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digitalmars.D - What is throwable

reply Steve Teale <steve.teale britseyeview.com> writes:
The language reference says:

Throw Statement
Throw an exception.

ThrowStatement:
	throw Expression ;

Expression is evaluated and must be an Object reference. The Object reference
is thrown as an exception.

But the following compiles and runs and catches the RottenEgg.


import std.stdio;

class RottenEgg
{
   string message;
   this(string s) { message = s; }
}

void main()
{
   try
   {
      throw new RottenEgg("something smelly");
   }
   catch (Exception e)
   {
      writefln("Caught exception ", e.toString()); 
   }
   catch (RottenEgg err)
   {
      writefln("Caught RottenEgg exception ", err.message);
   }
}

What is the point of class Throwable if you can throw any old class object?
Mar 19 2009
next sibling parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
Steve Teale wrote:

 ...
 
 What is the point of class Throwable if you can throw any old class object?



What class Throwable?

  -- Daniel
Mar 19 2009
next sibling parent reply Steve Teale <steve.teale britseyeview.com> writes:
Daniel Keep Wrote:


 
 
 Steve Teale wrote:

 ...
 
 What is the point of class Throwable if you can throw any old class object?



 What class Throwable?



dmd\src\druntime\src\compiler\dmd\object_.d


 
   -- Daniel
Mar 19 2009
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
Steve Teale wrote:

 Daniel Keep Wrote:
 

 Steve Teale wrote:

 ...

 What is the point of class Throwable if you can throw any old class object?






 dmd\src\druntime\src\compiler\dmd\object_.d
 

   -- Daniel






Ah.

Well, if I had to guess, I'd say it was because Throwable objects are
more useful than random objects of other types.  Object doesn't give you
msg, file, line, info or next.

  -- Daniel
Mar 19 2009
parent reply Steve Teale <steve.teale britseyeview.com> writes:
Daniel Keep Wrote:


 
 
 Steve Teale wrote:

 Daniel Keep Wrote:
 

 Steve Teale wrote:

 ...

 What is the point of class Throwable if you can throw any old class object?






 dmd\src\druntime\src\compiler\dmd\object_.d
 

   -- Daniel






 Ah.
 
 Well, if I had to guess, I'd say it was because Throwable objects are
 more useful than random objects of other types.  Object doesn't give you
 msg, file, line, info or next.
 
   -- Daniel



But doesn't it rather imply that if you want to be able to throw a class, then
it should be derived from Throwable?
Mar 19 2009
parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Steve Teale wrote:

 Daniel Keep Wrote:
 

 Steve Teale wrote:

 Daniel Keep Wrote:


 Steve Teale wrote:

 ...

 What is the point of class Throwable if you can throw any old class object?








   -- Daniel







 Well, if I had to guess, I'd say it was because Throwable objects are
 more useful than random objects of other types.  Object doesn't give you
 msg, file, line, info or next.

   -- Daniel



 But doesn't it rather imply that if you want to be able to throw a class, then
it should be derived from Throwable?



Yes, a check should be added to that effect. Later on we need to 
actually exploit that only certain types can be thrown in implementing 
exception chaining.

What is exception chaining? Consider:

void weird()
{
     scope(failure) throw new Exception1("hum");
     throw new Exception2("ho");
}

The C++ model terminates the app when an exception is thrown while the 
stack is unwound. Java (I was told without having checked) makes the 
latest exception thrown the "main" exception, and you get to also access 
the other exceptions by using primitives in class Exception. Is that true?

Anyhow, we should implement a model in D that allows such multiple 
throws. Then a few primitives defined in class Exception - e.g. next() - 
should make the entire chain accessible. At some point Walter started 
implementing that but other priorities took over. A great side project 
for anyone if you ask me :o).


