//

// 10.4.coroutine.cpp

// chapter 10 cpp20

// modern c++ tutorial

//

// created by changkun at changkun.de

// https://github.com/changkun/modern-cpp-tutorial

//

#include <coroutine>

#include <iostream>

#include <optional>

// A minimal lazy generator coroutine. C++20 provides only the language

// machinery (co_yield/co_await/co_return) plus the <coroutine> support

// library; the user (or a library) supplies the promise_type. C++23

// adds a ready-made std::generator.

template <typename T>

struct Generator {

struct promise_type {

T current;

Generator get_return_object() {

return Generator{handle::from_promise(*this)};

}

std::suspend_always initial_suspend() { return {}; }

std::suspend_always final_suspend() noexcept { return {}; }

std::suspend_always yield_value(T value) { current = value; return {}; }

void return_void() {}

void unhandled_exception() { std::terminate(); }

};

using handle = std::coroutine_handle<promise_type>;

handle h;

explicit Generator(handle h) : h(h) {}

~Generator() { if (h) h.destroy(); }

Generator(const Generator&) = delete;

Generator(Generator&& o) noexcept : h(o.h) { o.h = {}; }

std::optional<T> next() {

if (!h || h.done()) return std::nullopt;

h.resume();

if (h.done()) return std::nullopt;

return h.promise().current;

}

};

Generator<int> range(int a, int b) {

for (int i = a; i < b; ++i)

co_yield i;

}

int main() {

auto g = range(1, 5);

while (auto v = g.next()) std::cout << *v << ' '; // 1 2 3 4

std::cout << std::endl;

}