[Interest] QLineEdit upper case

pmqt71 pmqt71 at gmail.com
Fri Aug 8 23:38:55 CEST 2014

thanks Oliver,

I know about is-a and has-a but in this case I just want do what I can do
in 2 seconds with MFC just setting a property.
I've already spent so much time, so the faster solution is the best
solution for a so simple problem.
going for validator inheritance.


2014-08-08 20:14 GMT+02:00 Till Oliver Knoll <till.oliver.knoll at gmail.com>:

> Am 08.08.14 18:08, schrieb pmqt71:
> > ...
> > @Oliver:
> > yes my problem is that QLineEdit can have only 1 validator. I think
> > inherithance should be easier than implementin a ChainValidator, am I
> right?
> We could now fill pages of discussions about "Inheritance being evil",
> "is-a" vs "has-a" vs "uses-a" relations etc., and we'd end up having at
> least 10 times more opinions than possible ways of implementing the
> solution ;)
> (Just start by asking Google with "Inheritance is Evil" - and you get
> the idea ;))
> Here is just a starter link:
>   http://en.wikipedia.org/wiki/Composition_over_inheritance
> Personally I try to question inheritance as much as possible. The
> classic question is "Is a Square also a Rectangle"?
> The best practical explanation - and design question you should ask
> yourself whenever you have inheritance in mind - I ever read is: would
> an instance of a derived class pass all (possible and sensible) unit
> tests of a base class?
> Let's continue with the "Shall I derive my Square class from the
> Rectangle class?"-example (which is really a classic, but as such
> probably also of "academic value") and let us further assume there was a
> unit test which would verify the "area" method.
> So the test would look something like:
>   int width = 3;
>   int height = 5;
>   Rectangle rect;
>   // setup
>   rect->setWidth(width);
>   rect->setHeight(height);
>   // exercise
>   int area = rect.area();
>   // verify
>   assert(area, width * height);
> Now if you ran the same test, but with an instance of Square, that test
> would fail! Why? Because our expectation is that whenever you modify
> either width or height, the implementation of a Square would implicitly
> set the height or width respective to the same value (otherwise the
> instance would not be a square!).
> So after setting the height to 5, the width in above example would also
> become 5 and the area would be 25 != 15. Fail.
> Now would e.g. a UPPERCaseValidator inherited from a
> LettersOnlyValidator also falsify unit tests in the same way? Most likely.
> There is another reason why particularly in this example I would
> definitively opt for the "ChainValidator" (-> composition): it is way
> more flexible!
> Let's assume you had 3 validators A, B and C and an additional
> "UPPERCaseValidator" (4 classes in total). If you wanted to have for
> each A, B, C Validator a combination, you'd have to come up with another
> 3 classes (deriving each time from A, B and C respective)! So that would
> be 7 classes in total to maintain (and if you need to fix something in
> your UPPERCaseValidator implementation, you'd need to fix it 3 times in
> the worst case - you could share parts of the implementation, but
> still...).
> With a composition pattern you'd only need one extra class - a
> "ChainValidator" - and you could combine either A, B or C with your
> UPPERCaseValidator. Only one additional class, total 5 classes.
> "Well, 5 or 7 classes", what's the big deal anyway!
> Well, here is the deal: just add one more validator D, and your number
> of classes grows linearily (and so are the number of implementations to
> maintain): not only do you have another class D, but also a new derived
> class from D - total 9 classes! For every "base validator" class that
> you'd add you would have to implement yet another derived class
> (assuming off course that this would make sense to have an
> UPPERCaseValidator also validate the output of all A, B, C and D classes).
> But here's an even bigger deal: Let's assume your boss now tells you
> that you need to validate A, B - AND C! Or any combination thereof
> (boss: "Oh and yes, off course, don't forget about our new shiny
> UPPERCaseValidator class, add that to the mix, too! And the new
> validator D that we just purchased yesterday...").
> So would you want to come up with classes such as ABCUpperCaseValidator,
> CBAUpperCaseValidator, AUpperCaseValidator, BUpperCaseValidator,
> CUpperCaseValidator, ... (the names reflect the "inheritance chain")?
> Hopefully not! A combinatorial explosion of possible inheritance
> permutations!
> With the "ChainValidator" however you would simply assemble the
> validators like so (pseudo-syntax - you get the idea):
>   AValidator a;
>   BValidator b;
>   CValidator c;
>   UPPERCaseValidator d;
>   ChainValidator ca;
>   // validates a and d
>   ca.add(a);
>   ca.add(d);
>   state = ca.validate(theString, pos);
>   ca.clear();
>   // validats a, c and d
>   ca.add(a);
>   ca.add(c);
>   ca.add(d);
>   ca.validate(theOtherString, pos);
> No need to come up with new classes! And just one single
> UPPERCaseValidator implementation!
> (Again, whether the above actually works when it comes to validation, I
> don't know - clearly there would be combinations which would not make
> sense at all, but in the worst case you'd probably just get an Invalid
> state with /any/ input).
> Now all that said: the above might be complete "over-engineering" in
> your particular case! If you have 2 or 3 very specific cases to validate
> "in just that particular dialog" and "I will never ever have the need to
> ever re-use those inhertited validation classes anywhere - I swear!"
> then go for inheritance - it is not "per se evil" ;) Just know the
> consequences.
> Cheers, Oliver
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