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 09:00:06

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]  |  Fax [phone]
johannes.mueller-roemer_at_[hidden]  |  www.igd.fraunhofer.de

From: Boost-users [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Oliver Kowalke
Sent: Monday, February 16, 2015 14:20
To: 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