In C++ if I want to catch and rethrow any exception I can do this:

  try {

  } catch(...) {
    throw;
  }

The roughly equivalent syntax in D would be this:

  try {

  } catch {
    throw;
  }

But currently, 'throw' must be followed by an expression in D.  This works:

  try {

  } catch(Object o) {
    throw o;
  }

But it relies on undocumented (I believe) knowledge that all exceptions in D are objects and also subverts the need for a LastCatch statement. Is there any D equivalent of the C++ code above, barring the use of scope(failure)?  And why is there no way to rethrow the exception caught by a LastCatch statement?


Sean

The reason Object works is because the definition of Exception is

class Exception : Object
{
// blah
}

You can use

catch (Exception o) {
	throw o;
}

or

catch (Object o) {
	throw o;
}

or just not catch it at all.

Sean Kelly wrote:
> In C++ if I want to catch and rethrow any exception I can do this:
> 
>   try {
> 
>   } catch(...) {
>     throw;
>   }
> 
> The roughly equivalent syntax in D would be this:
> 
>   try {
> 
>   } catch {
>     throw;
>   }
> 
> But currently, 'throw' must be followed by an expression in D.  This works:
> 
>   try {
> 
>   } catch(Object o) {
>     throw o;
>   }
> 
> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects and also subverts the need for a LastCatch statement. Is there any D equivalent of the C++ code above, barring the use of scope(failure)?  And why is there no way to rethrow the exception caught by a LastCatch statement?
> 
> 
> Sean

Sean Kelly wrote:
> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects
In D, all classes derive from Object, and you can only throw classes, so the fact that you can only throw things derived from Object is implicit in the structure of the language.

 - Gregor Richards

Sean Kelly wrote:
> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects ...
> Sean

quote from the specification (statements throw)
"Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception."

So it is documented that Exceptions is objects.

Johan Granberg wrote:
> Sean Kelly wrote:
>> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects ...
>> Sean
> 
> quote from the specification (statements throw)
> "Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception."
> 
> So it is documented that Exceptions is objects.

Ah, good to know.  But then what is the purpose of catch{}?  Without the ability to rethrow, it seems to have marginal utility.


Sean

The purpose is to inform the user that there has been an error without overly interrupting the program.  I think the rationale is that after you have taken the time to handle an exception, why would you rethrow it?

If you disagree, you don't need to have the kind of catch.  You can always catch either Exception or Object and ignore the LastCatch.

Sean Kelly wrote:
> Johan Granberg wrote:
>> Sean Kelly wrote:
>>> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects ...
>>> Sean
>>
>> quote from the specification (statements throw)
>> "Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception."
>>
>> So it is documented that Exceptions is objects.
> 
> Ah, good to know.  But then what is the purpose of catch{}?  Without the ability to rethrow, it seems to have marginal utility.
> 
> 
> Sean

Sean Kelly wrote:
> Johan Granberg wrote:
> 
>> Sean Kelly wrote:
>>
>>> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects ...
>>> Sean
>>
>>
>> quote from the specification (statements throw)
>> "Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception."
>>
>> So it is documented that Exceptions is objects.
> 
> 
> Ah, good to know.  But then what is the purpose of catch{}?  Without the ability to rethrow, it seems to have marginal utility.

Sean; I thought you could rethrow:

catch (Exception e)
       throw e;

But then why have catch{} at all?  It's actually less useful than catch(Object) as it provides no facility to rethrow the exception, and it offers nothing over catch(Object) in what it can catch.  Also, it's possible that a LastCatch block may simply be intended to detect a failure condition and perform some operation (similar to stack(failure)) or that it may need to rethrow the exception if some global condition cannot be met.  Sure, you could use catch(Object) in these instances, but then again, what is the purpose of catch{}?  Surely it can't simply be to avoid typing "(Object o)"?

James Pelcis wrote:
> The purpose is to inform the user that there has been an error without overly interrupting the program.  I think the rationale is that after you have taken the time to handle an exception, why would you rethrow it?
> 
> If you disagree, you don't need to have the kind of catch.  You can always catch either Exception or Object and ignore the LastCatch.
> 
> Sean Kelly wrote:
>> Johan Granberg wrote:
>>> Sean Kelly wrote:
>>>> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects ...
>>>> Sean
>>>
>>> quote from the specification (statements throw)
>>> "Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception."
>>>
>>> So it is documented that Exceptions is objects.
>>
>> Ah, good to know.  But then what is the purpose of catch{}?  Without the ability to rethrow, it seems to have marginal utility.
>>
>>
>> Sean

kris wrote:
> Sean Kelly wrote:
>> Johan Granberg wrote:
>>
>>> Sean Kelly wrote:
>>>
>>>> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects ...
>>>> Sean
>>>
>>>
>>> quote from the specification (statements throw)
>>> "Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception."
>>>
>>> So it is documented that Exceptions is objects.
>>
>>
>> Ah, good to know.  But then what is the purpose of catch{}?  Without the ability to rethrow, it seems to have marginal utility.
> 
> Sean; I thought you could rethrow:
> 
> catch (Exception e)
>        throw e;

You can for the above case.  But D also supports:

    catch{ ... }

ie. without the "(Exception e)".  However, unlike C++, "throw;" in this instance is invalid so there's no way to rethrow the exception.

Basically, I'm wondering why this form of catch exists.  It can catch exceptions but not report anything about them (as it has no object reference), and it can't rethrow them.  I can't think of a single case where I would actually use this statement as it exists now.


Sean

I can't say for sure, but I think one of the uses of a LastCatch instead of catch (Object) is efficiency of the compiled code.  It knows that the code should be called without worrying about if it meets some criteria.  Essentially, I believe it's for optimization when all you need to know is that something went wrong.

Of course, I could be totally wrong.  If so, I agree that it serves no purpose and should be removed or deprecated.

Sean Kelly wrote:
> But then why have catch{} at all?  It's actually less useful than catch(Object) as it provides no facility to rethrow the exception, and it offers nothing over catch(Object) in what it can catch.  Also, it's possible that a LastCatch block may simply be intended to detect a failure condition and perform some operation (similar to stack(failure)) or that it may need to rethrow the exception if some global condition cannot be met.  Sure, you could use catch(Object) in these instances, but then again, what is the purpose of catch{}?  Surely it can't simply be to avoid typing "(Object o)"?
> 
> James Pelcis wrote:
>> The purpose is to inform the user that there has been an error without overly interrupting the program.  I think the rationale is that after you have taken the time to handle an exception, why would you rethrow it?
>>
>> If you disagree, you don't need to have the kind of catch.  You can always catch either Exception or Object and ignore the LastCatch.
>>
>> Sean Kelly wrote:
>>> Johan Granberg wrote:
>>>> Sean Kelly wrote:
>>>>> But it relies on undocumented (I believe) knowledge that all exceptions in D are objects ...
>>>>> Sean
>>>>
>>>> quote from the specification (statements throw)
>>>> "Expression is evaluated and must be an Object reference. The Object reference is thrown as an exception."
>>>>
>>>> So it is documented that Exceptions is objects.
>>>
>>> Ah, good to know.  But then what is the purpose of catch{}?  Without the ability to rethrow, it seems to have marginal utility.
>>>
>>>
>>> Sean