docs/doxygen/overviews/log.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        log
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows license
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /*!
  10
  11  @page log_overview wxLog classes overview
  12
  13  Classes: #wxLog,
  14
  15  #wxLogStderr,
  16
  17  #wxLogStream,
  18
  19  #wxLogTextCtrl,
  20
  21  #wxLogWindow,
  22
  23  #wxLogGui,
  24
  25  #wxLogNull,
  26
  27  #wxLogBuffer,
  28
  29  #wxLogChain,
  30
  31  #wxLogInterposer,
  32
  33  #wxLogInterposerTemp,
  34
  35  #wxStreamToTextRedirector
  36  This is a general overview of logging classes provided by wxWidgets. The word
  37  logging here has a broad sense, including all of the program output, not only
  38  non-interactive messages. The logging facilities included in wxWidgets provide
  39  the base @e wxLog class which defines the standard interface for a @e log
  40  target as well as several standard implementations of it and a family of
  41  functions to use with them.
  42  First of all, no knowledge of @e wxLog classes is needed to use them. For
  43  this, you should only know about @e wxLogXXX() functions. All of them have
  44  the same syntax as @e printf() or @e vprintf() , i.e. they take the
  45  format string as the first argument and respectively a variable number of
  46  arguments or a variable argument list pointer. Here are all of them:
  47
  48
  49  @b wxLogFatalError which is like @e wxLogError, but also
  50  terminates the program with the exit code 3 (using @e abort() standard
  51  function). Unlike for all the other logging functions, this function can't be
  52  overridden by a log target.
  53  @b wxLogError is the function to use for error messages, i.e. the
  54  messages that must be shown to the user. The default processing is to pop up a
  55  message box to inform the user about it.
  56  @b wxLogWarning for warnings - they are also normally shown to the
  57  user, but don't interrupt the program work.
  58  @b wxLogMessage is for all normal, informational messages. They also
  59  appear in a message box by default (but it can be changed, see below).
  60  @b wxLogVerbose is for verbose output. Normally, it is suppressed, but
  61  might be activated if the user wishes to know more details about the program
  62  progress (another, but possibly confusing name for the same function is @b wxLogInfo).
  63  @b wxLogStatus is for status messages - they will go into the status
  64  bar of the active or specified (as the first argument) #wxFrame if it has one.
  65  @b wxLogSysError is mostly used by wxWidgets itself, but might be
  66  handy for logging errors after system call (API function) failure. It logs the
  67  specified message text as well as the last system error
  68  code (@e errno or @e ::GetLastError() depending on the platform) and
  69  the corresponding error message. The second form of this function takes the
  70  error code explicitly as the first argument.
  71  @b wxLogDebug is @b the right function for debug output. It only
  72  does anything at all in the debug mode (when the preprocessor symbol
  73  __WXDEBUG__ is defined) and expands to nothing in release mode (otherwise).
  74  @b Tip: under Windows, you must either run the program under debugger or
  75  use a 3rd party program such as #DbgView
  76  to actually see the debug output.
  77  @b wxLogTrace as @b wxLogDebug only does something in debug
  78  build. The reason for making it a separate function from it is that usually
  79  there are a lot of trace messages, so it might make sense to separate them
  80  from other debug messages which would be flooded in them. Moreover, the second
  81  version of this function takes a trace mask as the first argument which allows
  82  to further restrict the amount of messages generated.
  83
  84
  85  The usage of these functions should be fairly straightforward, however it may
  86  be asked why not use the other logging facilities, such as C standard stdio
  87  functions or C++ streams. The short answer is that they're all very good
  88  generic mechanisms, but are not really adapted for wxWidgets, while the log
  89  classes are. Some of advantages in using wxWidgets log functions are:
  90
  91
  92  @b Portability It is a common practice to use @e printf()
  93  statements or cout/cerr C++ streams for writing out some (debug or otherwise)
  94  information.
  95  Although it works just fine under Unix, these messages go strictly nowhere
  96  under Windows where the stdout of GUI programs is not assigned to anything.
