Re: [Boost-users] Using Coroutines in the Visitor Pattern (use-case: BGL event visitor)

Subject: Re: [Boost-users] Using Coroutines in the Visitor Pattern (use-case: BGL event visitor)
From: Nat Goodspeed (nat_at_[hidden])
Date: 2016-01-19 16:24:33

On Tue, Jan 19, 2016 at 6:02 AM, alex <alexhighviz_at_[hidden]> wrote:

>>> I can see in the documentation that the constructor enters the coroutine-
>>function, but it is not clear to me why. Would it not have been neater if
>>this was avoided?

>>With a pull_type coroutine, the expectation is that every time it
>>suspends (until it exits), it has produced a value for its consumer.
>>If the coroutine constructor didn't enter the coroutine-function, the
>>first invocation would have to be a special case.

> To me it seems that the first invocation may either exit or produce a value,
> just as each subsequent invocation. I don't see the why the first invocation
> is special.

Okay, so let's consider this snippet:

    boost::coroutines::asymmetric_coroutine<int>::pull_type source(somefunc);
    while (source)
    {
        std::cout << source.get() << std::endl;
        source();
    }

Suppose the asymmetric_coroutine<int>::pull_type constructor did not
enter somefunc(). How would the operator bool() call invoked by the
while statement know whether there's a value?

>> Even if that were
>>desirable, how could you distinguish the case in which the coroutine
>>produces zero values?

> That case would just never enter the while loop.

But again: how do we know?