future.hpp

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#pragma once


#include <felspar/coro/coroutine.hpp>
#include <felspar/exceptions.hpp>
#include <felspar/test/source.hpp>

#include <optional>
#include <vector>


namespace felspar::coro {

Asynchronous future

A non-thread safe asynchronous future. This type is not used to define a coroutine (like a task is), but is used to enable communication to coroutines from other parts of the code. Typically you will find this type as an instance in a data structure.

It is able to have multiple coroutines wait on the future, with them all being resumed when a value is pushed. If the value has already been set then any new coroutine will continue without suspending.

The value can be set and read from non-coroutines as well. The value can only be set once.

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    template<typename T>
    class future {
        std::optional<T> m_value;
        std::vector<coroutine_handle<>> continuations;


      public:
        using value_type = T;

Query the future

 
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        bool has_value() const noexcept { return m_value.has_value(); }
        explicit operator bool() const noexcept { return has_value(); }

        value_type &
                value(source_location const &loc = source_location::current()) {
            if (not m_value) {
                throw felspar::stdexcept::logic_error{
                        "Future does not contain a value", loc};
            } else {
                return *m_value;
            }
        }
        value_type const &value(
                source_location const &loc = source_location::current()) const {
            if (not m_value) {
                throw felspar::stdexcept::logic_error{
                        "Future does not contain a value", loc};
            } else {
                return *m_value;
            }
        }

Coroutine interface

 
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        auto operator co_await() {
            struct awaitable {
                explicit awaitable(coro::future<value_type> &f) : fut{f} {}
                awaitable(awaitable const &) = delete;
                // TODO We could be movable
                awaitable(awaitable &&) = delete;
                ~awaitable() {
                    if (mine) { std::erase(fut.continuations, mine); }
                }

                awaitable &operator=(awaitable const &) = delete;
                awaitable &operator=(awaitable &&) = delete;


                coro::future<value_type> &fut;
                coroutine_handle<> mine = {};


                bool await_ready() const noexcept { return fut.has_value(); }
                void await_suspend(coroutine_handle<> h) {
                    mine = h;
                    fut.continuations.push_back(h);
                }
                value_type &await_resume() {
                    mine = {};
                    return *fut.m_value;
                }
            };
            return awaitable{*this};
        }

Set the future's value

 
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        void set_value(
                value_type t,
                source_location const &loc = source_location::current()) {
            if (m_value) {
                throw stdexcept::logic_error{
                        "The future already has a value set", loc};
            }
            m_value = std::move(t);
            for (auto h : continuations) { h.resume(); }
            continuations = {};
        }
    };

    template<>
    class future<void> {
        bool m_has_value = false;
        std::vector<coroutine_handle<>> continuations;


      public:
        using value_type = void;

Query the future

 
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        bool has_value() const noexcept { return m_has_value; }
        explicit operator bool() const noexcept { return has_value(); }

        void value(source_location const &loc = source_location::current()) {
            if (not m_has_value) {
                throw felspar::stdexcept::logic_error{
                        "Future does not contain a value", loc};
            }
        }
        void value(
                source_location const &loc = source_location::current()) const {
            if (not m_has_value) {
                throw felspar::stdexcept::logic_error{
                        "Future does not contain a value", loc};
            }
        }

Coroutine interface

 
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        auto operator co_await() {
            struct awaitable {
                explicit awaitable(coro::future<void> &f) : fut{f} {}
                awaitable(awaitable const &) = delete;
                // TODO We could be movable
                awaitable(awaitable &&) = delete;
                ~awaitable() {
                    if (mine) { std::erase(fut.continuations, mine); }
                }

                awaitable &operator=(awaitable const &) = delete;
                awaitable &operator=(awaitable &&) = delete;


                coro::future<void> &fut;
                coroutine_handle<> mine = {};


                bool await_ready() const noexcept { return fut.has_value(); }
                void await_suspend(coroutine_handle<> h) {
                    mine = h;
                    fut.continuations.push_back(h);
                }
                void await_resume() noexcept { mine = {}; }
            };
            return awaitable{*this};
        }

Set the future's value

 
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        void set_value(source_location const &loc = source_location::current()) {
            if (m_has_value) {
                throw stdexcept::logic_error{
                        "The future already has a value set", loc};
            }
            m_has_value = true;
            for (auto h : continuations) { h.resume(); }
            continuations = {};
        }
    };


}