Re: [boost] Custom logging backend difficulty

Subject: Re: [boost] Custom logging backend difficulty
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-10-27 16:30:55

On Monday 27 October 2014 23:30:09 you wrote:
> On Monday 27 October 2014 15:48:28 Kenneth Adam Miller wrote:
> > On Mon, Oct 27, 2014 at 3:03 PM, Kenneth Adam Miller <
> > 
> > kennethadammiller_at_[hidden]> wrote:
> > > Ah ok that clarifies somewhat;  I thought it was the formatter that I
> > > was
> > > having trouble with. I tried setting one, and then saw that the text was
> > > added to the console to test. But I'm doing
> > > boost::log::add_common_attributes(), and I print the string to std out
> > > just
> > > to make sure that I will get what I expect. As it turns out, I think
> > > that
> > > the backend is supposed to visit the record_view, and the record view
> > > has
> > > the default attributes and any others added.
> 
> The formatter just composes a string from the attribute values attached to
> the record, it's the sink backend that is supposed to do something with
> that string, like output to the console or send over the network.
> Text-based backends typically don't need to access the record view
> (although they're still able to) because the formatter already did all the
> job.
> 
> > > I'm trying to get those extra attributes; if I do BOOST_LOG(lg) << "
> > > blah"
> > > << endl; I get "blah" on my screen which I swap out the backend from the
> > > default. My backend just prints to the console.
> > > 
> > > How do I had the record_view items to the string to be sent?
> 
> With formatter, you just have to use it in the formatting expression. A few
> examples are here:
> 
> http://www.boost.org/doc/libs/1_56_0/libs/log/doc/html/log/tutorial/formatte
> rs.html
> 
> Basically, you need the attribute value name and type to do that.
> 
> > Reading some more at the top of this:
> > http://boost-log.sourceforge.net/libs/log/doc/html/log/tutorial/formatters
> > .h tml
> > 
> > I found that:
> > Even if you added attributes to the logging core or a logger, the
> > attribute
> > values will not reach the output unless you specify a formatter that will
> > use these values.
> > 
> > is what is holding me up. How can I specify a formatter that just gives
> > the
> > attribute name and then it's value, each separable by some delimiter? I
> > want to make certain that all attributes are consumed. Really, the reason
> > why I want this is, I don't know what the actual types are in the attr<
> > some_type > format, as demonstrated in several examples. This is kind of a
> > work around, but really I just want all the data that I enable at the
> > consumer level to make it by some default formatting method.
> 
> There is no built-in formatter in the library that just iterates through the
> values in the record, you have to write your own. It's simple, just write a
> function like described here:
> 
> http://www.boost.org/doc/libs/1_56_0/libs/log/doc/html/log/tutorial/formatte
> rs.html#log.tutorial.formatters.custom_formatting_functions
> 
> The record view has attribute_values() member, which returns an associative
> container of all attached values in pairs (name; value). Each value is an
> object of attribute_value class, from which you can extract the actual
> contained value through one of the approaches listed in the link I posted
> earlier. In particular, you can use visitation and something similar to a
> print_visitor class in the example here:
> 
> http://www.boost.org/doc/libs/1_56_0/libs/log/doc/html/log/detailed/attribut
> es.html#log.detailed.attributes.related_components.value_processing.visitati
> on
> 
> Of course, the formatter should insert its output into the provided stream,
> not std::cout.

PS: Please, don't top post.