> On Feb 20, 2018, at 8:58 AM, Chris Angelico <ros...@gmail.com> wrote: > >> On Wed, Feb 21, 2018 at 12:53 AM, Antoon Pardon <antoon.par...@vub.be> wrote: >> In C++ I can do something like: >> >> SomeClass MyVar; >> >> And after that the kind of possible assignments to MyVar are constraint. It >> makes the runtime throw an error when somewhere the program tries to assign >> something to MyVar that isn't allowed by SomeClass. >> >> You can't put such constraints on names in Python. >> >> In C++ I can do some like: >> Some_Class: MyVar; >> >> And after that, It will be impossible to assign a value to MyVar that >> doesn't meet the >> constraints imposed by the constructor/copy operator. You have put >> somekind of >> contract on the name MyVar, that limits the kind of things assignable to >> it. You >> can't put such constraints on a name in Python. > > Okay. Now create a constraint on a name in C++ such that it can only > accept integers representing A.D. years which, on the Gregorian > calendar, are leap years. (Using a dedicated integer-like type is > permitted.) It must accept all multiples of four, except those which > are multiples of one hundred, unless they're also multiples of four > hundred. > > That's what Steve asked for. Can you do it? Or is the C++ type system > not flexible enough for that? > > ChrisA > -- > https://mail.python.org/mailman/listinfo/python-list
Such a class would be fairly trivial to write in C++, or I suspect Python. In C++ you would have a constructor and possibilities an assignment operator that takes an integer, tests it for validity and throw/asserts if it is incorrect. (Depending on other requirements, you might allow or not implicit conversions) It probably also has a constructor and an assignment operator that takes a ‘LeapYear’ and just uses it. To be usable, it will need something to allow you to get the integer value out, might be implicit or explicit. The big issue with such a type, and why it doesn’t make much sense as a type, is that there is very little that can be done with such a type, as there is no interesting operation for which the type is at least mostly closed under, at best it is closed under +400*n -- https://mail.python.org/mailman/listinfo/python-list
