2012/6/13 lcaminiti <lorcaminiti@gmail.com>
suggestion. I read in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1962.html that you can only relax preconditions; that is, you mustn't make them stronger. The issue I see with this is that the "mustn't" is left entirely to the good will of the derived class designer and never checked by anyone. So
I don't think that is correct. In fact, there's no way the derived class designer can strength a base class precondition because the base class precondition is checked in logic-or. For dervied::f the subcontracted preconditions are:
derived::f::subcontracted_pre := base::f::pre or derived::f::pre
This is always true when base::f::pre is true and regardless of dervied::f::pre so the subcontracted precondition cannot be stronger (more demanding, false when the base is true) no matter what the derived class designer does with derived::f::pre. In fact, subcontracting allows the base class designer to always remain in control as the base class contract (precondition, postconditions, and class invariants) must always be satisfied by all derived classes no matter how you design the derived classes. (Of course, if you don't use the library macros do define the derived class then you're on your own, there's no way my library can enforce anything.)
IMO, a real good subcontracting example is:
http://contractpp.svn.sourceforge.net/viewvc/contractpp/releases/contractpp_...
This is a bit off topic, but I think there is something wrong with the postconditions of unique_identifiers::add(). If you require that the added indentifier must not be in the collection yet, why should you check in the postcondition if it was there before the addition: it is obvious it wasn't.
Keeping in mind the substitution principle: http://en.wikipedia.org/wiki/Liskov_substitution_principle If derived::f overrides base::f then derived::f must be used wherever base::f is used. Where base::f can be called, derived::f can be called. So where base::f preconditions are true, also derived::f (subcontracted) preconditions are true. That is ensured by subcontracting using the logic-or:
derived::f::subcontracted_pre := base::f::pre or dervied::f::pre
This is always true where base::f::pre is true so it always OK to call dervied::f where it was OK to call base::f (as the substitution principle says).
I think we interpret the "substitution principle" somewhat differently (tell me, if I am wrong). My interpretation is, that it is the author of the derived class (and the relaxed preconditions) that is responsible for making sure that certain semantic constraints are met -- not your framework. Overriding the preconditions so that they are not relaxation of base preconditions is such a semantic error made by the author of the derived class. Your framework makes the (successful) effort to work around this error (using the logic-or), but I believe it is not the right thing to do. The whole purpose of the concept of DbC (apart from documentation) is to detect any "logic" errors at run-time,report them, and ideally prevent the further execution of the program. The logic-or appears to me like doing exactly the opposite: hiding a logic error. Even if other frameworks in Eiffel or D do it this way, I would dare to say that they are doing it wrong.
'natural' it is stressed that one should not derive one type from another,
but when it comes to preconditions the problem is that the precondition in overriding function is not relaxed but in fact more constrained. This is the bug that your library could help detect. Of course it is not possible to assert in general that one predicate represents a relaxation of another, but what you could do is, when you check preconditions and find that the overriding precondition failed but the overridden precondition passed, signal this at run-time as design error. In other words:
if (neither base nor derived precondition holds) { report precondition violation } else { if (derived precondition fails but base precond holds) { report "precondition design" error } else { precondition is satisfied } }
Or perhaps your library already works like that?
This would be easy to implement because my library internally goes over nested if statements like the one above when evaluating subcontracted preconditions. However, IMO having yet another configuration macro to enable detecting this case and explaining this case to the user complicates the situation even more. Again, I think this design is simpler and sufficient for the user "if you want to deal with subcontracted preconditions then they are as from the substitution principle (and Eiffel), otherwise you can disable subcontracted preconditions all together".
You are probably right that since the relaxation of preconditions is likely not to be usefull at all, making too much effort to have it work this way or the other, is not the economically correct way to go. Assuming that the implementation of Boost.Contract (didn't see it yet) is as thorough as its documentation, I can only imagine how much effort you must have put into it, and still will have to. It is an impressive piece of work. Regards, &rzej