docs/doxygen/overviews/log.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        log.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows license
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /*!
  10
  11 @page overview_log wxLog Classes Overview
  12
  13 Classes:
  14 @li wxLog
  15 @li wxLogStderr
  16 @li wxLogStream
  17 @li wxLogTextCtrl
  18 @li wxLogWindow
  19 @li wxLogGui
  20 @li wxLogNull
  21 @li wxLogBuffer
  22 @li wxLogChain
  23 @li wxLogInterposer
  24 @li wxLogInterposerTemp
  25 @li wxStreamToTextRedirector
  26
  27 This is a general overview of logging classes provided by wxWidgets. The word
  28 logging here has a broad sense, including all of the program output, not only
  29 non-interactive messages. The logging facilities included in wxWidgets provide
  30 the base wxLog class which defines the standard interface for a @e log target
  31 as well as several standard implementations of it and a family of functions to
  32 use with them.
  33
  34 First of all, no knowledge of wxLog classes is needed to use them. For this,
  35 you should only know about @e wxLogXXX() functions. All of them have the same
  36 syntax as @e printf() or @e vprintf() , i.e. they take the format string as the
  37 first argument and respectively a variable number of arguments or a variable
  38 argument list pointer. Here are all of them:
  39
  40 @li wxLogFatalError which is like wxLogError, but also terminates the program
  41     with the exit code 3 (using @e abort() standard function). Unlike for all
  42     the other logging functions, this function can't be overridden by a log
  43     target.
  44 @li wxLogError is the function to use for error messages, i.e. the messages
  45     that must be shown to the user. The default processing is to pop up a
  46     message box to inform the user about it.
  47 @li wxLogWarning for warnings. They are also normally shown to the user, but
  48     don't interrupt the program work.
  49 @li wxLogMessage is for all normal, informational messages. They also appear in
  50     a message box by default (but it can be changed, see below).
  51 @li wxLogVerbose is for verbose output. Normally, it is suppressed, but might
  52     be activated if the user wishes to know more details about the program
  53     progress (another, but possibly confusing name for the same function is
  54     wxLogInfo).
  55 @li wxLogStatus is for status messages. They will go into the status bar of the
  56     active or specified (as the first argument) wxFrame if it has one.
  57 @li wxLogSysError is mostly used by wxWidgets itself, but might be handy for
  58     logging errors after system call (API function) failure. It logs the
  59     specified message text as well as the last system error code (@e errno or
  60     ::GetLastError() depending on the platform) and the corresponding error
  61     message. The second form of this function takes the error code explicitly
  62     as the first argument.
  63 @li wxLogDebug is @b the right function for debug output. It only does anything
  64     at all in the debug mode (when the preprocessor symbol __WXDEBUG__ is
  65     defined) and expands to nothing in release mode (otherwise). @b Tip: under
  66     Windows, you must either run the program under debugger or use a 3rd party
  67     program such as DebugView to actually see the debug output.
  68     - DebugView: http://www.microsoft.com/technet/sysinternals/Miscellaneous/DebugView.mspx
  69 @li wxLogTrace as wxLogDebug only does something in debug build. The reason for
  70     making it a separate function from it is that usually there are a lot of
  71     trace messages, so it might make sense to separate them from other debug
  72     messages which would be flooded in them. Moreover, the second version of
  73     this function takes a trace mask as the first argument which allows to
  74     further restrict the amount of messages generated.
  75
  76 The usage of these functions should be fairly straightforward, however it may
  77 be asked why not use the other logging facilities, such as C standard stdio
  78 functions or C++ streams. The short answer is that they're all very good
  79 generic mechanisms, but are not really adapted for wxWidgets, while the log
  80 classes are. Some of advantages in using wxWidgets log functions are:
  81
  82 @li @b Portability: It is a common practice to use @e printf() statements or
  83     cout/cerr C++ streams for writing out some (debug or otherwise)
  84     information. Although it works just fine under Unix, these messages go
  85     strictly nowhere under Windows where the stdout of GUI programs is not
  86     assigned to anything. Thus, you might view wxLogMessage() as a simple
  87     substitute for @e printf().
