Re: [Boost-users] [context] boost::coroutines::asymmetric_coroutine<T>::pull_type iterator does not properly model InputIterator

Subject: Re: [Boost-users] [context] boost::coroutines::asymmetric_coroutine<T>::pull_type iterator does not properly model InputIterator
From: Mueller-Roemer, Johannes Sebastian (Johannes.Sebastian.Mueller-Roemer_at_[hidden])
Date: 2015-02-16 10:02:30

Example of how valid semantics could be achieved (as replacement for the old operator++(int), modulo potential mistakes):

struct proxy_reference
{
    proxy_reference(typename iterator::value_type const & v) : v(v) {}
    typename iterator::value_type const & operator*() const { return v; }
private:
    typename iterator::value_type v;
};

proxy_reference operator++( int)
{
    proxy_reference tmp(**this);
    ++*this;
    return tmp;
}

--
Johannes S. Mueller-Roemer, MSc
Wiss. Mitarbeiter - Interactive Engineering Technologies (IET)

Fraunhofer-Institut für Graphische Datenverarbeitung IGD
Fraunhoferstr. 5  |  64283 Darmstadt  |  Germany
Tel [phone]  |  Fax [phone]
johannes.mueller-roemer_at_[hidden]  |  www.igd.fraunhofer.de

From: Mueller-Roemer, Johannes Sebastian
Sent: Monday, February 16, 2015 15:41
To: 'boost-users_at_[hidden]'
Subject: RE: [Boost-users] [context] boost::coroutines::asymmetric_coroutine<T>::pull_type iterator does not properly model InputIterator

The temporary variable would not have to store the execution state of the coroutine, only the yielded value. Offering a broken postfix increment is very dangerous and should be avoided. Also, if I can’t count on InputIterator features being supported, the following is a lie:

class iterator : public std::iterator< std::input_iterator_tag, typename remove_reference< R >::type >

(taken from asymmetric_coroutine.hpp)

--
Johannes S. Mueller-Roemer, MSc
Wiss. Mitarbeiter - Interactive Engineering Technologies (IET)

Fraunhofer-Institut für Graphische Datenverarbeitung IGD
Fraunhoferstr. 5  |  64283 Darmstadt  |  Germany
Tel [phone]  |  Fax [phone]
johannes.mueller-roemer_at_[hidden]<mailto:johannes.mueller-roemer_at_[hidden]>  |  www.igd.fraunhofer.de<http://www.igd.fraunhofer.de>

From: Boost-users [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Oliver Kowalke
Sent: Monday, February 16, 2015 15:28
To: boost-users_at_[hidden]<mailto:boost-users_at_[hidden]>
Subject: Re: [Boost-users] [context] boost::coroutines::asymmetric_coroutine<T>::pull_type iterator does not properly model InputIterator

coroutines behave different than ordinary containers - even a temp var, created inside iterator::operator++( int), will influence the state of the attached coroutine (increment operator)
you can't count on the feature set described for input iterators in the standard

2015-02-16 15:00 GMT+01:00 Mueller-Roemer, Johannes Sebastian <Johannes.Sebastian.Mueller-Roemer_at_[hidden]<mailto:Johannes.Sebastian.Mueller-Roemer_at_[hidden]>>:
If that is „as expected“ it still violates the requirements for an InputIterator. See the last row in Table 107 in 24.2.3 in the standard http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf

As-is the current “iterator” is not a valid iterator of any kind as InputIterator is already the weakest requirement.

Example of how it should work (using an istream_iterator, which is an InputIterator):
#include <iostream>
#include <sstream>
#include <iterator>

auto main(int, char **) -> int
{
      std::istringstream str("Hello , World !");
      for(auto it = std::istream_iterator<std::string>(str); it != std::istream_iterator<std::string>();)
            std::cout << *it++;
      std::cout << "\n";
}

Which outputs (as expected):
Hello,World!

The issue can be solved by storing a value in the iterator or returning a proxy object which dereferences to the original value.
--
Johannes S. Mueller-Roemer, MSc
Wiss. Mitarbeiter - Interactive Engineering Technologies (IET)

Fraunhofer-Institut für Graphische Datenverarbeitung IGD
Fraunhoferstr. 5  |  64283 Darmstadt  |  Germany
Tel [phone]<tel:%2B49%206151%20155-606>  |  Fax [phone]<tel:%2B49%206151%20155-139>
johannes.mueller-roemer_at_[hidden]<mailto:johannes.mueller-roemer_at_[hidden]>  |  www.igd.fraunhofer.de<http://www.igd.fraunhofer.de>

From: Boost-users [mailto:boost-users-bounces_at_[hidden]<mailto:boost-users-bounces_at_[hidden]>] On Behalf Of Oliver Kowalke
Sent: Monday, February 16, 2015 14:20
To: boost-users_at_[hidden]<mailto:boost-users_at_[hidden]>
Subject: Re: [Boost-users] [context] boost::coroutines::asymmetric_coroutine<T>::pull_type iterator does not properly model InputIterator

2015-02-16 13:46 GMT+01:00 Mueller-Roemer, Johannes Sebastian <Johannes.Sebastian.Mueller-Roemer_at_[hidden]<mailto:Johannes.Sebastian.Mueller-Roemer_at_[hidden]>>:
Compiling is not the issue.

it works as expected

Furthermore your example does not test the issue I mentioned, as it does not use *it++;

operator++ has higher precedence than operator* -> each increment triggers an jump/switch operation -> last increment invalidates coroutine


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