Andrei
Mar 19 2009
parent reply Ary Borenszweig <ary esperanto.org.ar> writes:
Andrei Alexandrescu wrote:

 Steve Teale wrote:

 Daniel Keep Wrote:


 Steve Teale wrote:

 Daniel Keep Wrote:


 Steve Teale wrote:

 ...

 What is the point of class Throwable if you can throw any old 
 class object?








   -- Daniel







 Well, if I had to guess, I'd say it was because Throwable objects are
 more useful than random objects of other types.  Object doesn't give you
 msg, file, line, info or next.

   -- Daniel



 But doesn't it rather imply that if you want to be able to throw a 
 class, then it should be derived from Throwable?



 Yes, a check should be added to that effect. Later on we need to 
 actually exploit that only certain types can be thrown in implementing 
 exception chaining.
 
 What is exception chaining? Consider:
 
 void weird()
 {
     scope(failure) throw new Exception1("hum");
     throw new Exception2("ho");
 }
 
 The C++ model terminates the app when an exception is thrown while the 
 stack is unwound. Java (I was told without having checked) makes the 
 latest exception thrown the "main" exception, and you get to also access 
 the other exceptions by using primitives in class Exception. Is that true?



You can't throw multiple exceptions in Java. What you can do is:

try {
   ...
} catch(SomeException e) {
   throw new AnotherException(e);
}

and then AnotherException is thrown with "e" being it's inner exception, 
and you can check that.


 
 Anyhow, we should implement a model in D that allows such multiple 
 throws.



What for?
Mar 19 2009
parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Ary Borenszweig wrote:

 Andrei Alexandrescu wrote:

 Steve Teale wrote:

 Daniel Keep Wrote:


 Steve Teale wrote:

 Daniel Keep Wrote:


 Steve Teale wrote:

 ...

 What is the point of class Throwable if you can throw any old 
 class object?








   -- Daniel







 Well, if I had to guess, I'd say it was because Throwable objects are
 more useful than random objects of other types.  Object doesn't give 
 you
 msg, file, line, info or next.

   -- Daniel



 But doesn't it rather imply that if you want to be able to throw a 
 class, then it should be derived from Throwable?



 Yes, a check should be added to that effect. Later on we need to 
 actually exploit that only certain types can be thrown in implementing 
 exception chaining.

 What is exception chaining? Consider:

 void weird()
 {
     scope(failure) throw new Exception1("hum");
     throw new Exception2("ho");
 }

 The C++ model terminates the app when an exception is thrown while the 
 stack is unwound. Java (I was told without having checked) makes the 
 latest exception thrown the "main" exception, and you get to also 
 access the other exceptions by using primitives in class Exception. Is 
 that true?



 You can't throw multiple exceptions in Java. What you can do is:
 
 try {
   ...
 } catch(SomeException e) {
   throw new AnotherException(e);
 }
 
 and then AnotherException is thrown with "e" being it's inner exception, 
 and you can check that.
 

 Anyhow, we should implement a model in D that allows such multiple 
 throws.



 What for?



For allowing destructors to throw.

Andrei
Mar 19 2009
parent reply Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:
Denis Koroskin wrote:

 On Thu, 19 Mar 2009 19:39:52 +0300, Andrei Alexandrescu 
 <SeeWebsiteForEmail erdani.org> wrote:
 

 Ary Borenszweig wrote:

 Andrei Alexandrescu wrote:

 Anyhow, we should implement a model in D that allows such multiple 
 throws.






 For allowing destructors to throw.

 Andrei



 Are you sure this is sound?



Yes. The notion that throwing destructors should terminate the 
application pronto is a prejudice we got from C++.

Andrei
Mar 19 2009
parent reply Don <nospam nospam.com> writes:
Andrei Alexandrescu wrote:

 Denis Koroskin wrote:

 On Thu, 19 Mar 2009 19:39:52 +0300, Andrei Alexandrescu 
 <SeeWebsiteForEmail erdani.org> wrote:


 Ary Borenszweig wrote:

 Andrei Alexandrescu wrote:

 Anyhow, we should implement a model in D that allows such multiple 
 throws.






