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

19 Jan 2016

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-----Original Message-----
From: Boost-users [mailto:boost-users-bounces@lists.boost.org] On Behalf
Of
Nat Goodspeed
Sent: 19 January 2016 21:25
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] Using Coroutines in the Visitor Pattern
(use-case: BGL
event visitor)
On Tue, Jan 19, 2016 at 6:02 AM, alex  wrote:
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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?
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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.
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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
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know whether there's a value?
OK, if the constructor would not enter the function, I would use it like
this:

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

I expect the following in the evaluation of the while condition

1.The operator () will enter somefunc().
2. somefunc() will either complete or produce an int, the state of source
will reflect this
3. The operator()  returns a reference to source. 
4. If the function has completed the bool() function will cast it to false,
if the function has returned an int the bool() function will cast it to
true.
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Even if that were
desirable, how could you distinguish the case in which the coroutine
produces zero values?
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That case would just never enter the while loop.
But again: how do we know?
Operator() will cause somefunc() to complete and hence bool() will return
false: source.get() will never be called.
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