std::coroutine_handle, std::noop_coroutine_handle

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General 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)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management launder (C++17) Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) 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) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (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) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) 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)

Range generators

generator (C++23)

 

std::coroutine_handle

Member functions

coroutine_handle::coroutine_handle (C++20)

coroutine_handle::operator= (C++20)

coroutine_handle::from_promise (C++20)

Conversion

coroutine_handle::operator coroutine_handle<> (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)
Conversion
operator coroutine_handle<> (C++20)	obtains a type-erased coroutine_handle (public member function)
Observers
done (C++20)	checks if the coroutine has completed (public member function)
operator bool (C++20)	checks 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)
from_promise [static] (C++20)	creates a coroutine_handle from the promise object of a coroutine (public static 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)	imports 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 <optional>
 
template<std::movable T>
class Generator
{
public:
    struct promise_type
    {
        Generator<T> get_return_object()
        {
            return Generator{Handle::from_promise(*this)};
        }
        static std::suspend_always initial_suspend() noexcept
        {
            return {};
        }
        static std::suspend_always final_suspend() noexcept
        {
            return {};
        }
        std::suspend_always yield_value(T value) noexcept
        {
            current_value = std::move(value);
            return {};
        }
        // Disallow co_await in generator coroutines.
        void await_transform() = delete;
        [[noreturn]]
        static void unhandled_exception() { throw; }
 
        std::optional<T> current_value;
    };
 
    using Handle = std::coroutine_handle<promise_type>;
 
    explicit Generator(const Handle coroutine) :
        m_coroutine{coroutine}
    {}
 
    Generator() = default;
    ~Generator()
    {
        if (m_coroutine)
            m_coroutine.destroy();
    }
 
    Generator(const Generator&) = delete;
    Generator& operator=(const Generator&) = delete;
 
    Generator(Generator&& other) noexcept :
        m_coroutine{other.m_coroutine}
    {
        other.m_coroutine = {};
    }
    Generator& operator=(Generator&& other) noexcept
    {
        if (this != &other)
        {
            if (m_coroutine)
                m_coroutine.destroy();
            m_coroutine = other.m_coroutine;
            other.m_coroutine = {};
        }
        return *this;
    }
 
    // Range-based for loop support.
    class Iter
    {
    public:
        void operator++()
        {
            m_coroutine.resume();
        }
        const T& operator*() const
        {
            return *m_coroutine.promise().current_value;
        }
        bool operator==(std::default_sentinel_t) const
        {
            return !m_coroutine || m_coroutine.done();
        }
 
        explicit Iter(const Handle coroutine) :
            m_coroutine{coroutine}
        {}
 
    private:
        Handle m_coroutine;
    };
 
    Iter begin()
    {
        if (m_coroutine)
            m_coroutine.resume();
        return Iter{m_coroutine};
    }
 
    std::default_sentinel_t end() { return {}; }
 
private:
    Handle m_coroutine;
};
 
template<std::integral T>
Generator<T> range(T first, const T last)
{
    while (first < last)
        co_yield first++;
}
 
int main()
{
    for (const char i : range(65, 91))
        std::cout << i << ' ';
    std::cout << '\n';
}

Output:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3460	C++20	the public base class of coroutine_handle could leave it in an undesired state	inheritance removed

See also

generator (C++23)

A view that represents synchronous coroutine generator (class template)