17 Language support library [support]

17.12 Coroutines [support.coroutine]

17.12.1 General [support.coroutine.general]

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The header <coroutine> defines several types providing compile and run-time support for coroutines in a C++ program.

17.12.2 Header <coroutine> synopsis [coroutine.syn]

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// all freestanding #include <compare> // see [compare.syn] namespace std { // [coroutine.traits], coroutine traits template<class R, class... ArgTypes> struct coroutine_traits; // [coroutine.handle], coroutine handle template<class Promise = void> struct coroutine_handle; // [coroutine.handle.compare], comparison operators constexpr bool operator==(coroutine_handle<> x, coroutine_handle<> y) noexcept; constexpr strong_ordering operator<=>(coroutine_handle<> x, coroutine_handle<> y) noexcept; // [coroutine.handle.hash], hash support template<class T> struct hash; template<class P> struct hash<coroutine_handle<P>>; // [coroutine.noop], no-op coroutines struct noop_coroutine_promise; template<> struct coroutine_handle<noop_coroutine_promise>; using noop_coroutine_handle = coroutine_handle<noop_coroutine_promise>; noop_coroutine_handle noop_coroutine() noexcept; // [coroutine.trivial.awaitables], trivial awaitables struct suspend_never; struct suspend_always; }

17.12.3 Coroutine traits [coroutine.traits]

17.12.3.1 General [coroutine.traits.general]

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Subclause [coroutine.traits] defines requirements on classes representing coroutine traits, and defines the class template coroutine_traits that meets those requirements.

17.12.3.2 Class template coroutine_traits [coroutine.traits.primary]

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The header <coroutine> defines the primary template coroutine_traits such that if ArgTypes is a parameter pack of types and if the qualified-id R​::​promise_type is valid and denotes a type ([temp.deduct]), then coroutine_traits<R, ArgTypes...> has the following publicly accessible member: using promise_type = typename R::promise_type;
Otherwise, coroutine_traits<R, ArgTypes...> has no members.
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Program-defined specializations of this template shall define a publicly accessible nested type named promise_type.

17.12.4 Class template coroutine_handle [coroutine.handle]

17.12.4.1 General [coroutine.handle.general]

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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.con], construct/reset constexpr coroutine_handle() noexcept; constexpr coroutine_handle(nullptr_t) noexcept; static coroutine_handle from_promise(Promise&); 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.conv], conversion constexpr operator coroutine_handle<>() const noexcept; // [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; // [coroutine.handle.promise], promise access Promise& promise() const; private: void* ptr; // exposition only }; }
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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 coroutine_handle object whose member address() returns a null pointer value does not refer to any coroutine.
Two coroutine_handle objects refer to the same coroutine if and only if their member address() returns the same non-null value.
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If a program declares an explicit or partial specialization of coroutine_handle, the behavior is undefined.

17.12.4.2 Construct/reset [coroutine.handle.con]

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

17.12.4.3 Conversion [coroutine.handle.conv]

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constexpr operator coroutine_handle<>() const noexcept;
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Effects: Equivalent to: return coroutine_handle<>​::​from_address(address());

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

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constexpr void* address() const noexcept;
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Returns: ptr.
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static constexpr coroutine_handle<> coroutine_handle<>::from_address(void* addr);
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Preconditions: addr was obtained via a prior call to address on an object whose type is a specialization of coroutine_handle.
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Postconditions: from_address(address()) == *this.
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static constexpr coroutine_handle<Promise> coroutine_handle<Promise>::from_address(void* addr);
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Preconditions: addr was obtained via a prior call to address on an object of type cv coroutine_handle<Promise>.
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Postconditions: from_address(address()) == *this.

