Re: [Boost-users] [Log] How to use a channel that is constant per file?

Subject: Re: [Boost-users] [Log] How to use a channel that is constant per file?
From: Timo Schmiade (the_isz_at_[hidden])
Date: 2013-09-03 17:25:51

Hi Immanuel,

thanks for your answer! Reading it made me realize that my biggest flaw
was trying to use the BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT macro.
Using it, I was accessing the very same logger instance in every file,
thus disallowing myself to have a per-file channel.

This is what I came up with in the end:

--- 8< ---

// logging.hxx

#ifndef __LOGGING_HXX__
#define __LOGGING_HXX__

#include <boost/log/trivial.hpp>
#include <boost/log/sources/severity_logger.hpp>
#include <boost/log/attributes/constant.hpp> // for make_constant

class MyLogger : public boost::log::sources::severity_logger_mt< boost::log::trivial::severity_level >
{
  public:
    MyLogger(const std::string& channel)
    {
      add_attribute(
          "Channel",
          boost::log::attributes::make_constant(channel));
    }
};

#define MY_LOG(severity) BOOST_LOG_SEV(::my_logger, boost::log::trivial::severity)

#define DECLARE_MY_LOG(channel) \
namespace \
{ \
  MyLogger my_logger(#channel); \
}

#endif // __LOGGING_HXX__

--- >8 ---

With this, I now declare my channel name once at the top of every file
(via the DECLARE_MY_LOG macro) and then use the MY_LOG macro anywhere in
the code to log.

Kind regards,

Timo

On Mon, Sep 02, 2013 at 11:10:24PM +0200, Timo Schmiade wrote:
> Hi all,
> 
> I want to use boost's log library to output messages to the console
> which feature a per-message severity and a per-file channel.
> 
> I already tried two approaches, none of which was satisfying:
> 
> === 1. approach ===
> 
> The class boost::log::sources::severity_channel_logger supports both
> severity and channel and I could define it in each source file like so:
> 
> --- 8< ---
> 
> BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT(
>     log,
>     boost::log::sources::severity_channel_logger<
>         boost::log::trivial::severity_level,
>         std::string
>     >
> )
> 
> --- >8 ---
> 
> Unfortunately, using it like this requires me to redundantly add the
> channel to each log message, which is very inconvenient because it is
> the same for all logs in the file.
> 
> --- 8< ---
> 
> // Message.cpp
> 
> BOOST_LOG_CHANNEL_SEV(log, Message, trace) << "Hello, world";
> 
> --- >8 ---
> 
> === 2. approach ===
> 
> I defined my logger like this:
> 
> --- 8< ---
> 
> BOOST_LOG_INLINE_GLOBAL_LOGGER_DEFAULT(
>     log,
>     boost::log::sources::severity_logger<
>         boost::log::trivial::severity_level
>     >
> )
> 
> --- >8 ---
> 
> Then, in each file, I manually set  the "Channel" attribute for the
> logger, like so:
> 
> --- 8< ---
> 
> // Message.cpp
> 
> log::get().add_attribute(
>     "Channel",
>     boost::log::attributes::make_constant("Message"));
> 
> --- >8 ---
> 
> This works, but as the "log" object seems to be stored globally, I would
> have to "invent" new names for the logger object in each file. This is
> both insecure and prevents the usage of macros (I think).
> 
> So here's the question:
> 
> What would be the recommended / a convenient way to add a per-file
> constant channel to log messages?
> 
> Thanks in advance!
> 
> Timo