 For allowing destructors to throw.

 Andrei



 Are you sure this is sound?



 Yes. The notion that throwing destructors should terminate the 
 application pronto is a prejudice we got from C++.
 
 Andrei



And it causes lots of problems, in my experience. Since it usually 
doesn't terminate the app, just the thread with the defective 
destructor. Which is unnecessarily difficult to debug.
Mar 19 2009
parent reply Sean Kelly <sean invisibleduck.org> writes:
== Quote from Don (nospam nospam.com)'s article

 Andrei Alexandrescu wrote:

 Denis Koroskin wrote:

 On Thu, 19 Mar 2009 19:39:52 +0300, Andrei Alexandrescu
 <SeeWebsiteForEmail erdani.org> wrote:


 Ary Borenszweig wrote:

 Andrei Alexandrescu wrote:

 Anyhow, we should implement a model in D that allows such multiple
 throws.






 For allowing destructors to throw.

 Andrei



 Are you sure this is sound?



 Yes. The notion that throwing destructors should terminate the
 application pronto is a prejudice we got from C++.

 Andrei



 doesn't terminate the app, just the thread with the defective
 destructor. Which is unnecessarily difficult to debug.



I've considered letting the user supply a custom "unhandled
exception" handler for threads.  It just seemed messy given that
calling join() on a thread now will rethrow the unhandled exception
if there was one.  I figured that the user would join threads he cared
about, and if one he didn't care about terminated unexpectedly the
perhaps that's not a problem.
Mar 19 2009
parent reply Sergey Gromov <snake.scaly gmail.com> writes:
Thu, 19 Mar 2009 20:44:51 +0000 (UTC), Sean Kelly wrote:


 == Quote from Don (nospam nospam.com)'s article

 Andrei Alexandrescu wrote:

 Denis Koroskin wrote:

 On Thu, 19 Mar 2009 19:39:52 +0300, Andrei Alexandrescu
 <SeeWebsiteForEmail erdani.org> wrote:


 Ary Borenszweig wrote:

 Andrei Alexandrescu wrote:

 Anyhow, we should implement a model in D that allows such multiple
 throws.






 For allowing destructors to throw.

 Andrei



 Are you sure this is sound?



 Yes. The notion that throwing destructors should terminate the
 application pronto is a prejudice we got from C++.

 Andrei



 doesn't terminate the app, just the thread with the defective
 destructor. Which is unnecessarily difficult to debug.



 I've considered letting the user supply a custom "unhandled
 exception" handler for threads.  It just seemed messy given that
 calling join() on a thread now will rethrow the unhandled exception
 if there was one.  I figured that the user would join threads he cared
 about, and if one he didn't care about terminated unexpectedly the
 perhaps that's not a problem.



The problem is when you have a worker thread which never terminates and
sends notifications to the main thread.  Like game logic thread versus
main GUI thread.  If logic thread throws, your game suddenly stops
working for no apparent reason.
Mar 19 2009
parent Sean Kelly <sean invisibleduck.org> writes:
== Quote from Sergey Gromov (snake.scaly gmail.com)'s article

 Thu, 19 Mar 2009 20:44:51 +0000 (UTC), Sean Kelly wrote:

 I've considered letting the user supply a custom "unhandled
 exception" handler for threads.  It just seemed messy given that
 calling join() on a thread now will rethrow the unhandled exception
 if there was one.  I figured that the user would join threads he cared
 about, and if one he didn't care about terminated unexpectedly the
 perhaps that's not a problem.



 sends notifications to the main thread.  Like game logic thread versus
 main GUI thread.  If logic thread throws, your game suddenly stops
 working for no apparent reason.



