std::coroutine_traits

 

C++

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Technical Specifications

 

Utilities library

Language support Type support (basic types, RTTI, type traits) Library feature-test macros (C++20) Dynamic memory management Program utilities Error handling Coroutine support (C++20) Variadic functions launder (C++17) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20) is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) integer_sequence (C++14) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_not_equalcmp_lesscmp_greatercmp_less_thancmp_greater_than (C++20)(C++20)(C++20)(C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) forward (C++11) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) apply (C++17) make_from_tuple (C++17) optional (C++17) any (C++17) variant (C++17) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

Coroutine support

Coroutine traits

coroutine_traits (C++20)

Coroutine handle

coroutine_handle (C++20)

No-op coroutines

noop_coroutine_promise (C++20)

noop_coroutine (C++20)

Trivial awaitables

suspend_never (C++20)

suspend_always (C++20)

 

Defined in header <coroutine>
template< class R, class... Args > struct coroutine_traits;		(since C++20)

Determines the promise type from the return type and parameter types of a coroutine. The standand library implementation provides a publicly accessible member type promise_type same as R::promise_type if the qualified-id is valid and denotes a type. Otherwise, it has no such member.

Program-defined specializations of coroutine_traits shall define a publicly accessible member type promise_type; otherwise, the behavior is undefined.

Template parameters

R	-	return type of the coroutine
Args	-	parameter types of the coroutine, including the implicit object parameter if the coroutine is a non-static member function

Member types

Type	Definition
promise_type	R::promise_type if it is valid, or provided by program-defined specializations

Possible implementation

template<class, class...> struct coroutine_traits {};   template<class R, class... Args> requires requires { typename R::promise_type } struct coroutine_traits<R, Args...> { using promise_type = R::promise_type; };

Notes

If the coroutine is a non-static member function, then the first type in Args... is the type of the implicit object parameter, and the rest are parameter types of the function (if any).

If std::coroutine_traits<R, Args...>::promise_type does not exist or is not a class type, the corresponding coroutine definition is ill-formed.

Users may define explicit or partial specializations of coroutine_traits dependent on program-defined types to avoid modification to return types.