[boost] [log][phoenix] Bound arguments not protoified?

Subject: [boost] [log][phoenix] Bound arguments not protoified?
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2013-02-02 11:26:27

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 https://boost-log.svn.sourceforge.net/svnroot/boost-
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).

Compilation error:

In file included from ./boost/phoenix/core/preprocessed/expression.hpp:12:0,
                 from ./boost/phoenix/core/expression.hpp:21,
                 from ./boost/phoenix/core/argument.hpp:14,
                 from ./boost/phoenix/core.hpp:17,
                 from ./boost/phoenix/phoenix.hpp:11,
                 from ./boost/phoenix.hpp:11,
                 from libs/log/example/doc/extension_app_launcher.cpp:13:
./boost/phoenix/core/preprocessed/expression_10.hpp: In instantiation of 
‘static boost::phoenix::expr_ext<Actor, Tag, A0, A1>::type 
boost::phoenix::expr_ext<Actor, Tag, A0, A1>::make(typename 
boost::call_traits<A0>::param_type, typename 
boost::call_traits<A1>::param_type) [with Actor = boost::phoenix::actor; Tag = 
boost::phoenix::detail::tag::function_eval; A0 = 
boost::phoenix::detail::function_ptr<1, std::basic_string<char>, 
std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)>; A1 = 
boost::log::v2s_mt_posix::expressions::attribute_keyword<boost::log::v2s_mt_posix::trivial::tag::severity>; 
boost::phoenix::expr_ext<Actor, Tag, A0, A1>::type = 
boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, 
boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, 
boost::proto::argsns_::term<boost::phoenix::detail::function_ptr<1, 
std::basic_string<char>, std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)> >, 0l>, 
boost::log::v2s_mt_posix::expressions::attribute_actor<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity, boost::phoenix::actor> >, 
2l> >; typename boost::call_traits<A0>::param_type = const 
boost::phoenix::detail::function_ptr<1, std::basic_string<char>, 
std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)>&; typename 
boost::call_traits<A1>::param_type = const 
boost::log::v2s_mt_posix::expressions::attribute_keyword<boost::log::v2s_mt_posix::trivial::tag::severity>&]’:
./boost/phoenix/bind/bind_function.hpp:45:83:   required from ‘const typename 
boost::phoenix::detail::expression::function_eval<boost::phoenix::detail::function_ptr<1, 
RT, RT (*)(T0)>, A0>::type boost::phoenix::bind(RT (*)(T0), const A0&) [with 
RT = std::basic_string<char>; T0 = const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&; A0 = 
boost::log::v2s_mt_posix::expressions::attribute_keyword<boost::log::v2s_mt_posix::trivial::tag::severity>; 
typename 
boost::phoenix::detail::expression::function_eval<boost::phoenix::detail::function_ptr<1, 
RT, RT (*)(T0)>, A0>::type = 
boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, 
boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, 
boost::proto::argsns_::term<boost::phoenix::detail::function_ptr<1, 
std::basic_string<char>, std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)> >, 0l>, 
boost::log::v2s_mt_posix::expressions::attribute_actor<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity, boost::phoenix::actor> >, 
2l> >]’
libs/log/example/doc/extension_app_launcher.cpp:92:91:   required from here
./boost/phoenix/core/preprocessed/expression_10.hpp:98:17: warning: missing 
braces around initializer for 
‘boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, 
boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, 
boost::proto::argsns_::term<boost::phoenix::detail::function_ptr<1, 
std::basic_string<char>, std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)> >, 0l>, 
boost::log::v2s_mt_posix::expressions::attribute_actor<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity, boost::phoenix::actor> >, 
2l> >::expr_type {aka 
boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, 
boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, 
boost::proto::argsns_::term<boost::phoenix::detail::function_ptr<1, 
