Investigating cobalt::generator

Standard C++ generator

The code below demonstrates how standard C++23 synchronous generator works:

#include <boost/cobalt.hpp>
#include <iostream>
#include <generator>

namespace cobalt = boost::cobalt;

std::generator<int> numbers()
{
    for (int i = 1; i <= 5; ++i)
    {
        co_yield i;
    }
}

cobalt::main co_main(int argc, char* argv[])
{
    for (int v : numbers())
    {
        std::cout << "Got value: " << v << std::endl;
    }

    co_return 0;
}

Cobalt generator

I replaced standard generator with cobalt generator:

cobalt::generator<int> numbers2()
{
    for (int i = 1; i <= 5; ++i)
    {
        co_yield i;
    }
}

and got the following warning:

warning C4033: 'numbers2' must return a value

so it wonts me to add a fake co_return and I also use the generator is a different way, because it is asynchronous:

cobalt::generator<int> numbers2()
{
    for (int i = 1; i <= 5; ++i)
    {
        co_yield i;
    }

    co_return 0;
}

cobalt::main co_main(int argc, char* argv[])
{
    auto gen = numbers2();  // создаём генератор

    while (auto val = co_await gen)
    {
        std::cout << "Got value: " << val << std::endl;
    }

    co_return 0;
}

This code works and prints the following:

Got value: 1
Got value: 2
Got value: 3
Got value: 4
Got value: 5

Using BOOST_COBALT_FOR macro

Replaced co_main with the following:

cobalt::main co_main(int argc, char* argv[])
{
    auto gen = numbers2();  // создаём генератор

    BOOST_COBALT_FOR(auto val, gen)
    {
        std::cout << "Got value: " << val << std::endl;
    }

    co_return 0;
}

and got the following output:

Got value: 1
Got value: 2
Got value: 3
Got value: 4
Got value: 5
Got value: 0

True asynchronous generator

cobalt::generator<int> numbers3(asio::any_io_executor exec)
{
    asio::steady_timer timer(exec);

    for (int i = 1; i <= 5; ++i)
    {
        timer.expires_after(500ms);
        co_await timer.async_wait(cobalt::use_op);
        co_yield i;
    }

    co_return 0;
}

cobalt::main co_main(int argc, char* argv[])
{
    auto exec = co_await cobalt::this_coro::executor;

    auto gen = numbers3(exec);

    BOOST_COBALT_FOR(auto val, gen)
    {
        std::cout << "Got value: " << val << std::endl;
    }

    co_return 0;
}

Multiple iterating coroutines

Tried to iterate on two spawned coroutines:

cobalt::generator<int> number_source(int count)
{
    auto exec = co_await cobalt::this_coro::executor;
    asio::steady_timer timer(exec);

    for (int i = 1; i <= count; ++i)
    {
        timer.expires_after(500ms);
        co_await timer.async_wait(cobalt::use_op);
        co_yield i;
    }

    co_return 0;
}

using GenPtr = std::shared_ptr<cobalt::generator<int>>;

cobalt::task<void> consumer(int id, GenPtr gen)
{
    auto exec = co_await cobalt::this_coro::executor;
    asio::steady_timer timer(exec);

    while (true)
    {
        auto val = co_await *gen;
        if (!val)
            break;  // the channel is closed

        std::cout << "Consumer " << id << " got: " << val << "\n";

        timer.expires_after(500ms);
        co_await timer.async_wait(cobalt::use_op);
    }

    std::cout << "Consumer " << id << " finished\n";

    co_return;
}

cobalt::main co_main(int argc, char* argv[])
{
    GenPtr gen = std::make_shared<cobalt::generator<int>>(number_source(10));
    
    auto exec = co_await cobalt::this_coro::executor;

    cobalt::spawn(exec, consumer(1, gen), boost::asio::detached);
    cobalt::spawn(exec, consumer(2, gen), boost::asio::detached);

    co_return 0;
}

but got a result that I did not expect, coroutine #2 did not print anything:

Consumer 1 got: 1
Consumer 1 got: 2
Consumer 1 got: 3
Consumer 1 got: 4
Consumer 1 got: 5
Consumer 1 got: 6
Consumer 1 got: 7
Consumer 1 got: 8
Consumer 1 got: 9
Consumer 1 got: 10
Consumer 1 finished

DeveloperNote.com

A software developer's blog