17 Language support library [language.support]

17.12 Coroutines [support.coroutine]

17.12.3 Class template coroutine_­handle [coroutine.handle]

namespace std {
  template<>
  struct coroutine_handle<void>
  {
    // [coroutine.handle.con], construct/reset
    constexpr coroutine_handle() noexcept;
    constexpr coroutine_handle(nullptr_t) noexcept;
    coroutine_handle& operator=(nullptr_t) noexcept;

    // [coroutine.handle.export.import], export/import
    constexpr void* address() const noexcept;
    static constexpr coroutine_handle from_address(void* addr);

    // [coroutine.handle.observers], observers
    constexpr explicit operator bool() const noexcept;
    bool done() const;

    // [coroutine.handle.resumption], resumption
    void operator()() const;
    void resume() const;
    void destroy() const;

  private:
    void* ptr;  // exposition only
  };

  template<class Promise>
  struct coroutine_handle : coroutine_handle<>
  {
    // [coroutine.handle.con], construct/reset
    using coroutine_handle<>::coroutine_handle;
    static coroutine_handle from_promise(Promise&);
    coroutine_handle& operator=(nullptr_t) noexcept;

    // [coroutine.handle.export.import], export/import
    static constexpr coroutine_handle from_address(void* addr);

    // [coroutine.handle.promise], promise access
    Promise& promise() const;
  };
}
An object of type coroutine_­handle<T> is called a coroutine handle and can be used to refer to a suspended or executing coroutine.
A default-constructed coroutine_­handle object does not refer to any coroutine.
If a program declares an explicit or partial specialization of coroutine_­handle, the behavior is undefined.

17.12.3.1 Construct/reset [coroutine.handle.con]

constexpr coroutine_handle() noexcept; constexpr coroutine_handle(nullptr_t) noexcept;
Ensures: address() == nullptr.
static coroutine_handle from_promise(Promise& p);
Requires: p is a reference to a promise object of a coroutine.
Returns: A coroutine handle h referring to the coroutine.
Ensures: addressof(h.promise()) == addressof(p).
coroutine_handle& operator=(nullptr_t) noexcept;
Ensures: address() == nullptr.
Returns: *this.

17.12.3.2 Export/import [coroutine.handle.export.import]

constexpr void* address() const noexcept;
Returns: ptr.
static constexpr coroutine_handle<> coroutine_handle<>::from_address(void* addr); static constexpr coroutine_handle<Promise> coroutine_handle<Promise>::from_address(void* addr);
Requires: addr was obtained via a prior call to address.
Ensures: from_­address(address()) == *this.

17.12.3.3 Observers [coroutine.handle.observers]

constexpr explicit operator bool() const noexcept;
Returns: address() != nullptr.
bool done() const;
Requires: *this refers to a suspended coroutine.
Returns: true if the coroutine is suspended at its final suspend point, otherwise false.

17.12.3.4 Resumption [coroutine.handle.resumption]

Resuming a coroutine via resume, operator(), or destroy on an execution agent other than the one on which it was suspended has implementation-defined behavior unless each execution agent either is an instance of std​::​thread or std​::​jthread, or is the thread that executes main.
Note
:
A coroutine that is resumed on a different execution agent should avoid relying on consistent thread identity throughout, such as holding a mutex object across a suspend point.
— end note
 ]
Note
:
A concurrent resumption of the coroutine may result in a data race.
— end note
 ]
void operator()() const; void resume() const;
Requires: *this refers to a suspended coroutine.
Effects: Resumes the execution of the coroutine.
If the coroutine was suspended at its final suspend point, behavior is undefined.
void destroy() const;
Requires: *this refers to a suspended coroutine.
Effects: Destroys the coroutine ([dcl.fct.def.coroutine]).

17.12.3.5 Promise access [coroutine.handle.promise]

Promise& promise() const;
Requires: *this refers to a coroutine.
Returns: A reference to the promise of the coroutine.

17.12.3.6 Comparison operators [coroutine.handle.compare]

constexpr bool operator==(coroutine_handle<> x, coroutine_handle<> y) noexcept;
Returns: x.address() == y.address().
constexpr strong_ordering operator<=>(coroutine_handle<> x, coroutine_handle<> y) noexcept;
Returns: compare_­three_­way()(x.address(), y.address()).

17.12.3.7 Hash support [coroutine.handle.hash]

template<class P> struct hash<coroutine_handle<P>>;
The specialization is enabled ([unord.hash]).