std::basic_string<char>, std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)> >, 0l>, 
boost::log::v2s_mt_posix::expressions::attribute_actor<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity, boost::phoenix::actor> >, 
2l>}’ [-Wmissing-braces]
./boost/phoenix/core/preprocessed/expression_10.hpp:98:17: warning: missing 
braces around initializer for 
‘boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, 
boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, 
boost::proto::argsns_::term<boost::phoenix::detail::function_ptr<1, 
std::basic_string<char>, std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)> >, 0l>, 
boost::log::v2s_mt_posix::expressions::attribute_actor<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity, boost::phoenix::actor> >, 
2l>::proto_child0 {aka 
boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, 
boost::proto::argsns_::term<boost::phoenix::detail::function_ptr<1, 
std::basic_string<char>, std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)> >, 0l>}’ [-Wmissing-
braces]
./boost/phoenix/core/preprocessed/expression_10.hpp:98:17: error: could not 
convert ‘boost::proto::make_expr<boost::phoenix::detail::tag::function_eval, 
boost::proto::domainns_::basic_default_domain, 
boost::phoenix::detail::function_ptr<1, std::basic_string<char>, 
std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)>, 
boost::log::v2s_mt_posix::expressions::attribute_keyword<boost::log::v2s_mt_posix::trivial::tag::severity> 
>((* & a0), (* & a1))’ from ‘const type {aka const 
boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, 
boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, 
boost::proto::argsns_::term<boost::phoenix::detail::function_ptr<1, 
std::basic_string<char>, std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)> >, 0l>, 
boost::log::v2s_mt_posix::expressions::attribute_keyword<boost::log::v2s_mt_posix::trivial::tag::severity> 
>, 2l>}’ to 
‘boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, 
boost::proto::argsns_::term<boost::phoenix::detail::function_ptr<1, 
std::basic_string<char>, std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)> >, 0l>::proto_child0 {aka 
boost::phoenix::detail::function_ptr<1, std::basic_string<char>, 
std::basic_string<char> (*)(const 
boost::log::v2s_mt_posix::value_ref<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::trivial::tag::severity>&)>}’
./boost/phoenix/core/preprocessed/expression_10.hpp:98:17: error: no matching 
function for call to 
‘boost::log::v2s_mt_posix::expressions::attribute_actor<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity, 
boost::phoenix::actor>::attribute_actor(<brace-enclosed initializer list>)’
./boost/phoenix/core/preprocessed/expression_10.hpp:98:17: note: candidates 
are:
In file included from ./boost/log/expressions.hpp:20:0,
                 from libs/log/example/doc/extension_app_launcher.cpp:16:
./boost/log/expressions/attr.hpp:174:14: note: 
boost::log::v2s_mt_posix::expressions::attribute_actor<T, FallbackPolicyT, 
TagT, ActorT>::attribute_actor(const base_type&) [with T = 
boost::log::v2s_mt_posix::trivial::severity_level; FallbackPolicyT = 
boost::log::v2s_mt_posix::fallback_to_none; TagT = 
boost::log::v2s_mt_posix::trivial::tag::severity; ActorT = 
boost::phoenix::actor; 
boost::log::v2s_mt_posix::expressions::attribute_actor<T, FallbackPolicyT, 
TagT, ActorT>::base_type = 
boost::phoenix::actor<boost::log::v2s_mt_posix::expressions::attribute_terminal<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity> >]
./boost/log/expressions/attr.hpp:174:14: note:   candidate expects 1 argument, 
0 provided
./boost/log/expressions/attr.hpp:156:7: note: 
boost::log::v2s_mt_posix::expressions::attribute_actor<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity, 
boost::phoenix::actor>::attribute_actor(const 
boost::log::v2s_mt_posix::expressions::attribute_actor<boost::log::v2s_mt_posix::trivial::severity_level, 
boost::log::v2s_mt_posix::fallback_to_none, 
boost::log::v2s_mt_posix::trivial::tag::severity, boost::phoenix::actor>&)
./boost/log/expressions/attr.hpp:156:7: note:   candidate expects 1 argument, 
0 provided