Re: [Boost-users] Limitations of BOOST_COROUTINES_V2 (pass by reference)

Subject: Re: [Boost-users] Limitations of BOOST_COROUTINES_V2 (pass by reference)
From: Mueller-Roemer, Johannes Sebastian (Johannes.Sebastian.Mueller-Roemer_at_[hidden])
Date: 2014-07-04 02:28:11

Sorry for the misunderstanding, the part in the parentheses was meant to point out that I currently do not require bidirectional data flow, so I don't have a use case for that ;)

The auto& is not the issue and works just fine in the BOOST_COROUTINES_V1 case (and if it were, it would be a compiler bug as auto& must match std::string const &).

Here is the error message using auto& and type=std::string const &:
In file included from D:/libs/mingw-amd64/boost_1_55_0/include/boost/optional.hpp:15:0,
                 from D:/libs/mingw-amd64/boost_1_55_0/include/boost/coroutine/v2/coroutine.hpp:17,
                 from D:/libs/mingw-amd64/boost_1_55_0/include/boost/coroutine/coroutine.hpp:11,
                 from D:/libs/mingw-amd64/boost_1_55_0/include/boost/coroutine/all.hpp:11,
                 from D:\projects\coro\coro_getline.cpp:4:
D:/libs/mingw-amd64/boost_1_55_0/include/boost/optional/optional.hpp: In instantiation of 'void boost::optional_detail::optional_base<T>::assign(const boost::optional<U>&) [with U = const std::basic_string<char>&; T = const std::basic_string<char>*]':
D:/libs/mingw-amd64/boost_1_55_0/include/boost/optional/optional.hpp:588:9:   required from 'boost::optional<T>& boost::optional<T>::operator=(const boost::optional<U>&) [with U = const std::basic_string<char>&; T = const std::basic_string<char>*]'
D:/libs/mingw-amd64/boost_1_55_0/include/boost/coroutine/v2/detail/pull_coroutine_base.hpp:227:17:   required from 'void boost::coroutines::detail::pull_coroutine_base<R&>::pull() [with R = const std::basic_string<char>]'
D:/libs/mingw-amd64/boost_1_55_0/include/boost/coroutine/v2/coroutine.hpp:1592:9:   required from 'boost::coroutines::pull_coroutine<R&>& boost::coroutines::pull_coroutine<R&>::operator()() [with R = const std::basic_string<char>]'
D:/libs/mingw-amd64/boost_1_55_0/include/boost/coroutine/v2/coroutine.hpp:1630:21:   required from 'void boost::coroutines::pull_coroutine<R&>::iterator::increment_() [with R = const std::basic_string<char>]'
D:/libs/mingw-amd64/boost_1_55_0/include/boost/coroutine/v2/coroutine.hpp:1666:24:   required from 'boost::coroutines::pull_coroutine<R&>::iterator& boost::coroutines::pull_coroutine<R&>::iterator::operator++() [with R = const std::basic_string<char>]'
D:\projects\coro\coro_getline.cpp:26:19:   required from here
D:/libs/mingw-amd64/boost_1_55_0/include/boost/optional/optional.hpp:293:88: error: invalid static_cast from type 'boost::optional_detail::types_when_is_ref<const std::basic_string<char>&>::raw_type {aka const std::basic_string<char>}' to type 'boost::optional_detail::optional_base<const std::basic_string<char>*>::value_type {aka const std::basic_string<char>*}'
              assign_value(static_cast<value_type>(rhs.get()), is_reference_predicate() );
                                                                                        ^
D:/libs/mingw-amd64/boost_1_55_0/include/boost/optional/optional.hpp:299:55: error: invalid static_cast from type 'boost::optional_detail::types_when_is_ref<const std::basic_string<char>&>::raw_type {aka const std::basic_string<char>}' to type 'boost::optional_detail::optional_base<const std::basic_string<char>*>::value_type {aka const std::basic_string<char>*}'
           construct(static_cast<value_type>(rhs.get()));
                                                       ^

Using std::string const & instead of auto results in an *identical* error message. An old-style iterator-based for loop produces a nearly identical error. FIY the compiler is GCC 4.9.0 Mingw-w64 (which does have some other issues with boost.context, as the result of the ml64 output produces " corrupt .drectve at end of def file" warnings at link time, but that is unrelated to this issue).

--
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

-----Original Message-----
From: Boost-users [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Nat Goodspeed
Sent: Friday, July 04, 2014 02:21
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] Limitations of BOOST_COROUTINES_V2 (pass by reference)

On Thu, Jul 3, 2014 at 7:55 AM, Mueller-Roemer, Johannes Sebastian <Johannes.Sebastian.Mueller-Roemer_at_[hidden]> wrote:

> aside from the fact that the new coroutines are – apparently 
> intentionally – only unidirectional now (currently not much of an 
> issue for me),

I'm really curious about any real use case that you, or anyone else, might have for bidirectional data flow with a coroutine.

> I noticed
> that it is apparently no longer possible to yield values by reference 
> as shown in the following example:

I must respectfully disagree. I have a working example program using the new syntax that yields temporary values from a polymorphic class hierarchy by const reference.

> auto lines_from_file(char const * fname) -> 
> boost::coroutines::coroutine<type>::pull_type
> {
>                 using namespace boost::coroutines;
>                 return coroutine<type>::pull_type{
>                                [fname](coroutine<type>::push_type& yield)
>                                {
>                                                std::string line;
>                                                std::ifstream stream(fname);
>                                                while(stream)
>                                                    std::getline(stream, line), yield(line);
>                                },
>                                attributes(fpu_not_preserved)
>                 };
> }
>
> int main(int, char**)
> {
>                 auto lines = lines_from_file("test.txt");
>                 unsigned count = 0;
>                 for(auto& line : lines)
>                                std::cout << std::setw(4) << ++count << ": "
> << line << "\n";
> }
>
> If one changes type to std::string const & the example no longer compiles.

Could it be that auto& isn't matching std::string const&? What if you change auto& to std::string const &?
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