Don wrote:
Andrei Alexandrescu wrote:
Walter and I just discussed the matter of inheriting constructors. Our
thought on the issue:
a) If a class doesn't define any constructors and adds no fields,
inherit constructors. Example:
class MyException : Exception {}
b) If a class defines at least one constructor, do not inherit
constructors.
c) If a class doesn't define any constructors but does add at least a
non-static field -> undecided.
What do you think?
Andrei
I would add: "and the class does not define a class invariant".
A constructor exists to establish the class invariant (even if that
invariant is implicit). For (a), the old invariant will still be satisfied.
In the case (c) there is a chance that the invariant will not be
appropriate. In (c), I suggest that it is valid to inherit constructors
if and only if the new class is a simple aggregate of the old class,
together with the new members (which may have invariants of their own).
If there's no constructor and no invariant, then either it's a simple
aggregate, or it's a bug.
Good point. I do think of classes that add fields that don't mess up the
invariant though:
class MyException : Exception {
private int sysCode;
invariant() { assert(sysCode < 100); }
}
If default initialization of the field puts it in a state that respects
the invariant, there isn't a problem.
Andrei
