std::coroutine_handle,std::noop_coroutine_handle(3)

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