This is very similar to lazy<T, Executor = void> , except that execution of the eager<T> returning function begins immediately, and if the function never suspends during the course of its execution, no suspend-resume cycle occurs. Functions which return eager<T> are therefore suitable for tasks which may require suspension, but will often complete immediately.

atomic_eager<T> is like eager<T>, except that the setting of the coroutine result performs an atomic release, whilst the checking of whether the coroutine has finished is an atomic acquire.

The Executor template parameter is purely for compatibility with third party software such as ASIO, and this awaitable can be directly used by ASIO.

Example of use (must be called from within a coroutinised function):

eager<int> func(int x)
{
  co_return x + 1;
}
...
// Executes like a non-coroutine function i.e. r is immediately set to 6.
int r = co_await func(5);

eager<T> has special semantics if T is a type capable of constructing from an exception_ptr or error_code – any exceptions thrown during the function’s body are sent via T, preferably via the error code route if error_from_exception( ) successfully matches the exception throw. This means that a basic_result<T, E, NoValuePolicy> or basic_outcome<T, EC, EP, NoValuePolicy> where one of its types is is compatible will have its .error() or .exception() set.

Note that eager<T> does not otherwise transport exception throws, and rethrows any exceptions thrown within the coroutine body through the coroutine machinery. This does not produce reliable consequences in current C++ compilers. You should therefore wrap the coroutine body in a try...catch if T is not able to transport exceptions on its own.

Requires: C++ coroutines to be available in your compiler.

Namespace: BOOST_OUTCOME_V2_NAMESPACE::awaitables

Header: <boost/outcome/coroutine_support.hpp>