cpp-coroutines

#include <coroutine>
#include <stdexcept>
#include <optional>
 
template <typename T>
struct Task {
    struct promise_type {
        std::optional<T> value;
        std::exception_ptr exception;
 
        Task get_return_object() {
            return Task{std::coroutine_handle<promise_type>::from_promise(*this)};
        }
 
        std::suspend_always initial_suspend() { return {}; }  // lazy start
        std::suspend_always final_suspend() noexcept { return {}; }
 
        void return_value(T v) { value = std::move(v); }
 
        void unhandled_exception() { exception = std::current_exception(); }
    };
 
    std::coroutine_handle<promise_type> handle;
 
    explicit Task(std::coroutine_handle<promise_type> h) : handle(h) {}
 
    Task(Task&&) = default;
    Task& operator=(Task&&) = default;
 
    ~Task() { if (handle) handle.destroy(); }
 
    T get() {
        handle.resume();                      // resume to completion
        if (handle.promise().exception)
            std::rethrow_exception(handle.promise().exception);
        return std::move(*handle.promise().value);
    }
};
 
// Usage
Task<int> compute() {
    co_return 42;
}
 
int main() {
    auto task = compute();
    int result = task.get();   // 42
}

template <typename T>
struct Generator {
    struct promise_type {
        T current_value;
 
        Generator get_return_object() {
            return Generator{std::coroutine_handle<promise_type>::from_promise(*this)};
        }
 
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() { throw; }
 
        std::suspend_always yield_value(T value) {
            current_value = value;
            return {};                     // suspend after yielding
        }
    };
 
    std::coroutine_handle<promise_type> handle;
 
    explicit Generator(std::coroutine_handle<promise_type> h) : handle(h) {}
    ~Generator() { if (handle) handle.destroy(); }
 
    struct iterator {
        std::coroutine_handle<promise_type> handle;
        bool done;
 
        iterator& operator++() {
            handle.resume();
            done = handle.done();
            return *this;
        }
        T operator*() const { return handle.promise().current_value; }
        bool operator!=(std::default_sentinel_t) const { return !done; }
    };
 
    iterator begin() {
        handle.resume();                   // advance to first yield
        return {handle, handle.done()};
    }
    std::default_sentinel_t end() { return {}; }
};
 
// Usage
Generator<int> iota(int start, int end) {
    for (int i = start; i < end; ++i)
        co_yield i;
}
 
for (int x : iota(0, 5)) {
    std::cout << x << ' ';   // 0 1 2 3 4
}

// An awaitable has three methods:
// await_ready() — true means don't suspend
// await_suspend(handle) — suspend: store handle, schedule resume
// await_resume() — return value of co_await expression
 
struct TimerAwaitable {
    int delay_ms;
 
    bool await_ready() const noexcept { return delay_ms <= 0; }
 
    void await_suspend(std::coroutine_handle<> h) {
        // Schedule h.resume() to be called after delay
        std::thread([h, this]() {
            std::this_thread::sleep_for(std::chrono::milliseconds(delay_ms));
            h.resume();
        }).detach();
    }
 
    void await_resume() const noexcept {}  // no return value
};
 
// suspend_always and suspend_never are built-in awaitables
std::suspend_always{};   // always suspends
std::suspend_never{};    // never suspends (no-op)

// Inspect frame size with Compiler Explorer (godbolt.org)
// Compile with: g++ -std=c++20 -O2 -S
// Look for: operator new call size in the generated asm
// Or: clang -std=c++20 -O2 -emit-llvm -S | grep "coro.size"
 
// Reduce frame size:
// 1. Don't keep large objects alive across co_await
struct Bad {
    std::vector<char> large_buf;   // whole vector lives in frame
    co_return large_buf.size();    // large_buf crosses suspension
};
 
// 2. Move data out before suspending
std::vector<char> buf = get_data();
size_t sz = buf.size();            // capture only what's needed
buf.clear();                       // release before suspension
co_await next_event;
// sz still valid; buf released

# Coroutines appear as regular stack frames after resume()
# To inspect a suspended coroutine:
 
(gdb) info locals
# Look for coroutine_handle variables
 
# Print the promise object
(gdb) p *(promise_type*)(handle.__handle_)
# GDB 14+ has coroutine-specific support
(gdb) info coroutines        # GCC coroutine support (experimental)
 
# Step through coroutine execution
(gdb) step     # enters co_await implementation
(gdb) finish   # returns from coroutine frame function
(gdb) next     # step over suspension point
 
# View all threads (coroutines running on thread pool)
(gdb) info threads
(gdb) thread 2
(gdb) bt

#include <boost/asio.hpp>
#include <boost/asio/co_spawn.hpp>
#include <boost/asio/awaitable.hpp>
 
namespace net = boost::asio;
 
net::awaitable<void> echo_session(net::ip::tcp::socket socket) {
    char buf[1024];
    for (;;) {
        std::size_t n = co_await socket.async_read_some(net::buffer(buf));
        co_await net::async_write(socket, net::buffer(buf, n));
    }
}
 
int main() {
    net::io_context io;
    net::co_spawn(io, listen_accept(io), net::detached);
    io.run();
}

#include <generator>
#include <ranges>
 
std::generator<int> fibonacci() {
    int a = 0, b = 1;
    while (true) {
        co_yield a;
        auto next = a + b;
        a = b;
        b = next;
    }
}
 
// Usage
for (int v : fibonacci() | std::views::take(10))
    printf("%d\n", v);

# Compile with debug info
g++ -std=c++20 -g -O0 -o app app.cpp
gdb ./app

(gdb) break my_coro
(gdb) run
(gdb) info frame                    # current stack frame
(gdb) info locals                   # promise, handle in scope
 
# Inspect coroutine frame pointer (compiler-specific mangling)
(gdb) p *(MyPromise*)h.address()    # h = coroutine_handle
 
# GCC coroutine support (GCC 14+)
(gdb) info coroutines
 
# Pretty-print promise state
(gdb) set print pretty on
(gdb) p promise