Felspar Coro

C++ coroutine library and toolkit

felspar-coro is a C++20 library to help you to use coroutines in your own libraries and applications. You should be able to use it with a recent clang or gcc compiler on Unix systems (e.g. Linux and Mac).

Using the library

The library is designed to be easy to use from CMake projects. To use it add it in your source tree somewhere and then use add_subdirectory to include it in your project. The library also requires the use of some other libraries which can get by using an include. So if you have the library in a folder called felspar-coro, you can add the following to your CMakeLists.txt:


To make use of the library add felspar-coro in your target_link_libraries directive.

target_link_libraries(your-app otherlib1 otherlib2 felspar-coro)

Coroutine functionality

A number of general coroutine awaitable types are provided, together with some useful utilities. The most important types if you want to just make use of coroutines are the task, the generator and the stream.


Used to turn a coroutine that co_yields values into an iterator that works with ranged for loops.

std::vector<std::size_t> fibs{};
for (auto f : take(10, fib())) { fibs.push_back(f); }
check(fibs.size()) == 10u;
check(fibs.front()) == 1u;
check(fibs.back()) == 55u;

Generators can be used from normal functions, not just coroutines. A generator coroutine is restricted to only using co_yield and not co_returning any value.

As well as supporting iteration, the use of next() directly on the generator is also supported (providing the same API structure as stream, but without the requirement to co_await the resulting values). This may provide a simpler API for cases where multiple generators are required to be used together.

auto t = take(3, fib());
check(t.next()) == 1u;
check(t.next()) == 1u;
check(t.next()) == 2u;


A coroutine whose result must be awaited. Like the lazy, the coroutine is not started until the return value is co_awaited.

When used from a coroutine, either co_await it directly or turn the task into an r-value with move:

auto val = co_await some_task();
auto task = some_task();
// other things
auto val = co_await std::move(task);

Note that the task code will not start executing until co_awaited.

The task can also be started from a non-coroutine using .get(). This also requires a r-value, so:

auto val some_task().get();
auto task = some_task();
// other things
auto val = std::move(task).get();

Note that get will block until the coroutine completes, so it is generally used at the top level where non-coroutine code starts coroutine execution. Never use .get() from within a coroutine or it will likely block forever.


A generator that must be called from a coroutine, but is allowed to use co_await in its internal implementation. Items are fetched from the stream one at a time using next and must be co_awaited. The returned value is a std::optional wrapper around the stream type and will be empty when the stream terminates.

felspar::coro::stream<int> numbers() {
    co_yield 1;
    co_yield 2;

felspar::coro::stream<int> doubler(felspar::coro::stream<int> in) {
    while (auto n = co_await in.next()) {
        co_yield *n * 2;


A basic lazily evaluated coroutine. Superficially very similar to a nullary lambda, but with an "only once" execution guarantee. The coroutine can be evaluated from either a normal function or a coroutine, and it's value is returned as if it was a nullary lambda using operator()().

felspar::coro::start and felspar::coro::eager

Starts and takes over ownership of new coroutines. starter can control many coroutines at once, and eager only one. They both allow new coroutines to be started in parallel. Posting of new coroutines into these structures does not need to happen from a coroutine.


A data bus that allows one or more coroutines to wait for a value to be produced.