The following code example is taken from the book
C++20 - The Complete Guide
by Nicolai M. Josuttis,
Leanpub, 2021
The code is licensed under a
Creative Commons Attribution 4.0 International License.
// raw code
#include <coroutine>
#include <exception> // for terminate()
#include <cassert> // for assert()
template<typename T>
class [[nodiscard]] Generator {
public:
// customization points:
struct promise_type {
T coroValue{}; // last value from co_yield
auto yield_value(T val) { // reaction to co_yield
coroValue = val; // - store value locally
return std::suspend_always{}; // - suspend coroutine
}
auto get_return_object() {
return std::coroutine_handle<promise_type>::from_promise(*this);
}
auto initial_suspend() { return std::suspend_always{}; }
void return_void() { }
void unhandled_exception() { std::terminate(); }
auto final_suspend() noexcept { return std::suspend_always{}; }
};
private:
std::coroutine_handle<promise_type> hdl; // native coroutine handle
public:
// constructors and destructor:
Generator(auto h) : hdl{h} { }
~Generator() { if (hdl) hdl.destroy(); }
// no copy or move supported:
Generator(const Generator&) = delete;
Generator& operator=(const Generator&) = delete;
// API to resume the coroutine and access its values:
// - iterator interface with begin() and end()
struct iterator {
std::coroutine_handle<promise_type> hdl; // nullptr on end
iterator(auto p) : hdl{p} {
}
void getNext() {
if (hdl) {
hdl.resume(); // RESUME
if (hdl.done()) {
hdl = nullptr;
}
}
}
int operator*() const {
assert(hdl != nullptr);
return hdl.promise().coroValue;
}
iterator operator++() {
getNext(); // resume for next value
return *this;
}
bool operator== (const iterator& i) const = default;
};
iterator begin() const {
if (!hdl || hdl.done()) {
return iterator{nullptr};
}
iterator itor{hdl}; // initialize iterator
itor.getNext(); // resume for first value
return itor;
}
iterator end() const {
return iterator{nullptr};
}
};