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


Why not do something like

my_log("this_module_logging_flag_notes") << "Doing something in
my_log("that_module_logging_flag_errors") << "Something went wrong in

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...


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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