17.12.4.5 Observers [coroutine.handle.observers]

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

17.12.4.6 Resumption [coroutine.handle.resumption]

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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 1: 
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 2: 
A concurrent resumption of the coroutine can result in a data race.
— end note]
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void operator()() const; void resume() const;
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Preconditions: *this refers to a suspended coroutine.
The coroutine is not suspended at its final suspend point.
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Effects: Resumes the execution of the coroutine.
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void destroy() const;
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Preconditions: *this refers to a suspended coroutine.
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Effects: Destroys the coroutine ([dcl.fct.def.coroutine]).

17.12.4.7 Promise access [coroutine.handle.promise]

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

17.12.4.8 Comparison operators [coroutine.handle.compare]

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

17.12.4.9 Hash support [coroutine.handle.hash]

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

17.12.5 No-op coroutines [coroutine.noop]

17.12.5.1 Class noop_coroutine_promise [coroutine.promise.noop]

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struct noop_coroutine_promise {};
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The class noop_coroutine_promise defines the promise type for the coroutine referred to by noop_coroutine_handle ([coroutine.syn]).

17.12.5.2 Class coroutine_handle<noop_coroutine_promise> [coroutine.handle.noop]

17.12.5.2.1 General [coroutine.handle.noop.general]

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namespace std { template<> struct coroutine_handle<noop_coroutine_promise> { // [coroutine.handle.noop.conv], conversion constexpr operator coroutine_handle<>() const noexcept; // [coroutine.handle.noop.observers], observers constexpr explicit operator bool() const noexcept; constexpr bool done() const noexcept; // [coroutine.handle.noop.resumption], resumption constexpr void operator()() const noexcept; constexpr void resume() const noexcept; constexpr void destroy() const noexcept; // [coroutine.handle.noop.promise], promise access noop_coroutine_promise& promise() const noexcept; // [coroutine.handle.noop.address], address constexpr void* address() const noexcept; private: coroutine_handle(unspecified); void* ptr; // exposition only }; }

17.12.5.2.2 Conversion [coroutine.handle.noop.conv]

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constexpr operator coroutine_handle<>() const noexcept;
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Effects: Equivalent to: return coroutine_handle<>​::​from_address(address());

17.12.5.2.3 Observers [coroutine.handle.noop.observers]

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constexpr explicit operator bool() const noexcept;
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Returns: true.
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constexpr bool done() const noexcept;
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Returns: false.

17.12.5.2.4 Resumption [coroutine.handle.noop.resumption]

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constexpr void operator()() const noexcept; constexpr void resume() const noexcept; constexpr void destroy() const noexcept;
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Effects: None.
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Remarks: If noop_coroutine_handle is converted to coroutine_handle<>, calls to operator(), resume and destroy on that handle will also have no observable effects.

17.12.5.2.5 Promise access [coroutine.handle.noop.promise]

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noop_coroutine_promise& promise() const noexcept;
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Returns: A reference to the promise object associated with this coroutine handle.

17.12.5.2.6 Address [coroutine.handle.noop.address]

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constexpr void* address() const noexcept;
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Returns: ptr.
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Remarks: A noop_coroutine_handle's ptr is always a non-null pointer value.

17.12.5.3 Function noop_coroutine [coroutine.noop.coroutine]

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noop_coroutine_handle noop_coroutine() noexcept;
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Returns: A handle to a coroutine that has no observable effects when resumed or destroyed.
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Remarks: A handle returned from noop_coroutine may or may not compare equal to a handle returned from another invocation of noop_coroutine.

17.12.6 Trivial awaitables [coroutine.trivial.awaitables]

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namespace std { struct suspend_never { constexpr bool await_ready() const noexcept { return true; } constexpr void await_suspend(coroutine_handle<>) const noexcept {} constexpr void await_resume() const noexcept {} }; struct suspend_always { constexpr bool await_ready() const noexcept { return false; } constexpr void await_suspend(coroutine_handle<>) const noexcept {} constexpr void await_resume() const noexcept {} }; }
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[Note 1: 
The types suspend_never and suspend_always can be used to indicate that an await-expression either never suspends or always suspends, and in either case does not produce a value.
— end note]