Coroutines (C++20)

A coroutine is a function that can suspend execution to be resumed later. Coroutines are stackless: they suspend execution by returning to the caller. This allows for sequential code that executes asynchronously (e.g. to handle non-blocking I/O without explicit callbacks), and also supports algorithms on lazy-computed infinite sequences and other uses.

A function is a coroutine if its definition does any of the following:

• uses the co_await operator to suspend execution until resumed

task<> tcp_echo_server() {
  char data[1024];
  for (;;) {
    size_t n = co_await socket.async_read_some(buffer(data));
    co_await async_write(socket, buffer(data, n));
  }
}

• uses the keyword co_yield to suspend execution returning a value

generator<int> iota(int n = 0) {
  while(true)
    co_yield n++;
}

• uses the keyword co_return to complete execution returning a value

lazy<int> f() {
  co_return 7;
}

Every coroutine must have a return type that satisfies a number of requirements, noted below.

Restrictions

Coroutines cannot use variadic arguments, plain return statements, or placeholder return types (auto or Concept).

Constexpr functions, constructors, destructors, and the main function cannot be coroutines.

Execution

Each coroutine is associated with.

• the promise object, manipulated from inside the coroutine. The coroutine submits its result or exception through this object.
• the coroutine handle, manipulated from outside the coroutine. This is a non-owning handle used to resume execution of the coroutine or to destroy the coroutine frame.
• the coroutine state, which is an internal, heap-allocated (unless the allocation is optimized out), object that contains
• the promise object
• the parameters (all copied by value)
• some representation of the current suspension point, so that resume knows where to continue and destroy knows what local variables were in scope
• local variables and temporaries whose lifetime spans the current suspension point

When a coroutine begins execution, it performs the following:

• allocates the coroutine state object using operator new (see below)
• copies all function parameters to the coroutine state: by-value parameters are moved or copied, by-reference parameters remain references (and so may become dangling if the coroutine is resumed after the lifetime of referred object ends)
• calls the constructor for the promise object. If the promise type has a constructor that takes all coroutine parameters, that constuctor is called, with post-copy coroutine arguments. Otherwise the default constructor is called.
• calls promise.get_return_object() and keeps the result in a local variable. The result of that call will be returned to the caller when the coroutine first suspends. Any exceptions thrown up to and including this step propagate back to the caller, not placed in the promise.
• calls promise.initial_suspend() and co_await's its result. Typical Promise types either return a suspend_always, for lazily-started coroutines, or suspend_never, for eagerly-started coroutines.
• when co_await promise.initial_suspend() resumes, starts executing the body of the coroutine

When a coroutine reaches a suspension point.

• the return object obtained earlier is returned to the caller/resumer, after implicit conversion to the return type of the coroutine, if necessary.

When a coroutine reaches the co_return statement, it performs the following:

• calls promise.return_void() for
• co_return;
• co_return expr where expr has type void
• falling off the end of a void-returning coroutine. The behavior is undefined if the Promise type has no Promise::return_void() member function in this case.
• or calls promise.return_value(expr) for co_return expr where expr has non-void type
• destroys all variables with automatic storage duration in reverse order they were created.
• calls promise.final_suspend() and co_await's the result.

If the coroutine ends with an uncaught exception, it performs the following:

• catches the exception and calls promise.unhandled_exception() from within the catch-block
• calls promise.final_suspend() and co_await's the result (e.g. to resume a continuation or publish a result). It's undefined behavior to resume a coroutine from this point.

When the coroutine state is destroyed either because it terminated via co_return or uncaught exception, or because it was destroyed via its handle, it does the following:

• calls the destructor of the promise object.
• calls the destructors of the function parameter copies.
• calls operator delete to free the memory used by the coroutine state
• transfers execution back to the caller/resumer.

Heap allocation

coroutine state is allocated on the heap via non-array operator new.

If the Promise type defines a class-level replacement, it will be used, otherwise global operator new will be used.

If the Promise type defines a placement form of operator new that takes additional parameters, and they match an argument list where the first argument is the size requested (of type std::size_t) and the rest are the coroutine function arguments, those arguments will be passed to operator new (this makes it possible to use leading-allocator-convention for coroutines).

The call to operator new can be optimized out (even if custom allocator is used) if.

