std::coroutine_handle, std::noop_coroutine_handle

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

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)

 

std::coroutine_handle

Member functions

coroutine_handle::coroutine_handle (C++20)

coroutine_handle::operator= (C++20)

coroutine_handle::from_promise (C++20)

Observers

coroutine_handle::done (C++20)

coroutine_handle::operator bool (C++20)

Control

coroutine_handle::operator()coroutine_handle::resume (C++20)(C++20)

coroutine_handle::destroy (C++20)

Promise access

coroutine_handle::promise (C++20)

Export/import

coroutine_handle::address (C++20)

coroutine_handle::from_address (C++20)

Non-member functions

operator==operator<=> (C++20)(C++20)

Helper classes

std::hash<std::coroutine_handle> (C++20)

 

Defined in header <coroutine>
template< class Promise = void > struct coroutine_handle;	(1)	(since C++20)
template<> struct coroutine_handle<void>;	(2)	(since C++20)
template<> struct coroutine_handle<std::noop_coroutine_promise>;	(3)	(since C++20)
using noop_coroutine_handle =     std::coroutine_handle<std::noop_coroutine_promise>;	(4)	(since C++20)

The class template coroutine_handle can be used to refer to a suspended or executing coroutine. Every specialization of coroutine_handle is a LiteralType.

1) Primary template, can be created from the promise object of type Promise.

2) Specialization std::coroutine_handle<void> erases the promise type. It is convertible from other specializations.

3) Specialization std::coroutine_handle<std::noop_coroutine_promise> refers to no-op coroutines. It cannot be created from a promise object.

On typical implementations, every specialization of std::coroutine_handle is TriviallyCopyable, and holds a pointer to the coroutine state as its only non-static data member.

The behavior of a program that adds specializations for coroutine_handle is undefined.

Member functions

(constructor) (C++20)	constructs a coroutine_handle object (public member function)
operator= (C++20)	assigns the coroutine_handle object (public member function)
from_promise [static] (C++20)	creates a coroutine_handle from the promise object of a coroutine (public static member function)
Observers
done (C++20)	check if the coroutine has completed (public member function)
operator bool (C++20)	check if the handle represents a coroutine (public member function)
Control
operator()resume (C++20)	resumes execution of the coroutine (public member function)
destroy (C++20)	destroys a coroutine (public member function)
Promise Access
promise (C++20)	access the promise of a coroutine (public member function)
Export/Import
address (C++20)	exports the underlying address, i.e. the pointer backing the coroutine (public member function)
from_address [static] (C++20)	import a coroutine from a pointer (public static member function)

Non-member functions

operator==operator<=> (C++20)

compares two coroutine_handle objects (function)

Helper classes

std::hash<std::coroutine_handle> (C++20)

hash support for std::coroutine_handle (class template specialization)

Notes

A coroutine_handle may be dangling, in which case the coroutine_handle must be used carefully in order to avoid undefined behavior.

Example

Run this code

#include <coroutine>
#include <iostream>
#include <ranges>
 
template<class T>
struct generator {
    struct promise_type {
        auto get_return_object() {
            return generator(std::coroutine_handle<promise_type>::from_promise(*this));
        }
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        void unhandled_exception() { throw; }
        std::suspend_always yield_value(T value) {
            current_value = std::move(value);
            return {};
        }
        void await_transform() = delete; // disallow co_await in generator coroutines
        T current_value;
    };
 
    generator(std::coroutine_handle<promise_type> h) : coro(h) {}
    generator(generator&& other) : coro(other.coro) { other.coro = {}; }
    ~generator() { if (coro) coro.destroy(); }
 
    // range-based for support
    struct iter {
        explicit iter(std::coroutine_handle<promise_type> h) : coro(h) {}
        void operator++() { coro.resume(); }
        T operator*() const { return coro.promise().current_value; }
        bool operator==(std::default_sentinel_t) const { return coro.done(); }
    private:
        std::coroutine_handle<promise_type> coro;
    };
    iter begin() { coro.resume(); return iter(coro); }
    std::default_sentinel_t end() { return {}; }
 
private:
    std::coroutine_handle<promise_type> coro;
};
 
generator<int> ints(int x) {
    for (int i = 0; i < x; ++i)
        co_yield i;
}
 
int main() {
    for (auto i : ints(5))
        std::cout << i << '\n';
}

Output:

0
1
2
3
4