The following code example is taken from the book
C++23 - The Complete Guide
by Nicolai M. Josuttis,
Leanpub, 2026
The code is licensed under a
Creative Commons Attribution 4.0 International License.
// raw code
#include <iostream>
#include <utility>
#include <generator>
#include "tracingpmr.hpp"
// coroutine using passed memory resource to hold its state:
std::pmr::generator<long> fibonacci(std::allocator_arg_t,
std::pmr::polymorphic_allocator<>,
int num)
{
// first two numbers are 1:
long a = 1;
long b = 1;
co_yield a; // SUSPEND with first number
co_yield b; // SUSPEND with second number
// loop over sums of previous two numbers:
for (int i = 0; i < num-2; ++i) {
auto sum = a + b;
co_yield sum; // SUSPEND with third to num-th number
a = std::exchange(b, sum);
}
}
int main()
{
// init a tracing memory resource:
TracingPMR pool;
{
std::cout << "*** init coroutine:\n";
auto gen = fibonacci(std::allocator_arg, &pool, 10);
std::cout << "*** call begin():\n";
auto pos = gen.begin();
std::cout << "*** loop with *pos and ++pos:\n";
while (pos != gen.end()) {
std::cout << *pos << ' ';
++pos; // RESUME
}
std::cout << '\n';
std::cout << "*** destroy coroutine:\n";
}
std::cout << "*** destroy pool\n";
}