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1\section{Log classes overview}\label{wxlogoverview}
2
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3Classes: \helpref{wxLog}{wxlog}, wxLogStderr,
4wxLogOstream, wxLogTextCtrl, wxLogWindow, wxLogGui, wxLogNull
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5
6This is a general overview of logging classes provided by wxWindows. The word
7logging here has a broad sense, including all of the program output, not only
8non interactive messages. The logging facilities included in wxWindows provide
9the base {\it wxLog} class which defines the standard interface for a {\it log
10target} as well as several standard implementations of it and a family of
11functions to use with them.
12
13First of all, no knowledge of {\it wxLog} classes is needed to use them. For
14this, you should only know about {\it wxLogXXX()} functions. All of them have
15the same syntax as {\it printf()}, i.e. they take the format string as the
16first argument and a variable number of arguments. Here are all of them:
2319d2b0 17
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18\begin{itemize}\itemsep=0pt
19\item{\bf wxLogFatalError} which is like {\it wxLogError}, but also
20terminates the program with the exit code 3 (using {\it abort()} standard
21function also terminates the program with this exit code).
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22\item{\bf wxLogError} is the function to use for error messages, i.e. the
23messages that must be shown to the user. The default processing is to pop up a
24message box to inform the user about it.
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25\item{\bf wxLogWarning} for warnings - they are also normally shown to the
26user, but don't interrupt the program work.
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27\item{\bf wxLogMessage} is for all normal, informational messages. They also
28appear in a message box by default (but it can be changed, see below). Notice
29that the standard behaviour is to not show informational messages if there are
30any errors later - the logic being that the later error messages make the
31informational messages preceding them meaningless.
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32\item{\bf wxLogVerbose} is for verbose output. Normally, it's suppressed, but
33might be activated if the user wishes to know more details about the program
34progress (another, but possibly confusing name for the same function is {\bf
0f358732 35wxLogInfo}).
5f3cd8a2 36\item{\bf wxLogStatus} is for status messages - they will go into the status
42ff6409 37bar of the active or specified (as the first argument) \helpref{wxFrame}{wxframe} if it has one.
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38\item{\bf wxLogSysError} is mostly used by wxWindows itself, but might be
39handy for logging errors after system call (API function) failure. It logs the
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40specified message text as well as the last system error
41code ({\it errno} or {\it ::GetLastError()} depending on the platform) and the corresponding error
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42message. The second form of this function takes the error code explitly as the
43first argument.
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44\item{\bf wxLogDebug} is {\bf the} right function for debug output. It only
45does anything at all in the debug mode (when the preprocessor symbol
46\_\_WXDEBUG\_\_ is defined) and expands to nothing in release mode (otherwise).
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47\item{\bf wxLogTrace} as {\bf wxLogDebug} only does something in debug
48build. The reason for making it a separate function from it is that usually
49there are a lot of trace messages, so it might make sense to separate them
50from other debug messages which would be flooded in them. Moreover, the second
51version of this function takes a trace mask as the first argument which allows
52to further restrict the amount of messages generated.
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53\end{itemize}
54
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55The usage of these functions should be fairly straightforward, however it may
56be asked why not use the other logging facilities, such as C standard stdio
57functions or C++ streams. The short answer is that they're all very good
58generic mechanisms, but are not really adapted for wxWindows, while the log
59classes are. Some of advantages in using wxWindows log functions are:
5f3cd8a2 60
2319d2b0 61\begin{itemize}\itemsep=0pt
42ff6409 62\item{\bf Portability} It's a common practice to use {\it printf()} statements or
5f3cd8a2 63cout/cerr C++ streams for writing out some (debug or otherwise) information.
0f358732 64Although it works just fine under Unix, these messages go strictly nowhere
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65under Windows where the stdout of GUI programs is not assigned to anything.
66Thus, you might view {\it wxLogMessage()} as a simple substitute for {\it
67printf()}.
42ff6409 68\item{\bf Flexibility} The output of wxLog functions can be redirected or
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69suppressed entirely based on their importance, which is either impossible or
70difficult to do with traditional methods. For example, only error messages, or
71only error messages and warnings might be logged, filtering out all
72informational messages.
42ff6409 73\item{\bf Completeness} Usually, an error message should be presented to the user
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74when some operation fails. Let's take a quite simple but common case of a file
75error: suppose that you're writing your data file on disk and there is not
76enough space. The actual error might have been detected inside wxWindows code
77(say, in {\it wxFile::Write}), so the calling function doesn't really know the
78exact reason of the failure, it only knows that the data file couldn't be
79written to the disk. However, as wxWindows uses {\it wxLogError()} in this
80situation, the exact error code (and the corresponding error message) will be
81given to the user together with "high level" message about data file writing
82error.
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83\end{itemize}
84
85After having enumerated all the functions which are normally used to log the
86messages, and why would you want to use them we now describe how all this
87works.
88
89wxWindows has the notion of a {\it log target}: it's just a class deriving
90from \helpref{wxLog}{wxlog}. As such, it implements the virtual functions of
91the base class which are called when a message is logged. Only one log target
92is {\it active} at any moment, this is the one used by \it{wxLogXXX()}
93functions. The normal usage of a log object (i.e. object of a class derived
94from wxLog) is to install it as the active target with a call to {\it
95SetActiveTarget()} and it will be used automatically by all subsequent calls
96to {\it wxLogXXX()} functions.
97
98To create a new log target class you only need to derive it from wxLog and
99implement one (or both) of {\it DoLog()} and {\it DoLogString()} in it. The
100second one is enough if you're happy with the standard wxLog message
101formatting (prepending "Error:" or "Warning:", timestamping \&c) but just want
102to send the messages somewhere else. The first one may be overridden to do
103whatever you want but you have to distinguish between the different message
104types yourself.
105
106There are some predefined classes deriving from wxLog and which might be
107helpful to see how you can create a new log target class and, of course, may
108also be used without any change. There are:
2319d2b0 109
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110\begin{itemize}\itemsep=0pt
111\item{\bf wxLogStderr} This class logs messages to a {\it FILE *}, using
112stderr by default as its name suggests.
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113\item{\bf wxLogStream} This class has the same functionality as wxLogStderr,
114but uses {\it ostream} and cerr instead of {\it FILE *} and stderr.
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115\item{\bf wxLogGui} This is the standard log target for wxWindows
116applications (it's used by default if you don't do anything) and provides the
117most reasonable handling of all types of messages for given platform.
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118\item{\bf wxLogWindow} This log target provides a "log console" which
119collects all messages generated by the application and also passes them to the
120previous active log target. The log window frame has a menu allowing user to
121clear the log, close it completely or save all messages to file.
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122\item{\bf wxLogNull} The last log class is quite particular: it doesn't do
123anything. The objects of this class may be instantiated to (temporarily)
124suppress output of {\it wxLogXXX()} functions. As an example, trying to open a
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125non-existing file will usually provoke an error message, but if for some
126reasons it's unwanted, just use this construction:
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127
128{\small
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129\begin{verbatim}
130 wxFile file;
131
132 // wxFile.Open() normally complains if file can't be opened, we don't want it
133 {
134 wxLogNull logNo;
135 if ( !file.Open("bar") )
136 ... process error ourselves ...
137 } // ~wxLogNull called, old log sink restored
138
139 wxLogMessage("..."); // ok
140\end{verbatim}
2319d2b0 141}
5f3cd8a2 142\end{itemize}
2319d2b0 143