Re: [boost] Re: recommend logging library?

From: John Torjo (john.lists_at_[hidden])
Date: 2004-03-22 01:37:12

Geoff Leyland wrote:

> Hi,
>
> Why not do something like
>
> my_log("this_module_logging_flag_notes") << "Doing something in 
> this_module";
> my_log("that_module_logging_flag_errors") << "Something went wrong in 
> that_module";
>
> where a log object is either (depending on a compile time setting) a 
> proxy for a stream or a not_logger than has
>

this is not such a hard thing to do ;) I've implemented some log classes 
like this in the (far) past ;)
But what you would actually want is an enumerator or something, so that 
you can write things like:

log<error>() << "some error";
log<activity>() << "some activity";

And with compile switches you can turn on/off error/activity/etc. logs.
But what about runtime? When it comes to logging, I really want a lot to 
be able to turn on/off such features at runtime.

So, maybe a better result would be:
log(error) << "some error;
log(activity) << "some activity";

and log(x) be a macro in the lines of 'if(x) x'

Best,
John

>
> You'd probably still need a macro for log so you could get a file and 
> line number, though.
>
> What would be REALLY nice would be if C++ had a built in stack tracing 
> facility.  In the past I've written a tracing library to keep track of 
> the stack (and turn tracing and logging on and off above and below 
> particular functions) and I once found 68000 stack tracing assembler 
> for the old mac os, which was really nice.  I should really get around 
> to implementing something like that again...
>
> cheers,
> Geoff
>
> On 19/03/2004, at 6:32 PM, John Torjo wrote:
>
>>
>>> I think it's the wrong approach. In our company we have implemented 
>>> logging through macros which has one very important property: When I 
>>> write
>>>
>>> LOG_DEBUG( whatever() );
>>>
>>> into my source, then whatever() is only evaluated when the 
>>> debug-logger is active. The macro basically expands to something 
>>> like this:
>>>
>>> if( Base::hasLogger( LogLevel::DEBUG ) )
>>>   Base::log( whatever(), LogLevel::DEBUG );
>>>
>>> I think this cannot be done without macros as normal function calls 
>>> (including operator<< for streams) need to evaluate their arguments 
>>> before they are called. As the difference in the application 
>>> execution speed can vary by a factor of 1000, this is a very 
>>> valuable optimization for us. My 2¢.
>>>
>> Man, you must be doing a LOT of logging ;)
>> I do quite a lot of logging myself, but I've never needed anything 
>> like above.
>>
>> Anyway, in my code I have functions, which return a log_stream (a 
>> thread-safe wrapper over a stream).
>>
>> And use it like this:
>>
>> activity_log() << "My app. started at" << time(0) << std::endl;
>> error_log() << "should have not arrived here" << std::endl;
>> // etc.
>>
>> And if I truly want, I can implement an operator bool(), and be able 
>> to disable it at runtime, like this:
>>
>> if (activity_log()) activity_log() << ...;
>>
>> Best,
>> John
>
>
>
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