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Subject: Re: [boost] [log][phoenix] Bound arguments not protoified?
From: Joel de Guzman (djowel_at_[hidden])
Date: 2013-02-06 09:31:50

On 2/6/13 3:17 PM, Andrey Semashev wrote:
> On Sat, Feb 2, 2013 at 8:26 PM, Andrey Semashev
> <andrey.semashev_at_[hidden]> wrote:
>> Hi,
>> 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:
>> 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);
>> // The function initializes the logging library
>> void init_logging()
>> {
>> // ...
>> sink->set_formatter
>> (
>> expr::stream
>> << phoenix::bind(&severity_level_as_urgency, logging::trivial::severity)
>> );
>> }
>> The intended behavior of this code is to invoke severity_level_as_urgency
>> function with severity level value extracted from a log record and put the
>> result of this function into a stream, thus forming a formatted representation
>> of the record.
>> 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 posted in the end of the post.
>> 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. Is that right?
>> In case if you want to see the complete code of the example and/or Boost.Log,
>> you can check it out from SVN:
>> svn co
>> log/branches/bleeding-edge boost-log
>> The example resides in libs/log/example/doc/extension_app_launcher.cpp. I have
>> modified it to call or_none() on trivial::severity so that the example
>> compiles; in order to reproduce the problem you have to remove this call (it
>> sould be performed by my protoify specialization instead).
> Ping? Anyone? Should I create a ticket for Boost.Phoenix?

That would be best, yes. Seems Thomas Heller is very busy these days.
I'll see if I can take over some of the support tasks.


Joel de Guzman

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