  88     You can also redirect the @e wxLogXXX calls to @e cout by just writing:
  89     @code
  90     wxLog* logger = new wxLogStream(&cout);
  91     wxLog::SetActiveTarget(logger);
  92     @endcode
  93     Finally, there is also a possibility to redirect the output sent to @e cout
  94     to a wxTextCtrl by using the wxStreamToTextRedirector class.
  95 @li @b Flexibility: The output of wxLog functions can be redirected or
  96     suppressed entirely based on their importance, which is either impossible
  97     or difficult to do with traditional methods. For example, only error
  98     messages, or only error messages and warnings might be logged, filtering
  99     out all informational messages.
 100 @li @b Completeness: Usually, an error message should be presented to the user
 101     when some operation fails. Let's take a quite simple but common case of a
 102     file error: suppose that you're writing your data file on disk and there is
 103     not enough space. The actual error might have been detected inside
 104     wxWidgets code (say, in wxFile::Write), so the calling function doesn't
 105     really know the exact reason of the failure, it only knows that the data
 106     file couldn't be written to the disk. However, as wxWidgets uses
 107     wxLogError() in this situation, the exact error code (and the corresponding
 108     error message) will be given to the user together with "high level" message
 109     about data file writing error.
 110
 111 After having enumerated all the functions which are normally used to log the
 112 messages, and why would you want to use them we now describe how all this
 113 works.
 114
 115 wxWidgets has the notion of a <em>log target</em>: it is just a class deriving
 116 from wxLog. As such, it implements the virtual functions of the base class
 117 which are called when a message is logged. Only one log target is @e active at
 118 any moment, this is the one used by @e wxLogXXX() functions. The normal usage
 119 of a log object (i.e. object of a class derived from wxLog) is to install it as
 120 the active target with a call to @e SetActiveTarget() and it will be used
 121 automatically by all subsequent calls to @e wxLogXXX() functions.
 122
 123 To create a new log target class you only need to derive it from wxLog and
 124 implement one (or both) of @e DoLog() and @e DoLogString() in it. The second
 125 one is enough if you're happy with the standard wxLog message formatting
 126 (prepending "Error:" or "Warning:", timestamping @&c) but just want to send
 127 the messages somewhere else. The first one may be overridden to do whatever
 128 you want but you have to distinguish between the different message types
 129 yourself.
 130
 131 There are some predefined classes deriving from wxLog and which might be
 132 helpful to see how you can create a new log target class and, of course, may
 133 also be used without any change. There are:
 134
 135 @li wxLogStderr: This class logs messages to a <tt>FILE *</tt>, using stderr by
 136     default as its name suggests.
 137 @li wxLogStream: This class has the same functionality as wxLogStderr, but uses
 138     @e ostream and cerr instead of <tt>FILE *</tt> and stderr.
 139 @li wxLogGui: This is the standard log target for wxWidgets applications (it is
 140     used by default if you don't do anything) and provides the most reasonable
 141     handling of all types of messages for given platform.
 142 @li wxLogWindow: This log target provides a "log console" which collects all
 143     messages generated by the application and also passes them to the previous
 144     active log target. The log window frame has a menu allowing user to clear
 145     the log, close it completely or save all messages to file.
 146 @li wxLogBuffer: This target collects all the logged messages in an internal
 147     buffer allowing to show them later to the user all at once.
 148 @li wxLogNull: The last log class is quite particular: it doesn't do anything.
 149     The objects of this class may be instantiated to (temporarily) suppress
 150     output of @e wxLogXXX() functions. As an example, trying to open a
 151     non-existing file will usually provoke an error message, but if for some
 152     reasons it is unwanted, just use this construction:
 153     @code
 154     wxFile file;
 155
 156     // wxFile.Open() normally complains if file can't be opened, we don't want it
 157     {
 158         wxLogNull logNo;
 159         if ( !file.Open("bar") )
 160         {
 161             // ... process error ourselves ...
 162         }
 163     } // ~wxLogNull called, old log sink restored
 164
 165     wxLogMessage("..."); // ok
 166     @endcode
 167
 168 The log targets can also be combined: for example you may wish to redirect the
 169 messages somewhere else (for example, to a log file) but also process them as
 170 normally. For this the wxLogChain, wxLogInterposer, and wxLogInterposerTemp can
 171 be used.
 172
 173 */
 174