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 +49 6151 155-606 | Fax +49 6151 155-139
johannes.mueller-roemer@igd.fraunhofer.de | www.igd.fraunhofer.de
From: Mueller-Roemer, Johannes Sebastian
Sent: Monday, February 16, 2015 15:41
To: 'boost-users@lists.boost.org'
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 +49 6151 155-606 | Fax +49 6151 155-139
johannes.mueller-roemer@igd.fraunhofer.de |
www.igd.fraunhofer.de
From: Boost-users [mailto:boost-users-bounces@lists.boost.org]
On Behalf Of Oliver Kowalke
Sent: Monday, February 16, 2015 15:28
To: boost-users@lists.boost.org
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@igd.fraunhofer.de>:
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
+49 6151 155-606 | Fax
+49 6151 155-139
johannes.mueller-roemer@igd.fraunhofer.de
| www.igd.fraunhofer.de
From: Boost-users
[mailto:boost-users-bounces@lists.boost.org]
On Behalf Of Oliver Kowalke
Sent: Monday, February 16, 2015 14:20
To: boost-users@lists.boost.org
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@igd.fraunhofer.de>:
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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