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