NAME

std::coroutine_handle,std::noop_coroutine_handle - std::coroutine_handle,std::noop_coroutine_handle

Synopsis

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 = (4) (since C++20)
std::coroutine_handle<std::noop_coroutine_promise>;

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 constructs a coroutine_handle object
(C++20) (public member function)
operator= assigns the coroutine_handle object
(C++20) (public member function)
Conversion
operator coroutine_handle<> obtains a type-erased coroutine_handle
(C++20) (public member function)

Observers

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

Non-member functions

operator== compares two coroutine_handle objects
operator<=> (function)
(C++20)

Helper classes

std::hash<std::coroutine_handle> hash support for std::coroutine_handle
(C++20) (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 inheritance removed
could leave it in an undesired state