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promise.adoc

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cobalt/promise.hpp

A promise is an eager coroutine that can co_await and co_return values. That is, it cannot use co_yield.

cobalt::promise<void> delay(std::chrono::milliseconds ms)
{
  asio::steady_timer tim{co_await cobalt::this_coro::executor, ms};
  co_await tim.async_wait(cobalt::use_op);
}

cobalt::main co_main(int argc, char *argv[])
{
  co_await delay(std::chrono::milliseconds(50));
  co_return 0;
}

Promises are by default attached. This means, that a cancellation is sent when the promise handles goes out of scope.

A promise can be detached by calling detach or by using the prefix + operator. This is a runtime alternative to using detached. A detached promise will not send a cancellation on destruction.

cobalt::promise<void> my_task();

cobalt::main co_main(int argc, char *argv[])
{
  +my_task(); // (1)
  co_await delay(std::chrono::milliseconds(50));
  co_return 0;
}

  1. By using + the task gets detached. Without it, the compiler would generate a nodiscard warning.

Executor

The executor is taken from the thread_local get_executor function, unless a asio::executor_arg is used in any position followed by the executor argument.

cobalt::promise<int> my_gen(asio::executor_arg_t, asio::io_context::executor_type exec_to_use);

Memory Resource

The memory resource is taken from the thread_local get_default_resource function, unless a std::allocator_arg is used in any position followed by a polymorphic_allocator argument.

cobalt::promise<int> my_gen(std::allocator_arg_t, pmr::polymorphic_allocator<void> alloc);

Outline

link:../../include/boost/cobalt/promise.hpp[role=include]

  1. Supports [interrupt_await]

  2. This allows to create promise running in parallel with a simple +my_task() expression.

  3. This allows code like while (p) co_await p;