scope + destructor with Exception parameter for RAII

[Sean Kelly]

Pragma wrote:
> BCS wrote:
>> Sean Kelly wrote:
>>> Leandro Lucarella wrote:
>>>> What do you think of adding an optional parameter (exception) to the 
>>>> destructor, defaulting to null, to indicate the destructor was 
>>>> called then unwinding because an exception was thrown? Then you can 
>>>> almost forget about scope(exit/success/failure) and you have a RAII 
>>>> as complete  as Python's 'with' statement.
>>>
>>> I don't like it, personally.  It doesn't seem a good idea for a dtor 
>>> to alter its behavior based on whether an exception is in flight, and 
>>> exceptions should never be thrown from dtors anyway.  Doing so makes 
>>> writing correct code far too complicated.
>>>
>>> One thing I have done for Ares, however, is to terminate the program 
>>> if one exception is thrown while another is in flight.  I think 
>>> DMD/Phobos does not do this currently, and instead either ignores the 
>>> new exception, or substitutes it for the in-flight exception (I can't 
>>> remember which).
>>>
>>>
>>> Sean
>>
>>
>> I think this works under DMD
>>
>> try
>> {
>>     throw new Error("some stuff");
>> }
>> catch(Error e)
>> {
>>     throw new Error("more stuff\n"~e.toString);
>> }
>>
>>
>> It could be vary handy to have this ability to swap out one exception 
>> for another.
> 
> Agreed.  Although I don't know if this interferes with Sean's 
> interpretation of the spec or not.  I've done this in a few places 
> myself, and I find it quite useful.

The above example isn't the same thing.  The in-flight exception is 
caught and a different one is rethrown.  What I consider illegal is 
something like this:

     try
     {
         throw new Exception( "A" );
     }
     finally
     {
         throw new Exception( "B" );
     }

What exception is in-flight after the finally block completes?  And what 
happens to the other exception?  The only correct behavior here is to 
terminate the application.


Sean