0; i != sz; i++) {
            auto state = std::move(pending_states[i]);
            state->invoke_all_callback();
        }
        pending_states.erase(pending_states.begin(), pending_states.begin()+sz);
    }
};

---

• 之所以这里使用下标循环，是因为迭代过程中还可能有callback继续往pending_states里面新增元素
• 之所以不用while !empty()而是预先获取size，是为了避免调度是callback内无限post callback导致无限循环
• 之所以调用前先move出来，是为了避免调用callback期间callback继续往pending_states里面新增元素导致容器扩容，内容物失效

这里对state调用了invoke_all_callback，显然这是一个虚函数，需要给SharedState补上

---

class SharedStateBase : public std::enable_shared_from_this<SharedStateBase> {
    // ...
    // private
    virtual void invoke_all_callback() = 0;
};

template<class T>
class SharedState : public SharedStateBase {
    // ...
    // private
    virtual void invoke_all_callback() override {
        callback_posted = false;
        size_t sz = finish_callbacks.size();
        for (size_t i = 0; i != sz; i++) {
            auto v = std::move(finish_callbacks[i]);
            v(value);
        }
        finish_callbacks.erase(finish_callbacks.begin(), finish_callbacks.begin()+sz);
    }
};

---

这里在invoke_all_callbacks里面，使用了和上面schedular poll里面类似的代码结构，可以让本帧callback里新增的callback post到下一帧调用

好了，全部功能都齐了，下面可以测试了，完整的代码在文章最后贴出

测试

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---

int main() {
    Schedular schedular;
    Promise<int> promise(schedular);
    Future<int> future = promise.get_future();
    std::cout << "future get
";
    promise.set_result(10);
    std::cout << "promise result set
";
    future.add_finish_callback([](int v) {
        std::cout << "callback 1 got result " << v << "
";
    });
    std::cout << "future callback add
";
    std::cout << "tick 1
";
    schedular.poll();
    std::cout << "tick 2
";
    future.add_finish_callback([](int v) {
        std::cout << "callback 2 got result " << v << "
";
    });
    std::cout << "future callback 2 add
";
    schedular.poll();

    std::cout << "
";

    Promise<double> promise2(schedular);
    promise2.set_result(12.34);
    std::cout << "promise result2 set
";
    Future<double> future2 = promise2.get_future();
    std::cout << "future2 get
";
    future2.add_finish_callback([&](double v) {
        std::cout << "future2 callback 1 got result" << v << "
";
        future2.add_finish_callback([](double v) {
            std::cout << "future2 callback 2 got result" << v << "
";
        });
        std::cout << "future2 callback 2 add inside callback
";
    });
    std::cout << "future2 callback add
";
    std::cout << "tick 3
";
    schedular.poll();
    std::cout << "tick 4
";
    schedular.poll();
}

---

输出

---

future get
promise result set
future callback add
tick 1
callback 1 got result 10
tick 2
future callback 2 add
callback 2 got result 10

promise result2 set
future2 get
future2 callback add
tick 3
future2 callback 1 got result12.34
future2 callback 2 add inside callback
tick 4
future2 callback 2 got result12.34

---

怎么样，是不是很简单呢，赶紧自己回家造一个吧！

附录

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完整代码

---

#include <vector>
#include <memory>
#include <iostream>
#include <functional>

template<class T>
class Future;

template<class T>
class Promise;

class Schedular;

class SharedStateBase : public std::enable_shared_from_this<SharedStateBase> {
    friend class Schedular;
public:
    virtual ~SharedStateBase() = default;
private:
    virtual void invoke_all_callback() = 0;
};

template<class T>
class SharedState : public SharedStateBase {
    friend class Future<T>;
    friend class Promise<T>;
public:
    SharedState(Schedular& schedular)
        : schedular(&schedular)
    {}
    SharedState(const SharedState&) = delete;
    SharedState(SharedState&&) = delete;
    SharedState& operator=(const SharedState&) = delete;
    SharedState& operator=(SharedState&&) = delete;

private:
    template<class U>
    void set(U&& v) {
        if (settled) {
            return;
        }
        settled = true;
        value = std::forward<U>(v);
        post_all_callbacks();
    }

