C++: Are there reasons to use exceptions in non exceptional situations

Question

Let me give you the following example: Imagine you have a window class which can have child windows. Each child window holds a weak pointer to its parent window, each window has a list of shared ptrs to its children. Now i have the situation that if the child window is destructed, I can't know if it is destructed because the parent window was closed, or because the child window itself was closed. Therefore the solution I came up with was to dereference the parent windows weak pointer in the child windows destructor in a try{}catch{} block to see if the parent window still exists (pseudocode):

 //we try to retain the parent window here
 try
 {
     //creates a shared pointer from the weak pointer, throws BadReferenceCounterException on fail
     WindowPtr parent = m_parentWindow.retain();

     //check if there is a parent at all, otherwise 0
     if(parent)
     {
         parent->removeChildWindow(this);
     }
  }
  catch(const BadReferenceCounterException & _ec)
  {
     std::cout<<"Catched expected exception, if Parent was closed before me!"<<std::endl;
  }

In this situation I could not think of another way to solve this simply because there is no way to know the context in which the destructor was called. Is this a stupid thing to do because I use an exception in a sort of expected situation? I know this is not a thing you want to do in performance critical code so this is not the point of the question.

Thank you!

Sounds like holy war territory to me. I'll lob the first volley: your weak pointers could allow their validity to be checked without ever throwing. `boost::weak_ptr::lock` returns a null `boost::shared_ptr` on such access. — Managu, Jan 24 '12 at 14:46
Does it work as desired? Perform well enough? Is it understandable? Then it’s a valid solution, IMHO. — Managu, Jan 24 '12 at 14:55
The question in the title sounds like something for Programmers.SE, whereas the problem you present in the body is something for Stack Overflow. Do you want an answer to your specific design issue, or do you want a discussion of the broader topic? You can't have both at once. — Rob Kennedy, Jan 24 '12 at 14:55
@Managu it does, It just felt like something horribly wrong to do from what I read about exceptions before :) — moka, Jan 24 '12 at 14:56
Moka, please use the "@" symbol to reply to people who have commented. That way, they will be notified that you replied to them, and others will have a better idea what you're talking about when your responses don't include much context. — Rob Kennedy, Jan 24 '12 at 14:57

score 5 · Accepted Answer · edited May 23 '17 at 09:59

5

You seem to be using faulty shared/weak pointer pair. Standard (both in C++11 and in boost) shared/weak pointers allow you to do it without exceptions:

typedef std::shared_ptr<Window> WindowPtr;
typedef std::weak_ptr<Window> WeakWindowPtr;

WeakWindowPtr m_parentWindow;
//...

WindowPtr parent = m_parentWindow.lock();
if (parent)
{
  // here you know parent wasn't destroyed and you can access it via parent
}

You may also want to check out code samples I've given in this stackoverflow answer

edited May 23 '17 at 09:59

Community

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answered Jan 24 '12 at 14:48

bronekk

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And due to a handy quirk in c++ syntax you can say `if (WindowPtr parent = m_parentWindow.lock()) { /* use parent */ }` and `parent` will have just the right scope. – bames53 Jan 30 '12 at 06:41

C++: Are there reasons to use exceptions in non exceptional situations

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