  97  Thus, you might view @e wxLogMessage() as a simple substitute for @e printf().
  98  You can also redirect the @e wxLogXXX calls to @e cout by just writing:
  99
 100
 101  @code
 102  wxLog *logger=new wxLogStream();
 103    wxLog::SetActiveTarget(logger);
 104  @endcode
 105
 106
 107  Finally, there is also a possibility to redirect the output sent to @e cout
 108  to a #wxTextCtrl by using the
 109  #wxStreamToTextRedirector class.
 110  @b Flexibility The output of wxLog functions can be redirected or
 111  suppressed entirely based on their importance, which is either impossible or
 112  difficult to do with traditional methods. For example, only error messages, or
 113  only error messages and warnings might be logged, filtering out all
 114  informational messages.
 115  @b Completeness Usually, an error message should be presented to the user
 116  when some operation fails. Let's take a quite simple but common case of a file
 117  error: suppose that you're writing your data file on disk and there is not
 118  enough space. The actual error might have been detected inside wxWidgets code
 119  (say, in @e wxFile::Write), so the calling function doesn't really know the
 120  exact reason of the failure, it only knows that the data file couldn't be
 121  written to the disk. However, as wxWidgets uses @e wxLogError() in this
 122  situation, the exact error code (and the corresponding error message) will be
 123  given to the user together with "high level" message about data file writing
 124  error.
 125
 126
 127  After having enumerated all the functions which are normally used to log the
 128  messages, and why would you want to use them we now describe how all this
 129  works.
 130  wxWidgets has the notion of a @e log target: it is just a class deriving
 131  from #wxLog. As such, it implements the virtual functions of
 132  the base class which are called when a message is logged. Only one log target
 133  is @e active at any moment, this is the one used by @e wxLogXXX()
 134  functions. The normal usage of a log object (i.e. object of a class derived
 135  from wxLog) is to install it as the active target with a call to @e SetActiveTarget() and it will be used automatically by all subsequent calls
 136  to @e wxLogXXX() functions.
 137  To create a new log target class you only need to derive it from wxLog and
 138  implement one (or both) of @e DoLog() and @e DoLogString() in it. The
 139  second one is enough if you're happy with the standard wxLog message
 140  formatting (prepending "Error:" or "Warning:", timestamping c) but just want
 141  to send the messages somewhere else. The first one may be overridden to do
 142  whatever you want but you have to distinguish between the different message
 143  types yourself.
 144  There are some predefined classes deriving from wxLog and which might be
 145  helpful to see how you can create a new log target class and, of course, may
 146  also be used without any change. There are:
 147
 148
 149  @b wxLogStderr This class logs messages to a @e FILE *, using
 150  stderr by default as its name suggests.
 151  @b wxLogStream This class has the same functionality as wxLogStderr,
 152  but uses @e ostream and cerr instead of @e FILE * and stderr.
 153  @b wxLogGui This is the standard log target for wxWidgets
 154  applications (it is used by default if you don't do anything) and provides the
 155  most reasonable handling of all types of messages for given platform.
 156  @b wxLogWindow This log target provides a "log console" which
 157  collects all messages generated by the application and also passes them to the
 158  previous active log target. The log window frame has a menu allowing user to
 159  clear the log, close it completely or save all messages to file.
 160  @b wxLogBuffer This target collects all the logged messages in an
 161  internal buffer allowing to show them later to the user all at once.
 162  @b wxLogNull The last log class is quite particular: it doesn't do
 163  anything. The objects of this class may be instantiated to (temporarily)
 164  suppress output of @e wxLogXXX() functions. As an example, trying to open a
 165  non-existing file will usually provoke an error message, but if for some
 166  reasons it is unwanted, just use this construction:
 167
 168  @code
 169  wxFile file;
 170
 171    // wxFile.Open() normally complains if file can't be opened, we don't want it
 172    {
 173      wxLogNull logNo;
 174      if ( !file.Open("bar") )
 175        ... process error ourselves ...
 176    } // ~wxLogNull called, old log sink restored
 177
 178    wxLogMessage("..."); // ok
 179  @endcode
 180
 181
 182
 183  The log targets can also be combined: for example you may wish to redirect the
 184  messages somewhere else (for example, to a log file) but also process them as
 185  normally. For this the #wxLogChain, #wxLogInterposer and
 186  #wxLogInterposerTemp can be used.
 187
 188  */
 189
 190