    T& get() { return value; }

    void add_finish_callback(std::function<void(T&)> callback) {
        finish_callbacks.push_back(std::move(callback));
        post_all_callbacks();
    }

    void post_all_callbacks();

    virtual void invoke_all_callback() override {
        callback_posted = false;
        size_t sz = finish_callbacks.size();
        for (size_t i = 0; i != sz; i++) {
            auto v = std::move(finish_callbacks[i]);
            v(value);
        }
        finish_callbacks.erase(finish_callbacks.begin(), finish_callbacks.begin()+sz);
    }

    bool has_owner = false;
    bool settled = false;
    bool callback_posted = false;
    Schedular* schedular = nullptr;
    T value;
    std::vector<std::function<void(T&)>> finish_callbacks;
};

template<class T>
class Promise {
public:
    Promise(Schedular& schedular)
        : _schedular(&schedular)
        , _state(std::make_shared<SharedState<T>>(*_schedular))
    {}

    Future<T> get_future();

    template<class U>
    void set_result(U&& value) {
        if (_state->settled) {
            throw std::invalid_argument("already set result");
        }
        _state->set(std::forward<U>(value));
    }
private:
    Schedular* _schedular;
    std::shared_ptr<SharedState<T>> _state;
};

template<class T>
class Future {
    friend class Promise<T>;
private:
    Future(std::shared_ptr<SharedState<T>> state)
        : _state(std::move(state))
    {
    }
public:

    void add_finish_callback(std::function<void(T&)> callback) {
        _state->add_finish_callback(std::move(callback));
    }
private:
    std::shared_ptr<SharedState<T>> _state;
};

template<class T>
Future<T> Promise<T>::get_future() {
    return Future<T>(_state);
}

class Schedular {
    template<class T>
    friend class SharedState;
public:
    Schedular() = default;
    Schedular(Schedular&&) = delete;
    Schedular(const Schedular&) = delete;
    Schedular& operator=(Schedular&&) = delete;
    Schedular& operator=(const Schedular&) = delete;

    void poll() {
        size_t sz = pending_states.size();
        for (size_t i = 0; i != sz; i++) {
            auto state = std::move(pending_states[i]);
            state->invoke_all_callback();
        }
        pending_states.erase(pending_states.begin(), pending_states.begin()+sz);
    }
private:
    void post_call_state(std::shared_ptr<SharedStateBase> state) {
        pending_states.push_back(std::move(state));
    }

    std::vector<std::shared_ptr<SharedStateBase>> pending_states;
};

template<class T>
void SharedState<T>::post_all_callbacks() {
    if (callback_posted) {
        return;
    }
    callback_posted = true;
    schedular->post_call_state(shared_from_this());
}

int main() {
    Schedular schedular;
    Promise<int> promise(schedular);
    Future<int> future = promise.get_future();
    std::cout << "future get
";
    promise.set_result(10);
    std::cout << "promise result set
";
    future.add_finish_callback([](int v) {
        std::cout << "callback 1 got result " << v << "
";
    });
    std::cout << "future callback add
";
    std::cout << "tick 1
";
    schedular.poll();
    std::cout << "tick 2
";
    future.add_finish_callback([](int v) {
        std::cout << "callback 2 got result " << v << "
";
    });
    std::cout << "future callback 2 add
";
    schedular.poll();

    std::cout << "
";

    Promise<double> promise2(schedular);
    promise2.set_result(12.34);
    std::cout << "promise result2 set
";
    Future<double> future2 = promise2.get_future();
    std::cout << "future2 get
";
    future2.add_finish_callback([&](double v) {
        std::cout << "future2 callback 1 got result" << v << "
";
        future2.add_finish_callback([](double v) {
            std::cout << "future2 callback 2 got result" << v << "
";
        });
        std::cout << "future2 callback 2 add inside callback
";
    });
    std::cout << "future2 callback add
";
    std::cout << "tick 3
";
    schedular.poll();
    std::cout << "tick 4
";
    schedular.poll();
}

---