7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.2 Unary expressions [expr.unary] Await [expr.await]

The co_await expression is used to suspend evaluation of a coroutine ([dcl.fct.def.coroutine]) while awaiting completion of the computation represented by the operand expression.
Suspending the evaluation of a coroutine transfers control to its caller or resumer.
An await-expression shall appear only as a potentially-evaluated expression within the compound-statement of a function-body or lambda-expression, in either case outside of a handler ([except.pre]).
An await-expression shall not appear in a default argument ([dcl.fct.default]).
An await-expression shall not appear in the initializer of a block variable with static or thread storage duration.
A context within a function where an await-expression can appear is called a suspension context of the function.
Evaluation of an await-expression involves the following auxiliary types, expressions, and objects:
  • p is an lvalue naming the promise object ([dcl.fct.def.coroutine]) of the enclosing coroutine and P is the type of that object.
  • Unless the await-expression was implicitly produced by a yield-expression ([expr.yield]), an initial await expression, or a final await expression ([dcl.fct.def.coroutine]), a search is performed for the name await_transform in the scope of P ([class.member.lookup]).
    If this search is performed and finds at least one declaration, then a is p.await_transform(cast-expression); otherwise, a is the cast-expression.
  • o is determined by enumerating the applicable operator co_await functions for an argument a ([over.match.oper]), and choosing the best one through overload resolution ([over.match]).
    If overload resolution is ambiguous, the program is ill-formed.
    If no viable functions are found, o is a.
    Otherwise, o is a call to the selected function with the argument a.
    If o would be a prvalue, the temporary materialization conversion ([conv.rval]) is applied.
  • e is an lvalue referring to the result of evaluating the (possibly-converted) o.
  • h is an object of type std​::​coroutine_handle<P> referring to the enclosing coroutine.
  • await-ready is the expression e.await_ready(), contextually converted to bool.
  • await-suspend is the expression e.await_suspend(h), which shall be a prvalue of type void, bool, or std​::​coroutine_handle<Z> for some type Z.
  • await-resume is the expression e.await_resume().
The await-expression has the same type and value category as the await-resume expression.
The await-expression evaluates the (possibly-converted) o expression and the await-ready expression, then:
  • If the result of await-ready is false, the coroutine is considered suspended.
    • If the type of await-suspend is std​::​coroutine_handle<Z>, await-suspend.resume() is evaluated.
      [Note 1: 
      This resumes the coroutine referred to by the result of await-suspend.
      Any number of coroutines can be successively resumed in this fashion, eventually returning control flow to the current coroutine caller or resumer ([dcl.fct.def.coroutine]).
      — end note]
    • Otherwise, if the type of await-suspend is bool, await-suspend is evaluated, and the coroutine is resumed if the result is false.
    • Otherwise, await-suspend is evaluated.
    If the evaluation of await-suspend exits via an exception, the exception is caught, the coroutine is resumed, and the exception is immediately rethrown ([except.throw]).
    Otherwise, control flow returns to the current coroutine caller or resumer ([dcl.fct.def.coroutine]) without exiting any scopes ([stmt.jump]).
    The point in the coroutine immediately prior to control returning to its caller or resumer is a coroutine suspend point.
  • If the result of await-ready is true, or when the coroutine is resumed other than by rethrowing an exception from await-suspend, the await-resume expression is evaluated, and its result is the result of the await-expression.
[Note 2: 
With respect to sequencing, an await-expression is indivisible ([intro.execution]).
— end note]
[Example 1: template <typename T> struct my_future { /* ... */ bool await_ready(); void await_suspend(std::coroutine_handle<>); T await_resume(); }; template <class Rep, class Period> auto operator co_await(std::chrono::duration<Rep, Period> d) { struct awaiter { std::chrono::system_clock::duration duration; /* ... */ awaiter(std::chrono::system_clock::duration d) : duration(d) {} bool await_ready() const { return duration.count() <= 0; } void await_resume() {} void await_suspend(std::coroutine_handle<> h) { /* ... */ } }; return awaiter{d}; } using namespace std::chrono; my_future<int> h(); my_future<void> g() { std::cout << "just about to go to sleep...\n"; co_await 10ms; std::cout << "resumed\n"; co_await h(); } auto f(int x = co_await h()); // error: await-expression outside of function suspension context int a[] = { co_await h() }; // error: await-expression outside of function suspension context — end example]