Examples

Examples

Code used to prepare Dietmar’s C++Now 2025 presentation

The example program c++now-affinity.cpp uses demo::thread_loop to demonstrate the behavior of scheduler affinity: the idea is that scheduler affinity causes the coroutine to resume on the same scheduler as the one the coroutine was started on. The program implements three coroutines which have most of their behavior in common:

  1. Each coroutine is executed from main using sync_wait(fun(loop.get_scheduler())).

  2. Each coroutine prints the id of the thread it is executing on prior to any co_await and after all co_await expression.

  3. Each coroutine co_await`s the result of `scheduler(sched) | then([]{ …​ }) where the function passed to then just prints the thread id.

  4. work2 additionally changes the coroutines scheduler to be an inline_scheduler and later restores the original scehduler using change_coroutine_scheduler.

  5. While work1 and work2 use the default scheduler (task_scheduler a type-erased scheduler which gets initialized from the receiver’s environment’s get_scheduler), work3 sets the coroutine’s scheduler up to be inline_scheduler, effectively causing the coroutine to resume wherever the `co_await’s expression resumed.

The output of the program is something like the below:

before id=0x1fd635f00
then id  =0x16b64b000
after id =0x1fd635f00

before id=0x1fd635f00
then id  =0x16b64b000
after1 id=0x16b64b000
then id  =0x16b64b000
after2 id=0x1fd635f00

before id=0x1fd635f00
then id  =0x16b64b000
after id =0x16b64b000

It shows that:

  1. The thread on which the `then’s function is executed is always the same and different from the thread each of the coroutines started on.

  2. For work1 the co_await resumes on the same thread as the one the coroutine was started on.

  3. For work2 the first co_await after schedule(sched) resumes on the thread used by sched. After restoring the original scheduler the co_await resumes on the original thread.

  4. For work3 the co_await resumes on the thread used by sched as the inline_scheduler doesn’t do any actual scheduling.

This demo shows how to configure task’s environment argument to use a different allocator than the default `std::allocator<std::byte>. To do so it defines an environment type with_allocator which defines a nested type alias allocator_type to be std::pmr::polymorphic_allocator<std::byte>.

The coroutine coro shows how to use read_env to extract the used allocator object to potentially use it for any allocation purposes within the coroutine. There are two uses of coro, the first one using the default which just uses std::pmr::polymorphic_allocator<std::byte>() to allocate memory. The second use explicitly specifies the memory resource std::pmr::new_delete_resource() to initialized the use std::pmr::polymorphic_allocator<std::byte>.

The example c++now-basic.cpp shows some basic use of a task:

  1. The coroutine basic just co_await`s the awaiter `std::suspend_never{} which immediately completes. This use demonstrates that any awaiter can be co_await`ed by a `task<…​>.

  2. The coroutine await_sender demonstrates the results of co_await`ing various senders. It uses variations of `just* to show the different results:

    • co_await`ing a sender completing with `set_value_t(), e.g., just(), produces an expression with type void.

    • co_await`ing a sender completing with `set_value_t(T), e.g., just(1), produces an expression with type T.

    • co_await`ing a sender completing with set_value_t(T0, …​, Tn), e.g., `just(1, true), produces an expression with type tuple<T0, …​, Tn>.

    • co_await`ing a sender completing with `set_error_t(E), e.g., just_error(1), results in an exception of type E being thrown.

    • co_await`ing a sender completing with `set_stopped_t(), e.g., just_stopped(), results in the corouting never getting resumed although all local objects are properly destroyed.

The example c++now-cancel.cpp shows a coroutine co_await`ing `just_stopped() which results in the coroutine getting cancelled. The coroutine will complete with set_stopped().

The example c++now-errors.cpp shows examples of how to handle errors within a coroutine:

  • The coroutine error_result simply co_await`s a sender producing an error (`just_error(17)). When a co_await`ed sender completes with `set_error_t(T) an exception of type T is thrown and the error needs to be handled with a try/catch block. Otherwise the coroutine itself completes with set_error_t(exception_ptr) where the exception_ptr hold the thrown exception object.

  • The coroutine expected uses a sender algorithm as_expected which is implemented at the top of the example to turn the result of the co_await`ed sender into an object of type `expected<T, E>, avoiding an exception from being thrown.

The example c++now-query.cpp shows how to define and use a custom environment element.

  1. The coroutine with_env uses a simple environment named context which just defines a custom query for get_value to obtain a value. The value itself gets initialized from the environment of the receiver used with the task.

  2. The coroutine with_fancy_env uses an environment which embed a an object depending the type of the environment of the receiver used with the task. While the type accessed from within the task needs to be type-erased, the actually stored value can depend on the environment of the upstream receiver.

The example c++now-result-types.cpp shows the result types of successful senders using variatons of just:

  • co_await just() doesn’t produce a value, i.e., the type of the expression is void.

  • co_await just(1) produces an int.

  • co_await just(1, true) produces a tuple<int, bool>.

The example c++now-return.cpp shows various ways of returning normally (without an error) for a task. Some of the coroutines are set up to produce specific error results although none of them are actually use:

  • default_return shows that the default return type for task<> is void.

  • void_return explicitly specifies a void return type.

  • int_return specifies the return type as int and returns an int value.

  • error_return specifies the return type as int and also specifies custom error results.

  • no_error_return specifies the return type as int and also specifies that the coroutine can’t produce any error.

The example c++now-stop_token.cpp shows how to get a stop token in side a task and how to use it to cancel active work. It doesn’t actually complete with a set_stopped() but completes with set_value().

  • In the coroutine co_await read_env(get_stop_token) is used to get a stop token.

  • In the loop the value of token.stop_requested() is checked to determine if the loop should continue.

  • In main an inplace_stop_source is used to have something which can be stopped.

  • When running the coroutine stopping on a separate thread, the environment is changed using write_env to use stop token from `main’s stop source in the environment.

  • After sleeping for a bit, source.request_stop() is called to trigger cancellation of the coroutine.

The example c++now-with_error.cpp shows how a coroutine can be exited reporting an error without throwing an exception. To do so, the coroutine uses co_yield with_error(e). By default the task only declares set_error_t(exception_ptr). To return other errors, an environment declaring a suitable set_error_t(E) completion using the error_types alias is used.

Tools Used By The Examples

Technically demo::thread_loop is a class public`ly derived from `execution::run_loop which is also owning a std::thread. The std::thread is constructed with a function object calling run() on the demo::thread_loop object. Destroying the object calls finish() and then join()`s the `std::thread: the destructor will block until the execution::run_loop’s `run() returns.

The important bit is that work executed on the demo::thread_loop's scheduler will be executed on a corresponding std::thread.