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.
#include <coroutine>
#include <exception> // for terminate()
class IntGen {
public:
// customization points:
struct promise_type {
int currentValue; // last value from co_yield
auto yield_value(int value) { // reaction on co_yield
currentValue = value; // - store value locally
return std::suspend_always{}; // - suspend coroutine
}
// the usual callbacks:
auto get_return_object() {
return std::coroutine_handle<promise_type>::from_promise(*this);
}
auto initial_suspend() { return std::suspend_always{}; }
auto final_suspend() noexcept { return std::suspend_always{}; }
void unhandled_exception() { std::terminate(); }
void return_void() { }
};
private:
// internal coroutine handle:
std::coroutine_handle<promise_type> coroHdl;
public:
// constructors and destructor:
IntGen(auto h) : coroHdl{h} {
}
~IntGen() {
if (coroHdl) {
coroHdl.destroy();
}
}
// no copying, but moving is supported:
IntGen(const IntGen&) = delete;
IntGen(IntGen&& rhs)
: coroHdl(rhs.coroHdl) {
rhs.coroHdl = nullptr;
}
IntGen& operator=(const IntGen&) = delete;
// API:
// - resume() to resume the coroutine
// - getValue() to get the last value from co_yield
bool resume() const {
if (!coroHdl) {
return false; // nothing (more) to process
}
coroHdl.resume(); // RESUME
return !coroHdl.done();
}
int getValue() const {
return coroHdl.promise().currentValue;
}
};