Re: Stream Operator Overloading Design Choices
VirGin wrote:
I have a logging class. I want to update it to perform the following
by overloading the insertion operator:
- I want to prepend the time to the stream. Whether I'm writing to a
stringstream, cout, cerr, whatever.
- I want to be able to drop certain lines that might otherwise be
written to the stream. This would allow me to set a logging level when
I start writing to the stream.
- In certain situations, I want to be able to insert the result of
some other operation into the stream. I can do this already.
- I want to put this in a library file that I can link to from
different projects when I need to.
Illustration of the different goals
oLog << "Some Event" << endl;
- Use the default log message level and if the message should be
logged, prepend the time to the message and process it.
oLog(a_message_level) << "Some Other Event" << endl;
oLog(different_level) << "Details regarding Some Other Event" <<
endl;
- Process the two lines based on the logging level. Drop one or both
if they don't meet whatever criteria is set.
class LogIt : public ofstream
{
public:
...
various member definitions
...
template <class T>
friend LogIt & operator<<(LogIt &oObj, const T &xVal);
friend LogIt & operator<<(
LogIt &oObj,
std::ostream &(*el) > (std::ostream &));
private:
...
various private member definitions
...
};
I'm not sure you want to derive here. I tend to have an
ostream* member, which I use. Depending on whether output is
desired or not, the oLog function returns an instance of log it
with this pointer set to null, or to a valid output stream. (In
my case, the "valid output stream" uses a filtering streambuf
which can fan the output out to several different destinations,
including special streambuf's to output to syslog or to email,
as well as a file or cerr or cout.)
template <class T>
LogIt & operator<<(LogIt &oObj, const T &xVal)
{
cout << "Prefix_Rez\t" << xVal;// << endl;
return oObj;
}
This *isn't* where you want to put the prefix. The prefix
should be handled by the special streambuf as well. In
particular, the special streambuf has a flag, isStartOfLine,
which is initialized true, and then set as a function of each
character output. Something like the following, for example:
int
LogStreambuf::overflow( int ch )
{
if ( myIsStartOfLine ) {
myDest->sputn( prefix ) ;
}
int result = myDest->sputc( ch ) ;
myIsStartOfLine = (ch == '\n') ;
return result ;
}
LogIt & operator<<(LogIt &oObj, std::ostream &(*el)(std::ostream &))
{
(*el)(oObj);
return oObj;
}
I'm testing the above with:
oLog01 << "Line One" << 5 << 5.55 << endl;
oLog01 << "Line Two" << endl;
oLog01 << 333 << endl;
As you can probably see, I end up with the "Prefix_Rez" string every
time the insertion operator is called. the second problem is that the
endl isn't ending the line.
The second is curious. You have the special function so that
endl should be called. All endl should do is more or less:
std::ostream&
endl( std::ostream& dest )
{
dest.put( '\n' ) ;
dest.flush() ;
return dest ;
}
Off hand, I don't see why this wouldn't work with your code.
Except, of course: you derive from an fstream, but your template
<< operator outputs to cout. Whereas the specialization for
endl does output to the base class. Are you sure that you've
opened the file in the base class correctly.
Again, I prefer using a member, with something like:
template< typename T >
LogIt&
operator<<( LogIt& dest, T const& obj )
{
if ( dest.stream != NULL ) {
dest << obj ;
}
return dest ;
}
(Note that this has the added advantage that if logging is
turned off here, you don't convert the obj to text.)
Also, you'll probably want a specialization for std::ios_base&
(*)( std::ios_base& ) as well, for some of the other
manipulators.
I've played with different samples, methods and ideas.
So.
- Design-wise, what is a "good" way to specify different logging
levels for different messages?
- Also design-wise, what is a good way to only process the insertion
operator one time for each message, regardless of the number of times
that the insertion operator might be used while building a given
message?
- What is the correct method of passing endl and ending that line?
In my own code, I actually have different ostream's for
different logging levels, with an array of ostream* indexed by
the logging level (and simply a null pointer if logging is
disactivated for that level). The oLog function returns a
temporary LogIt object, initialized with the corresponding
pointer. In my case, I've gone a step further: my LogIt object
supports copy (necessary if it is to be a return value) in a
special way, with an instance counter, so that the last
destructor of the copy will be recognized. In this way, I can
inform the special streambuf of both the start and the end of
the log record: this allows things like using a different prefix
in follow-up lines in the log, automatically appending a '\n' at
the end of the record if the client forgets it, generating an
explicit flush for streambuf's which need it to ensure atomicity
(e.g. email or syslog), and getting and releasing a mutex lock
in a multi-threaded envirionment.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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