ZCE Exceptions

metalenkist · 04-03-2010, 09:07 AM

Hey all, at the moment I'm studying for my ZCE (Zend Certified Engineer) exam and there and there are a few questions that are real headbreakers for me. One is about exceptions and hopefully can one of you explain why the output of the following code is "Didn't Caught Exception".

Already thanks a lot.

Code:

<?php
class MyException extends Exception {}
class AnotherException extends MyException {}

class Foo {
  public function something() {
    throw new AnotherException();
  }
  public function somethingElse() {
    throw new MyException();
  }
}

$a = new Foo();

try {
  try { $a->something();}
    catch(AnotherException $e) { $a->somethingElse();}
    catch(MyException $e) { print "Caught Exception";}
} catch(Exception $e) {
  print "Didn't catch the Exception!";
}

?>

Sergei Steshenko · 04-03-2010, 09:26 AM

Quote:

Originally Posted by metalenkist

Hey all, at the moment I'm studying for my ZCE (Zend Certified Engineer) exam and there and there are a few questions that are real headbreakers for me. One is about exceptions and hopefully can one of you explain why the output of the following code is "Didn't Caught Exception".

Already thanks a lot.

Code:

<?php
class MyException extends Exception {}
class AnotherException extends MyException {}

class Foo {
  public function something() {
    throw new AnotherException();
  }
  public function somethingElse() {
    throw new MyException();
  }
}

$a = new Foo();

try {
  try { $a->something();}
    catch(AnotherException $e) { $a->somethingElse();}
    catch(MyException $e) { print "Caught Exception";}
} catch(Exception $e) {
  print "Didn't catch the Exception!";
}

?>

For starters, reformat the code:

Code:

try
  {
  try
    {
    $a->something();
    }
  catch(AnotherException $e)
    {
    $a->somethingElse();
    }

  catch(MyException $e)
    {
    print "Caught Exception";
    }
  }
catch(Exception $e)
  {
  print "Didn't catch the Exception!";
  }

graemef · 04-03-2010, 10:11 AM

I think that you need to put another try block around the call to $a->somethingElse();

Sergei Steshenko · 04-03-2010, 10:32 AM

Quote:

Originally Posted by graemef

I think that you need to put another try block around the call to $a->somethingElse();

Yes, because it causes yet another exception.

metalenkist · 04-03-2010, 01:09 PM

At the mockup exam you get 4 options and you have to pick one. I taught myself the rule:

if a catch block throws an exception then ignore that catch block & and look to the upper catch.

Code:

catch(AnotherException $e)
    {
    $a->somethingElse();
    }

will throw error go to the next catch block

Code:

catch(MyException $e)
    {
    print "Caught Exception";
    }

This will never be caught cause it will catch exception from the MyException class.

Code:

catch(Exception $e)
  {
  print "Didn't catch the Exception!";
  }

This last catch, catches every error.

That's why I think the result is "Didn't catch the Exception!".

graemef · 04-03-2010, 09:42 PM

Okay I think that I misunderstood your original question.

As I understand it your thinking is correct.

When an exception is thrown within a try block the catch clauses will be tested in turn for a match (this works in the same way that a case clause within a switch statement works).

In the code that you have the catch clause itself throws an exception. So that throw looks for a try block, which means it needs to step out of its own try block into the containing try block and then look through the catch clauses for a match.

Earlier I compared the catch and cases clauses, the difference between them is when a match is found with the case clause it will still continue (unless you insert a break or return statement) whilst with the catch once a match is found it will stop. That is why the order of the catch clause is important. You need to place them in order of most specific through to most general.

Hope that helps some.

metalenkist · 04-07-2010, 04:33 PM

w00t, Yesterday I had the exam and I passed *woohooo*

Thank you very very much with helpen me out with this question.

There was a similar question at the exam.