• The lifetime of the coroutine state is strictly nested within the lifetime of the caller, and
• the size of coroutine frame is known at the call site

in that case, coroutine state is embedded in the caller's stack frame (if the caller is an ordinary function) or coroutine state (if the caller is a coroutine).

If allocation fails, the coroutine throws std::bad_alloc, unless the Promise type defines the member function Promise::get_return_object_on_allocation_failure(). If that member function is defined, allocation uses the nothrow form of operator new and on allocation failure, the coroutine immediately returns the object obtained from Promise::get_return_object_on_allocation_failure() to the caller.

Promise

The Promise type is determined by the compiler from the return type of the coroutine using std::coroutine_traits.

If the coroutine is defined as task<float> foo(std::string x, bool flag);, then its Promise type is std::coroutine_traits<task<float>, std::string, bool>::promise_type.

If the coroutine is a non-static member function, such as task<void> my_class::method1(int x) const;, its Promise type is std::coroutine_traits<task<void>, const my_class&, int>::promise_type.

co_await

The unary operator co_await suspends a coroutine and returns control to the caller. Its operand is an expression whose type must either define operator co_await, or be convertible to such type by means of the current coroutine's Promise::await_transform.

First, expr is converted to an awaitable as follows:

• if expr is produced by an initial suspend point, a final suspend point, or a yield expression, the awaitable is expr, as-is.
• otherwise, if the current coroutine's Promise type has the member function await_transform, then the awaitable is promise.await_transform(expr)
• otherwise, the awaitable is expr, as-is.

Then, the awaiter object is obtained, as follows:

• if overload resolution for operator co_await gives a single best overload, the awaiter is the result of that call (awaitable.operator co_await() for member overload, operator co_await(static_cast<Awaitable&&>(awaitable)) for the non-member overload)
• otherwise, if overload resolution finds no operator co_await, the awaiter is awaitable, as-is
• otherwise, if overload resolution is ambiguous, the program is ill-formed

If the expression above is a prvalue, the awaiter object is a temporary materialized from it. Otherwise, if the expression above is an glvalue, the awaiter object is the object to which it refers.

Then, awaiter.await_ready() is called (this is a short-cut to avoid the cost of suspension if it's known that the result is ready or can be completed synchronously). If its result, contextually-converted to bool is false then.

The coroutine is suspended (its coroutine state is populated with local variables and current suspension point).

awaiter.await_suspend(handle) is called, where handle is the coroutine handle representing the current coroutine. Inside that function, the suspended coroutine state is observable via that handle, and it's this function's responsibility to schedule it to resume on some executor, or to be destroyed (returning false counts as scheduling)

• if await_suspend returns void, control is immediately returned to the caller/resumer of the current coroutine (this coroutine remains suspended), otherwise
• if await_suspend returns bool,
• the value true returns control to the caller/resumer of the current coroutine
• the value false resumes the current coroutine.
• if await_suspend returns a coroutine handle for some other coroutine, that handle is resumed (by a call to handle.resume()) (note this may chain to eventually cause the current coroutine to resume)
• if await_suspend throws an exception, the exception is caught, the coroutine is resumed, and the exception is immediately re-thrown

Finally, awaiter.await_resume() is called, and its result is the result of the whole co_await expr expression.

If the coroutine was suspended in the co_await expression, and is later resumed, the resume point is immediately before the call to awaiter.await_resume().

Note that because the coroutine is fully suspended before entering awaiter.await_suspend(), that function is free to transfer the coroutine handle across threads, with no additional synchronization. For example, it can put it inside a callback, scheduled to run on a threadpool when async I/O operation completes. This also means the current coroutine may resume and finish on that threadpool, concurrently, while still inside await_suspend(), and so await_suspend() should not expect the awaiter (the *this object) to be accessible after the handle was published to other threads.

Note: the awaiter object is part of coroutine state (as a temporary whose lifetime crosses a suspension point) and is destroyed before the co_await expression finishes. It can be used to maintain per-operation state as required by some async I/O APIs without resorting to additional heap allocations.

The standard library defines two trivial awaitables: std::suspend_always and std::suspend_never.

co_yield

Yield-expression returns a value to the caller and suspends the current coroutine: it is the common building block of resumable generator functions.

It is equivalent to.

co_await promise.yield_value(expr)

A typical generator's yield_value would store (copy/move or just store the address of, since the argument's lifetime crosses the suspension point inside the co_await) its argument into the generator object and return std::suspend_always, transferring control to the caller/resumer.

Library support