From:	"MARTE" <MARTE AT htu DOT tu-graz DOT ac DOT at>
Organization:	Students Union Tech.Univ.Graz
To:	djgpp AT delorie DOT com
Date:	Tue, 3 Sep 1996 19:31:35 +0100
Subject:	Re: exception handling
Reply-To:	MARTE AT htu DOT tu-graz DOT ac DOT at
Message-Id:	<829B804BDD@htu.tu-graz.ac.at>

On 27 Aug 96 at 18:35, Mihai Moise wrote:
> Michael Day wrote:
> > 2.0? I'm doing something like this:
> >  try {
> >   throw 10;
> >  }
> >  catch (int i) {
> > 
> >  }
> >  catch (...) {
> > 
> >  }
> > 

Just to add to the confusion:
The syntax above has nothing to do with UNIX style exceptions.
Instead it describes a new way of error handling introduced in C++
which are also know as software-exceptions and have been ( in think )
taken from ADA.
The idea is the following (and quite simple):
instead of having each function return defined values to indicate an 
error or setting a variable like errno, a function when it encounters
an error "throw"s an exception. Such an exception can be any object 
of any type, meaning it can contain user defined data apart from the 
type definition.
As soon as the exception is throw, the program is aborted and resumed
at the next catch statement with an apropriate type.
an appropriate type would be the class of the thrown object or any 
base class of this class.

Let's consider the following example (not compiled):

// class definitions
class Ex                   // this defines the bse exception class
{
public:
   Ex() {};
   ~Ex() {};
};

class ExInt: public Ex    // this defines a specialized exception 
{                         // class containing an int error number
public:
  ExInt(int i)
  {
    err = i;
  };
  ~ExInt() {};
  int GetError() const
  {
    return err;
  };
private:
  int err;
};

class Test                 // the test class
{
public:
   Test(){ptr = 0;};
   ~Test(){};
   void SetPtr(void *p)
   {
      if (p == NULL)
         throw ExInt(10); // if we get a NULL pointer we raise an 
                          // exception
      ptr = p;     
   };
protected:
   void *ptr;
};

void main()
{
   Test a;
   try {                 // tell the compiler to watch for
                         // exceptions
         a.SetPtr((void *)&a); // do something
         a.SetPtr(NULL); // this is not allowed, an exception happens
   }
   catch(ExInt i) {      // continue here if an ExInt exception
                         // happens, the object throw is called 'i'
                         // in this example  
         printf("Error: %d\n",i.GetError());
   };
   Test *b = NULL;
   try {
       int v = 0;        // v gets destroyed when the exception 
                         // happens
       b = new Test;
       b->SetPtr(NULL);  // cause an exception
   }
   catch(Ex) {           // we are not interested in the Error code
                         // this catches all exceptions derived from
                         // class Ex 
       printf("an error occured\n");
       if (b != NULL) delete b;
   }
   catch (...) {         // this catches all exceptions
                         // in this case it will not execute,
                         // as the Exception is caught by the previous
                         // catch block 
       if (b != NULL) delete b; 
   };
};

------ End Example ------

Actually exception handling can get quite complex, but when properly
used can minimize the amount "if (error) return error;" statements
often seen in C/C++ programs significantly and therefore make your
programs much more readable.

A few Caveats:
Throwing from a constructor/destructor is not allowed
I don't know if the GNU library does this, but when "new" runs
    out of memory an exception gets thrown accoring to the draft 
    standard.
Signals won't get caught.
Be carefull of side effects when Objects on the stack get destroyed 
    due to an exception.

For a further discussion of this topic (which is quite off topic here)
see:
news:comp.std.c++
news:comp.lang.c++
book:The C++ Programming Language (Second Edition) by Bjarne Stoustrup
     (Chapter "Exception Handling" page 293)
other:The C++ Standard Draft Working Paper
      e.g.: http://www.bby.com.au/%7Egnb/wp/
 
mfg
Stefan Marte

......................................................................
. Stefan Marte:  marte AT htu DOT tu-graz DOT ac DOT at                             .
......................................................................
"It is by caffeine alone I set my mind in motion,
 It is by the beans of Java that thoughts acquire speed, 
 The hands acquire shaking, the shaking becomes a warning, 
 It is by caffeine alone I set my mind in motion."

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