The simplest C++20 coroutine library, specifically designed to solve the problem of callback-hell, supports integration with any asynchronous I/O framework

Copy the include folder to your build tree and use a C++20 compiler.

require cmake version >= 3.12

FetchContent_Declare(
    sco
    GIT_REPOSITORY https://github.com/kkHAIKE/sco.git
    GIT_TAG        main
)

if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.14)
    FetchContent_MakeAvailable(sco)
else()
    FetchContent_GetProperties(sco)
    if (NOT sco_POPULATED)
        FetchContent_Populate(sco)
        add_subdirectory(${sco_SOURCE_DIR} ${sco_BINARY_DIR})
    endif()
endif()

# link with your target
target_link_libraries(your_target sco::sco)

• very tiny/simple and no dependencies.
• support all async frameworks.
• support 3rd-party libraries have implemented the awaiter interface.
• very useful for bridging between async/coroutines frameworks and 3rd-party libraries.
• sco::call_with_callback wraps any async function to make it available for use within a coroutine.
• sco::all will wait for all coroutines to complete.

#include <sco/sco.hpp>

// a plus async function return value after 1 second
void plus_async(int a, int b, const std::function<void(int)>& cb);

// wWrap the async function into a coroutine using sco:async.
sco::async<int> plus(int a, int b) {
    int c{};
    co_await sco::call_with_callback(&plus_async, a, b, sco::cb_tie<void(int)>(c));
    co_return c;
}

// This is the root coroutine called from a thread.
sc::async<void> root_co(int a, int b) {
    int c = co_await plus(a, b);
    std::cout << a << " + " << b << " = " << c << std::endl;
    // void coroutine must call co_return explicitly.
    co_return;
}

// Some async frameworks will obtain a thread from a thread pool and call this function.
void thread_func_like_handler() {
    root_co(1, 2).start_root_in_this_thread();
    // This thread may quickly return when the first `co_await` is being called.
}

Is equivalent to:

void thread_func_like_handler() {
    plus_async(1, 2, [](int c) {
        std::cout << "1 + 2 = " << c << std::endl;
    });
}

more samples in example.cpp

httpcache is a more practical example that uses libhv and redis++.

• sco::async<T> is a coroutine that returns a value of type T.
• sco::async<> (aka sco::async<void>) can be obtained by converting from any sco::async<T>.
• start_root_in_this_thread will start the coroutine in the current thread.
• is a FutureLike type.

• sco::call_with_callback wraps any async function to make it available for use within a coroutine.
• require std::co_tie to tie the callback parameters to the coroutine variables.
• sco::wmove use move assignment instead of regular assignment.

  std::co_tie<void(NoCopy)> cb{sco::wmove(x)};

• is return a FutureLike type.

• sco::all will wait for all coroutines to complete.
• use with sco::async container:

  std::vector<sco::async<int>> coroutines;
  coroutines.emplace_back(plus(1, 2));
  coroutines.emplace_back(plus(3, 4));
  
  auto ret = co_await sco::all(coroutines.begin(), coroutines.end());
  
  std::cout << ret[0] << std::endl; // 3
  std::cout << ret[1] << std::endl; // 7

• use with any FutureLike:

  int r{};
  auto ret = co_await sco::all(
      plus(1, 2),
      call_with_callback(&plus_async, 3, 4, sco::cb_tie<void(int)>(r)),
      plus(5, 6),
  );
  
  std::cout << ret[0] << std::endl; // 3
  std::cout << r << std::endl; // 7
  // the 2nd `FutureLike` return type is void,
  // so the return value is ignored.
  std::cout << ret[1] << std::endl; // 11

• can use with 3rd-party libraries have implemented the awaiter interface.

• function signature must be like void (*)(Args..., const std::function<void(Ret...)>&, Args...).
• The fact that this callback function is called means that the asynchronous function has completed, thereby transferring control flow.
• the callback function must be called exactly once if no exception is thrown.

• if use reference type as parameter, the reference may be invalid after the frist co_await.

  sc::async<void> root_co(Type& x) {
      co_await other_async(x);
      // `x` may be invalid.
      co_return;
  }
  
  void thread_func_like_handler() {
      Type x;
      root_co(x).start_root_in_this_thread();
  }

MIT