So wrap your logic in a try/catch:

void worker() {
    try {
        doStuff();
    } catch( Throwable t ) {
        notifyProgramOfFailure( t );
    }
}

auto t = new Thread( &worker );

I think a case could be made for simply terminating the
app on an unhandled exception in a thread, but logic
errors are the programmer's responsibility.
Mar 19 2009
prev sibling next sibling parent Jesse Phillips <jessekphillips gmail.com> writes:
On Thu, 19 Mar 2009 06:09:40 -0400, Steve Teale wrote:


 Daniel Keep Wrote:
 
 

 
 Steve Teale wrote:

 Daniel Keep Wrote:
 
 

 Steve Teale wrote:

 ...

 What is the point of class Throwable if you can throw any old class
 object?






 dmd\src\druntime\src\compiler\dmd\object_.d
 

   -- Daniel






 Ah.
 
 Well, if I had to guess, I'd say it was because Throwable objects are
 more useful than random objects of other types.  Object doesn't give
 you msg, file, line, info or next.
 
   -- Daniel



 But doesn't it rather imply that if you want to be able to throw a
 class, then it should be derived from Throwable?



No, it just means that a Throwable object will provide a specific set of 
information.
Mar 19 2009
prev sibling parent "Denis Koroskin" <2korden gmail.com> writes:
On Thu, 19 Mar 2009 19:39:52 +0300, Andrei Alexandrescu
<SeeWebsiteForEmail erdani.org> wrote:


 Ary Borenszweig wrote:

 Andrei Alexandrescu wrote:

 Steve Teale wrote:

 Daniel Keep Wrote:


 Steve Teale wrote:

 Daniel Keep Wrote:


 Steve Teale wrote:

 ...

 What is the point of class Throwable if you can throw any old  
 class object?








   -- Daniel







 Well, if I had to guess, I'd say it was because Throwable objects are
 more useful than random objects of other types.  Object doesn't give  
 you
 msg, file, line, info or next.

   -- Daniel



 But doesn't it rather imply that if you want to be able to throw a  
 class, then it should be derived from Throwable?



 Yes, a check should be added to that effect. Later on we need to  
 actually exploit that only certain types can be thrown in implementing  
 exception chaining.

 What is exception chaining? Consider:

 void weird()
 {
     scope(failure) throw new Exception1("hum");
     throw new Exception2("ho");
 }

 The C++ model terminates the app when an exception is thrown while the  
 stack is unwound. Java (I was told without having checked) makes the  
 latest exception thrown the "main" exception, and you get to also  
 access the other exceptions by using primitives in class Exception. Is  
 that true?



  try {
   ...
 } catch(SomeException e) {
   throw new AnotherException(e);
 }
  and then AnotherException is thrown with "e" being it's inner  
 exception, and you can check that.


 Anyhow, we should implement a model in D that allows such multiple  
 throws.






 For allowing destructors to throw.

 Andrei



Are you sure this is sound?
Mar 19 2009
prev sibling parent reply Sean Kelly <sean invisibleduck.org> writes:
Steve Teale wrote:

 
 What is the point of class Throwable if you can throw any old class object?



I think a case could be made for allowing only children of Throwable to 
be thrown.  It's possible to throw integers and strings in C++, for 
example, which is totally ridiculous.  However, at the moment Throwable 
simply serves as the common base class for Exception and Error, and 
holds file and line info as well as the stack trace hooks.
Mar 19 2009
parent Steve Teale <steve.teale britseyeview.com> writes:
Sean Kelly Wrote:


 Steve Teale wrote:

 
 What is the point of class Throwable if you can throw any old class object?



 I think a case could be made for allowing only children of Throwable to 
 be thrown.  It's possible to throw integers and strings in C++, for 
 example, which is totally ridiculous.  However, at the moment Throwable 
 simply serves as the common base class for Exception and Error, and 
 holds file and line info as well as the stack trace hooks.



It's exactly what you would expect based on the languages that D has borrowed
from, but if it does not mean throwable, what's the point?

I still don't know what the significance of Exception and Error might be.
Mar 19 2009