Re: [boost] Re: recommend logging library?

From: Geoff Leyland (gley001_at_[hidden])
Date: 2004-03-21 17:26:12

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

template <class OUTPUT_THINGGY> not_logger &operator<<(const 
OUTPUT_THINGGY &o) { return *this; }

so when logging is compiled off, the statement evaluates to no code.

And where "this_module_logging_flag_notes" is a key to a 
std::map<std::string, bool> that tells you whether logging is on for 
that key or not.

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