id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
7996	phoenix::bind does not protoify the bound arguments	Andrey Semashev	Thomas Heller	"I have a problem with Boost.Phoenix v3 bind implementation when used with attribute keywords from Boost.Log. For some reason phoenix::bind does not apply proto::detail::protoify to the bound arguments which breaks compilation of some formatting expressions in Boost.Log. 

Here's an example:

{{{
#include <boost/phoenix.hpp>
#include <boost/log/trivial.hpp>
#include <boost/log/expressions.hpp>
#include <boost/log/utility/value_ref.hpp>

namespace logging = boost::log;
namespace expr = boost::log::expressions;
namespace phoenix = boost::phoenix;

// Custom severity level formatting function
std::string severity_level_as_urgency(
    logging::value_ref< logging::trivial::severity_level, logging::trivial::tag::severity > const& level)
{
    if (!level || level.get() == logging::trivial::info)
        return ""normal"";
    logging::trivial::severity_level lvl = level.get();
    if (lvl < logging::trivial::info)
        return ""low"";
    else
        return ""critical"";
}

int main(int, char*[])
{
    logging::formatter fmt =
        expr::stream << boost::phoenix::bind(&severity_level_as_urgency, logging::trivial::severity);

    return 0;
}
}}}

The example creates a formatter function object that should call severity_level_as_urgency function to convert the severity level to string and put its result into a stream.

trivial::severity is a keyword of type expr::attribute_keyword< ... >. 
Keywords themselves should never actually be embedded into phoenix 
expressions, instead I have attribute_actor that implements all the necessary work to extract attribute values. I have specialized proto::detail::protoify template for attribute_keyword (including references and reference_wrappers thereof) so that it is automatically converted to attribute_actor whenever it participates in expressions. But it doesn't work with the above code, Boost.Phoenix embeds attribute_keyword as is into the expression. This results 
in the error I attached to the ticket.

I realize that proto::detail::protoify is not for public use and may not be intended for my use case but I did not find any other way to implement what I described. Anyway, I think bind should treat bound arguments as child subexpressions and protoify them.

To compile the test code you will have to checkout Boost.Log from SVN:

{{{
svn co https://boost-log.svn.sourceforge.net/svnroot/boost-log/branches/bleeding-edge -r 822 boost-log
}}}

Then the relevant directories have to be linked/copied into the Boost tree.


PS: The issue came from the mailing list:

[http://lists.boost.org/Archives/boost/2013/02/200701.php]

"	Bugs	new	To Be Determined	phoenix	Boost Release Branch	Problem		phoenix bind proto log