ADA Exception Handling

ADA Exception Handling

[Stefan Papp]

Hello,

I am currently working on a project on Ada reengineering. My duty is to find 
out, whether it is possible to presentate exception in Ada graphically. I would 
be very happy, if somebody could give me some hints on ADA exceptions. 


As I am not an ADA programmer I have no real experience in ADA exceptions. Is 
it possible to compare the ADA exception handling to the e.h. from c++ or JAVA?

Could anyone tell, if there is something special I need to know about ADA 
excpetions?

Thank you

Stefan Papp

Re: ADA Exception Handling

[Corey Ashford]

Stefan Papp wrote in message <36370fab.0@alijku02.edvz.uni-linz.ac.at>...
>Hello,
>
>I am currently working on a project on Ada reengineering. My duty is to find
>out, whether it is possible to presentate exception in Ada graphically. I would
>be very happy, if somebody could give me some hints on ADA exceptions.

What sort of graphical representation do you have in mind?  I can see something
like illustrating the call graph of a particular application with exception
raising points and their handlers.  Did you have something else in mind?

>
>
>As I am not an ADA programmer I have no real experience in ADA exceptions. Is
>it possible to compare the ADA exception handling to the e.h. from c++ or JAVA?
>

Yes.  The main difference is that in C++ or Java, the programmer throws
and catches objects of any type, whereas in Ada, h programmer throws only
something of the predefined type "exception", and the handlers that catch
the exception must catch it by its name rather than by its type.

Dynamically, however, the concept is similar.  The details are a bit different.
For example, in C++, an exception can be reraised from outside the exception
handler (a.k.a catch block) - for example in a function which is called by the handler.  This
isn't possible in Ada.  On the other hand, C++ doesn't have built-in tasking
and so doesn't have to deal with the complications of exceptions being raised
during a rendezvous with another task.

Hope this gets you started.

- Corey

>Could anyone tell, if there is something special I need to know about ADA
>excpetions?
>
>Thank you
>
>Stefan Papp
>

Re: ADA Exception Handling

[Tucker Taft]

Corey Ashford (corey@remove.these.four.words.rational.com) wrote:

: Yes.  The main difference is that in C++ or Java, the programmer throws
: and catches objects of any type, whereas in Ada, h programmer throws only
: something of the predefined type "exception", and the handlers that catch
: the exception must catch it by its name rather than by its type.

Another way of looking at it is that an exception declaration
in Ada is effectively declaring a new exception "type," and you still 
handle exceptions by specifying the type of interest (or "others").  
There is a related concept called an exception "occurrence" which 
is essentially an instance of one of these exception "types."  
An exception occurrence is created implicitly as part of a "raise" 
statement, and can be given a name in a handler using the syntax:

    when E : others =>
       -- Inside here, "E" denotes and exception occurrence.

Using the operations in Ada.Exceptions, it is possible to add information
into the occurrence created by a "raise" by calling the Raise_Exception
operation explicitly (the information needs to be encoded as a string).

: Dynamically, however, the concept is similar.  The details are a bit different.
: For example, in C++, an exception can be reraised from outside the exception
: handler (a.k.a catch block) - for example in a function which is called by the handler.  This
: isn't possible in Ada.  

Actually, it is possible in Ada 95.  You can use the above mentioned
syntax to give a name to a handled exception, and then save the
exception occurrence in a global using one of the operations of
Ada.Exceptions, and then later re-raise the globally-saved occurrence
(using Ada.Exceptions.Reraise_Occurrence).

Chances are, though, this is much more information than you
need to use exceptions in most programs...

: ... On the other hand, C++ doesn't have built-in tasking
: and so doesn't have to deal with the complications of exceptions being raised
: during a rendezvous with another task.

: Hope this gets you started.

: - Corey

: >Could anyone tell, if there is something special I need to know about ADA
: >excpetions?
: >
: >Thank you
: >
: >Stefan Papp
: >

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA
An AverStar Company

Re: ADA Exception Handling

[Corey Ashford]

Thanks for the comments.

I thought about adding the info about the newer Ada95 exception features,
but I figured it was a bit much for the level of information he
originally requested.

I also thought about describing Ada exceptions as types too, but the syntax
doesn't really make that explanation terribly helpful.  If Ada95 had done
something like this:

subtype Foo is Ada.Exception;
...
raise Foo;

the "exceptions as types" would be easier to understand, perhaps.

- Corey

Re: ADA Exception Handling

[Robert A Duff]

"Corey Ashford" <corey@remove.these.four.words.rational.com> writes:

> For example, in C++, an exception can be reraised from outside the exception
> handler (a.k.a catch block) - for example in a function which is
> called by the handler.  This
> isn't possible in Ada.

It is possible in Ada, using exception occurrences and the
Ada.Exceptions package.

I agree that for the most part, Ada and C++ exceptions are more-or-less
the same.

I'd say the biggest difference between Ada and C++ exceptions is that
C++ allows you to add information to an exception occurrence in a
type-safe way; that is, you declare that information at the point of the
exception *type*, and the raising and handling have to agree.  You can
add information in Ada, too, but it's not type-safe -- you must encode
the information as a String.  Sigh.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.

Re: ADA Exception Handling

[Jerry van Dijk]

Stefan Papp (Stefan.papp@fhs-hagenberg.ac.at) wrote:

: I am currently working on a project on Ada reengineering. My duty is to find 
: out, whether it is possible to presentate exception in Ada graphically. I would 
: be very happy, if somebody could give me some hints on ADA exceptions. 

What do you mean by presenting an exception graphically ?

: As I am not an ADA programmer I have no real experience in ADA exceptions. Is 
: it possible to compare the ADA exception handling to the e.h. from c++ or JAVA?

In c++ and Java, exceptions are objects and are caught by type. in contrast Ada
exceptions are more like interrupts or signals causing an alternate control flow.

Any starters book on Ada will have a better explanation.

Jerry.

-- 
-- Jerry van Dijk  | email: jdijk@acm.org
-- Leiden, Holland | member Team-Ada
-- Ada & Win32: http://stad.dsl.nl/~jvandyk