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| Subject | Re: is "abstract" supposed to do something? | |||||||||||||||||||||
| From | Regan Heath <regan@netwin.co.nz> | |||||||||||||||||||||
| Date | Tue, 13 Jul 2004 10:18:01 +1200 | |||||||||||||||||||||
| Newsgroups | digitalmars.D | |||||||||||||||||||||
On Mon, 12 Jul 2004 13:54:31 -0400, Jarrett Billingsley <kb3ctd2@yahoo.com> wrote: > in C++, there is a concept called an "abstract class", which is achieved > by > having a class with at least one pure virtual function (function > declaration > =0). you are not allowed to create any instances of an abstract class. > > i thought the keyword "abstract" would cause this behavior on a class > without the "implied" kind that you get with C++. apparently not! > > defining a class as "abstract" does not seem to do anything. D allows > you > to create instances of it. deriving from the abstract class does not > change > any behavior. > > looking through the manual, i cannot find any explanation as to what the > abstract keyword does. > > all i'm doing is this: > > abstract class Parent > { > int foo() { return 10; } > } > > class Child : Parent > { > int foo() { return 20; } > } > > i'm still allowed to create instances of Parent and call its foo(). > > am i doing something wrong here? am i not supposed to have any kind of > function declaration in the Parent class for it to be treated as > abstract? > or does abstract do something completely different? or is it even > implemented ;) It is implemented, it just behaves differently to what you're expecting. From what I can see.. class Parent { int foo(); } class Child : Parent { int foo() { return 20; } } The above will not compile, *unless* I put 'abstract' in front of the class, or the method 'foo' eg abstract class Parent { int foo(); } or class Parent { abstract int foo(); } so abstract *allows* a class to have undefined method bodies. It does not *prohibit* instantiation of a class as long as it has all it's method bodies. I *do* like the fact that you do not need to place abstract on all the methods of a class, one abstract on the class definition will affect all methods in the class. I *do not* like the fact that you cannot provide bodies and still *prohibit* instantiation of the abstract class. These are my gut reactions, maybe it's time for a rethink, perhaps D has it right, if a class has all it's method bodies, why prohibit it's instantiation? In which case placing abstract in front of the class or method (with a body) should give an error. Regan. -- Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/ | ||||||||||||||||||||||
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