This is very similar to eager<T, Executor = void> , except that execution of the lazy<T> returning function suspends immediately. Functions which return lazy<T> are therefore suitable for tasks which you need to instantiate right now, but whose execution will occur elsewhere e.g. in a separate kernel thread. Because of the very common use case of using worker threads to execute the body of lazily executed coroutines, most people will want to use atomic_lazy<T> instead of lazy<T>.

atomic_lazy<T> is like lazy<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.

lazy<T> has similar semantics to std::lazy<T>, which is being standardised. See https://wg21.link/P1056 Add lazy coroutine (coroutine task) type.

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):

lazy<int> func(int x)
{
  co_return x + 1;
}
...
// Always suspends perhaps causing other coroutines to execute, then resumes.
int r = co_await func(5);

lazy<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 lazy<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>