Senders/Receivers

2025-05-05

C++26 introduces senders/receivers as a generic framework for organizing (asynchronous) execution. Let's look into them.

• sender: A description of work that produces values which are sent to receivers.
• receiver: A consumer of values sent by senders.

In order to produce actual work, a sender must be connected to a receiver, and the result of such a connect operation is the task to be executed, which we refer to as operation_state.

struct my_sender {
  template <typename Receiver>
  auto connect(Receiver&& recv) -> my_operation_state { /* ... */ }
};

Receivers receive values via set_value:

struct my_receiver {
  auto set_value(auto&& values) -> void {}
};

Operation states represent the actual work that need to be executed. So they need to have a start method.

struct my_operation_state {
  auto start() -> void {}
};

Now we can put together these three pieces and start the execution:

my_sender{}.connect(my_receiver{}).start()

The just sender

Our first sender is the just sender. It simply sends a given value to receivers.

Let's first write the operation state of just. When a just sender operates, it should give a value to a receiver.

template <typename R, typename V>
struct just_operation {
  R r;
  V v;
  auto start() -> void {
    std::move(r).set_value(std::move(v));
  }
};

It's then trivial to write down the just sender.

template <typename T>
struct just_sender {
  T t;
  template <typename R>
  auto connect(R&& r) -> just_operation<R, T> {
    return {std::forward<R>(r), std::move(t)};
  }
};

template <typename T>
auto just(T&& t) -> just_sender<T> {
  return {std::forward<T>(t)};
}

then: a sender adapter