std::coroutine_handle, std::noop_coroutine_handle

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

Compiler support
Freestanding and hosted
Language
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

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)

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)

std::coroutine_handle, std::noop_coroutine_handle

Member functions

Conversion

Observers

Control

Promise Access

Export/Import

Non-member functions

Helper classes

Notes

Example

Defect reports

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