Boost Log - why doesnt this compile?

Question

I've been thoroughly confused by the following compile error for about the last three hours. Can anybody tell me what is going on here?

I attempted to define the log::formatter (marked below) as its own variable, so it could be re-used in a couple places. However, I get a compile error when trying to re-use it.

However, if I get rid of that variable entirely, and instead copy-and-paste the code, it compiles. What the heck? Is there any way to do what I want?

boost::shared_ptr<log::core> logger = log::core::get();

logger->set_logging_enabled( enabled );
logger->set_filter(trivial::severity >= level);                                                                                                                           

logger->add_global_attribute(
    "TimeStamp", attr::local_clock());

logger->add_global_attribute(
    "ProcessID", attr::current_process_id());

logger->add_global_attribute(
    "ThreadID", attr::current_thread_id());

// want this to be it's own variable
log::formatter fmt = expr::stream
    << "[" << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%d.%m.%Y %H:%M:%S.%f") << "] "
    << "[" << expr::attr< attr::current_process_id::value_type >("ProcessID") << "] "
    << "[" << expr::attr< attr::current_thread_id::value_type >("ThreadID") << "] "
    << "[" << expr::attr< std::string >("Channel") << "] "
    << "[" << expr::attr< severity_level >("Severity") << "]: "
    << expr::smessage
    ;   

// so it can be reused here, but this is a compiler error
log::add_console_log(
    std::clog, keywords::format=fmt);

// and here, too. But this is also a compiler error
log::add_file_log(
    "test.log", keywords::format=fmt);

Compile error (with clang++) is:

In file included from ../src/util/logging/Logging.cpp:34:                                                                                                                 
In file included from /opt/boost-1.54.0/include/boost/log/utility/setup/console.hpp:22:
/opt/boost-1.54.0/include/boost/log/detail/sink_init_helpers.hpp:107:21: error: no matching function for call to 'acquire_formatter'
    s.set_formatter(aux::acquire_formatter(args[keywords::format]));
                    ^~~~~~~~~~~~~~~~~~~~~~
/opt/boost-1.54.0/include/boost/log/utility/setup/console.hpp:76:5: note: in instantiation of function template specialization 'boost::log::v2_mt_posix::aux::setup_format
boost::log::v2_mt_posix::sinks::synchronous_sink<boost::log::v2_mt_posix::sinks::basic_text_ostream_backend<char> >, boost::parameter::aux::tagged_argument<boost::log::v2
posix::keywords::tag::format, boost::log::v2_mt_posix::basic_formatter<char> > >' requested here
    aux::setup_formatter(*pSink, args,
    ^   
/opt/boost-1.54.0/include/boost/log/utility/setup/console.hpp:136:12: note: in instantiation of function template specialization 'boost::log::v2_mt_posix::aux::add_consol
g<char, boost::parameter::aux::tagged_argument<boost::log::v2_mt_posix::keywords::tag::format, boost::log::v2_mt_posix::basic_formatter<char> > >' requested here
    return aux::add_console_log(strm, arg1);
           ^   
../src/util/logging/Logging.cpp:121:2: note: in instantiation of function template specialization 'boost::log::v2_mt_posix::add_console_log<char, boost::parameter::aux::t
d_argument<boost::log::v2_mt_posix::keywords::tag::format, boost::log::v2_mt_posix::basic_formatter<char> > >' requested here
        log::add_console_log(
        ^   
/opt/boost-1.54.0/include/boost/log/detail/sink_init_helpers.hpp:80:33: note: candidate template ignored: failed template argument deduction
inline basic_formatter< CharT > acquire_formatter(const CharT* formatter)
                                ^   
/opt/boost-1.54.0/include/boost/log/detail/sink_init_helpers.hpp:85:33: note: candidate template ignored: failed template argument deduction
inline basic_formatter< CharT > acquire_formatter(std::basic_string< CharT, TraitsT, AllocatorT > const& formatter)
                                ^   
/opt/boost-1.54.0/include/boost/log/detail/sink_init_helpers.hpp:91:5: note: candidate template ignored: disabled by 'enable_if' [with FormatterT = boost::log::v2_mt_posi
asic_formatter<char>]
    phoenix::is_actor< FormatterT >,
    ^   

However, if I instead change it to this, it compiles:

boost::shared_ptr<log::core> logger = log::core::get();

logger->set_logging_enabled( enabled );
logger->set_filter(trivial::severity >= level);

logger->add_global_attribute(
    "TimeStamp", attr::local_clock());

logger->add_global_attribute(
    "ProcessID", attr::current_process_id());

logger->add_global_attribute(
    "ThreadID", attr::current_thread_id());

// copy and paste the expression
log::add_console_log(                                                                                                                                                     
    std::clog,
    keywords::format =
    (
        expr::stream
            << "[" << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%d.%m.%Y %H:%M:%S.%f") << "] "
            << "[" << expr::attr< attr::current_process_id::value_type >("ProcessID") << "] "
            << "[" << expr::attr< attr::current_thread_id::value_type >("ThreadID") << "] "
            << "[" << expr::attr< std::string >("Channel") << "] "
            << "[" << expr::attr< severity_level >("Severity") << "]: "
            << expr::smessage
    )
);

// copy and paste the expression again
log::add_file_log(
    "test.log",
    keywords::format =
    (
        expr::stream
            << "[" << expr::format_date_time< boost::posix_time::ptime >("TimeStamp", "%d.%m.%Y %H:%M:%S.%f") << "] "
            << "[" << expr::attr< attr::current_process_id::value_type >("ProcessID") << "] "
            << "[" << expr::attr< attr::current_thread_id::value_type >("ThreadID") << "] "
            << "[" << expr::attr< std::string >("Channel") << "] "
            << "[" << expr::attr< severity_level >("Severity") << "]: "
            << expr::smessage
    )
);

Answer 1

The log::formatter type is a wrapper around the actual (quite complicated) expression type. For some reason it cannot be used with add_file/console_log.

If you're using C++11 the auto keyword will avoid the wrapper class:

auto fmt = expr::stream
  << ...

Without C++11 a template function parameter will work:

template <class F>
void add_logs(const F & fmt)
{
  log::add_console_log(std::clog, keywords::format = fmt);
  log::add_file_log("test.log", keywords::format = fmt);
}

add_logs(expr::stream
  << ...
  );