Re: ExceptionHandler class & list of error messages ?

"mlimber" <>
12 Jul 2006 06:23:51 -0700
<> wrote:

Hi everyone... I have a TExceptionHandler class that is uses in the
code to thow exceptions. Whenever an exception is thrown the
TExceptionHander constructor takes an error code (int) as an argument.
I was hoping to create a map<int, const char*> that would be used in
the showError member function of the TExceptionHandler class where the
key (int) would be the error code and const char* the message printed
out to the console.

My question is the following. What would be the best way of creating
that map ? Here is why I am asking. I believe when you throw an
exception you actually create a copy of the object TExceptionHandler.
So if the map is a member variable of the class and that each entry in
the map is intialised with a string (as an error message) it would be a
waste of CPU time & memory (just stupide to create that list eveytime a
TExceptionHandler object is created).

Is the best way, to make the TExceptionHandler class a singleton ? The
map could a static member variable and would be intialised only once.
Or should I make the map a global variable (like an array of const
char* or string) and just use that global variable in the showError()
function ?

I am sure there's a good way of doing that ;-)

I looked on the net and the groups' archives but couldn't find anything
related on that precise aspect of exception handling.

Why not make it a private static member in your class:

 class TException
   : std::exception
   enum ErrorCode

   typedef std::map<ErrorCode, const char*> MsgMap;

   // Only one constructor is needed
     const ErrorCode code,
     const int severity = GIE_ERRORPRINT )
     : m_code( error )
     , m_severity( severity )

   virtual const char* what() const
     return msgMap_[ code_ ];

   int severity() const
     return severity_;

   const static MsgMap msgMap_;
   const ErrorCode code_;
   const int severity_;

Then in the .cpp file, use an initializer helper class to initialize

 namespace // anonymous
   class MsgMapInitializer
     TException::MsgMap m_;
     operator TException::MsgMap() const { return m_; }

     MsgMapInitializer& Add( const int i, const char* s )
       m_[i] = s;
       return *this;

 const TException::MsgMap TException::msgMap_ = MsgMapInitializer()
   .Add( TException::SOMETHING_HAPPENED, "Msg 1" )
   .Add( TException::SOMETHING_ELSE, "Msg 2" )
   .Add( TException::SOMETHING_STRANGE, "Msg 3" );

Then you can use it like this:

   throw TException( TException::SOMETHING_ELSE, 100 );
 catch( const std::exception& e )
   std::cerr << e.what() << std::endl; // Or whatever


  void showError()
     // print out the error message base of the error code
     cout << globalErrorMessageList[ m_error ] << endl; << is that the
best option ?

Typically it is best to inherit from std::exception and override the
virtual function what() to return an error message. Then the user can
do whatever s/he wants with it rather than being forced to use cout
(which is often impractical in GUI and embedded applications).

  int m_code;
  int m_severity;
  const char* m_message;

Why store the message locally? Just look it up when its time to use it
(probably only once).

Cheers! --M

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