Coroutines2 - why yield runs when no source called - c++

Question

I am learning how to use boost coroutines2 library. I have read some tutorials and started experimenting with it. But then I found something very confusing. Please take a look at this basic example.
#include <boost/coroutine2/all.hpp>
#include <iostream>
using namespace std;
typedef boost::coroutines2::coroutine<int> cr;
void creator(cr::push_type& yield)
{
cout << "First time." << endl;
yield(1);
cout << "Second time. " << endl;
yield(2);
}
int main()
{
cr::pull_type source{creator};
source();
}
The result is, naturally, this:
First time.
Second time.
But, to my surprise, when I remove the 'source' call in the main function, the result is just the same! (According to tutorials, the coroutine is called first time at construction time, so it is ok that it is called, but should be now called only once!)
#include <boost/coroutine2/all.hpp>
#include <iostream>
using namespace std;
typedef boost::coroutines2::coroutine<int> cr;
void creator(cr::push_type& yield)
{
cout << "First time." << endl;
yield(1);
cout << "Second time. " << endl;
yield(2);
}
int main()
{
cr::pull_type source{creator};
}
The result is still:
First time.
Second time.
When I remove the second 'yield' in the coroutine, the result is also the same:
#include <boost/coroutine2/all.hpp>
#include <iostream>
using namespace std;
typedef boost::coroutines2::coroutine<int> cr;
void creator(cr::push_type& yield)
{
cout << "First time." << endl;
yield(1);
cout << "Second time. " << endl;
}
int main()
{
cr::pull_type source{creator};
}
Result:
First time.
Second time.
How is that possible? How does it work? I expected that when I don't call the coroutine, then even if there is another 'yield' waiting, nothing will happen.
And I find also strange this behaviour:
When I add another 'source' statements in the main, the code still prints the same, as at the beginning!
#include <boost/coroutine2/all.hpp>
#include <iostream>
using namespace std;
typedef boost::coroutines2::coroutine<int> cr;
void creator(cr::push_type& yield)
{
cout << "First time." << endl;
yield(1);
cout << "Second time. " << endl;
yield(2);
}
int main()
{
cr::pull_type source{creator};
source();
source();
}
Result:
First time.
Second time.
No error, even when sourcing more times than there are 'yield's.
Only after adding one more 'source' in the main function do I receive a runtime error (This application has requested the Runtime to terminate it in an unusual way...)
int main()
{
cr::pull_type source{creator};
source();
source();
source();
}
Could someone help me please with understanding this behaviour?

Answer 1

The reason lies in a bug in Boost. I checked that in Boost 1.65.1 everything works fine. Here is the proof: https://wandbox.org/permlink/pRuSgnwa3VPdqNUk