What you think about throwing an exception for not found in C++?

Question

I know most people think that as a bad practice but when you are trying to make your class public interface only work with references, keeping pointers inside and only when necessary, I think there is no way to return something telling that the value you are looking doesn't exist in the container.

class list {
    public:
        value &get(type key);
};

Let's think that you don't want to have dangerous pointers being saw in the public interface of the class, how do you return a not found in this case, throwing an exception?

What is your approach to that? Do you return an empty value and check for the empty state of it? I actually use the throw approach but I introduce a checking method:

class list {
   public:
      bool exists(type key);
      value &get(type key);
};

So when I forget to check that the value exists first I get an exception, that is really an exception.

How would you do it?

Answer 1

Accessor?

The "iterator" idea proposed before me is interesting, but the real point of iterators is navigation through a container. Not as an simple accessor.

I agree with paercebal, an iterator is to iterate. I don't like the way STL does. But the idea of an accessor seems more appealing. So what we need? A container like class that feels like a boolean for testing but behaves like the original return type. That would be feasible with cast operators.

template <T> class Accessor {
    public:
        Accessor(): _value(NULL) 
        {}

        Accessor(T &value): _value(&value)
        {}

        operator T &() const
        {
            if (!_value)
               throw Exception("that is a problem and you made a mistake somewhere.");
            else
               return *_value;
        }

        operator bool () const
        {
            return _value != NULL;
        }

    private:
        T *_value;
};

Now, any foreseeable problem? An example usage:

Accessor <type> value = list.get(key);

if (value) {
   type &v = value;

   v.doSomething();
}