Wednesday, May 9, 2012

throw exception from dtor - justified use case

[[moreEffC++]] advocates preventing exception leaving a dtor, either virtual or nonvirtual.

However, in practice, there are reasons to break the rule. Suppose I have

try{
  int i1, i2;// temp variables,

  i1 = myobj->eat();
  myobj->drink(&i1);
  myobj->cook(&i1);
  i2 = myobj->wash(&i1);
//....
  delete myobj;
}catch(business_exception & be){
//handle exception using the temp variables
}

Since eat(), drink() etc and dtor all throw the same business_exception, this code is clean and maintainable. If we need to throw another exception from those 3 functions, we can easily add the code. The same exception handler is used as a catch-all.

It would be messy to pass the temp variables into myobj dtor and replicate the same exception logic therein.

So in this case, I'd make myobj dtor throw business_exception.

Now, as described in [[moreEffC++]] myobj dtor is invoked as part of stack unwinding due to another exception? System will trigger terminate(). Well, in this case, I know that's a fatal scenario and I do want system to crash anyway, so the terminate() behaviour is not unacceptable.

In other words, myobj's class is written such that its dtor should throw exception only under normal object destruction and should never be part of an exceptional stack unwinding. In such a case, no one should misuse this class in an exception-unsafe context. If they ignore the design purpose of this class, they could get this dtor invoked as part of an exceptional stack unwinding, and the consequence is something they must deal with.

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