WWA Coro — Yet Another C++20 Coroutine Library

This project provides a set of coroutine-based asynchronous utilities and generators for C++20. It includes synchronous and asynchronous generators, tasks, and various utilities to facilitate coroutine-based programming.

Features

• Synchronous Generators: Create coroutine-based generators that produce values synchronously.
• Asynchronous Generators: Create coroutine-based generators that produce values asynchronously.
• Tasks: Manage coroutine-based tasks that produce values of a specified type.

Getting Started

Prerequisites

• C++20 compatible compiler
• CMake 3.23 or higher

Installation

cmake -B build
cmake --build build
sudo cmake --install build

Configuration Options

Option	Description	Default
BUILD_TESTS	Build tests	ON
BUILD_EXAMPLES	Build examples	ON
BUILD_DOCS	Build documentation	ON
BUILD_INTERNAL_DOCS	Build internal documentation	OFF
ENABLE_MAINTAINER_MODE	Maintainer mode (enable more compiler warnings, treat warnings as errors)	OFF
USE_CLANG_TIDY	Use clang-tidy during build	OFF

The BUILD_DOCS (public API documentation) and BUILD_INTERNAL_DOCS (public and private API documentation) require Doxygen and, optionally, dot (a part of Graphviz).

The USE_CLANG_TIDY option requires clang-tidy.

Build Types

Build Type	Description
Debug	Build with debugging information and no optimization.
Release	Build with optimization for maximum performance and no debugging information.
RelWithDebInfo	Build with optimization and include debugging information.
MinSizeRel	Build with optimization for minimum size.
ASAN	Build with AddressSanitizer enabled for detecting memory errors.
LSAN	Build with LeakSanitizer enabled for detecting memory leaks.
UBSAN	Build with UndefinedBehaviorSanitizer enabled for detecting undefined behavior.
TSAN	Build with ThreadSanitizer enabled for detecting data races.
Coverage	Build with code coverage analysis enabled.

ASAN, LSAN, UBSAN, TSAN, and Coverage builds are only supported with GCC or clang.

Coverage build requires gcov (GCC) or llvm-gcov (clang) and gcovr.

Running Tests

To run the tests, use the following command:

ctest -T test --test-dir build/test

or run the following binary:

./build/test/coro_test

The test binary uses Google Test library. Its behavior can be controlled via environment variables and/or command line flags.

Run coro_test --help for the list of available options.

Usage Examples

The documentation is available at https://sjinks.github.io/coro-cpp/.

Eager Coroutine (eager_task)

An eager coroutine start execution immediately upon creation and does not suspend upon completion. From the caller's perspective, it runs synchoronously; the control is returned to the caller only after the coroutine finishes.

#include <wwa/coro/eager_task.h>
 
wwa::coro::eager_task my_task()
{
    co_await some_other_coroutine();
}

See examples/eager_task.cpp

Eager coroutines can co_await other coroutines but they cannot be co_await'ed.

Task (task<T>)

Tasks are lightweight coroutines that start executing when they are awaited; they can optionally return values (the type of the return value is determined by the template parameter T). Use tasks to create your coroutines, and use co_await or co_yield within tasks to perform asynchronous operations.

#include <wwa/coro/task.h>
 
wwa::coro::task<int> task1()
{
    co_return 123;
}
 
wwa::coro::task<int> task2()
{
    co_return 456;
}
 
wwa::coro::task<int> sum()
{
    const auto a = co_await task1();
    const auto b = co_await task2();
    co_return a + b;
}
 
wwa::coro::task<> print()
{
    std::cout << "The result is " << co_await sum() << "\n";
}

It is possible to turn any task (or any awaitable) into a fire-anf-forget eager coroutine. For the example above,

#include <wwa/coro/eager_task.h>
#include <wwa/coro/task.h>
 
wwa::coro::run_awaitable(print);

See examples/task.cpp.

Generators (generator<T>)

Generators are special coroutines that produce sequences of values of a specified type (T). These values are produced lazily and synchronously.

The coroutine body is able to yield values of type T using the co_yield keyword. However, the coroutine body is not able to use the co_await keyword; values must be produced synchronously.

Generators can be used with range-based for loops and ranges.

#include <wwa/coro/generator.h>
 
wwa::coro::generator<int> fibonacci(int n)
{
    int a = 0;
    int b = 1;
 
    if (n > 0) {
        co_yield a;
    }
 
    if (n > 1) {
        co_yield b;
    }
 
    for (int i = 2; i < n; ++i) {
        auto s = a + b;
        co_yield s;
        a = b;
        b = s;
    }
}
 
// ...
 
std::cout << "The first 10 Fibonacci numbers are: ";
for (auto n : fibonacci(10)) {
    std::cout << n << ' ';
}

See examples/generator.cpp.

Asynchronous Generators (async_generator<T>)

Generators are special coroutines that produce sequences of values of a specified type (T). These values are produced lazily and asynchronously.

Unlike the synchronous counterpart, the coroutine body is able to use both co_await and co_yield expressions.

// Simulates an asynchronous task
wwa::coro::task<int> get_next_value(int n)
{
    co_return n + 1;
}
 
wwa::coro::async_generator<int> async_first_n(int n)
{
    int v = 0;
    while (v < n) {
        co_yield v;
        v = co_await get_next_value(v);
    }
}
 
// ...
 
auto gen = async_first_n(5);
auto it  = co_await gen.begin();  // IMPORTANT! co_await is required
auto end = gen.end();
 
while (it != end) {
    std::cout << *it << "\n";
    co_await ++it;                // IMPORTANT! co_await is required
}

See examples/async_generator.cpp.

Unfortunately, it is impossible to use asynchronous iterators directly in range-based for loops. However, this limitation can be overcome with the help of eager coroutines.

There is a helper adapter, sync_generator_adapter<T>, that turns an asynchronous generator into a quasi-synchronous one:

#include <wwa/coro/async_generator.h>
#include <wwa/coro/sync_generator_adapter.h>
 
wwa::coro::async_generator<int> async_iota(int start = 0)
{
    for (int i = start;; ++i) {
        co_yield i;
    }
}
 
// ...
 
wwa::coro::sync_generator_adapter<int> sync_iota(async_iota(10));
 
for (auto& n : sync_iota | std::views::take(5)) {
    std::cout << n << "\n";
}

See examples/sync_generator_adapter.cpp.