Re: Why do abstract class functions require definitions?

[Marc Schütz via Digitalmars-d-learn]

On Wednesday, 16 September 2015 at 09:31:25 UTC, Jacob Carlborg 
wrote:
On 2015-09-16 10:49, FiveNights wrote:
Every so often I'll get a compiler error that isn't 
particularly clear
on what's wrong and eventually I'll figure out that what's 
causing it is
having a function in an abstract class somewhere that isn't 
defined:

abstract class SomeClass {
     int someVariable;
     void someFunction();
}
the solution is usually:
void someFunction(){}

Usually the abstract class is a converted interface, but it 
turned out
that I needed to include a variable for it to work out and I 
just wasn't
keen on remembering to put a mixin in each derived class.

I'm just wondering why I can't have an undefined function in 
an abstract
class? I'd the compiler to say, "Hey, you forgot to put 
'someFunction()'
in 'SomeDerrivedClass', go do something about that." when I 
end a
function with a semi-colon in the base class and don't have it 
in the
derrived. Everything just seems to break in cryptic ways 
unless I curly
brace the function ending.

I'm guessing you see a link error. The reason you see that 
instead of a compile error is because D supports separate 
compilation. Meaning that the method could be implemented in a 
different library that are resolved during link time.

As already answered in another post, the solution is to prefix 
the method declaration with "abstract".

Wouldn't the following behaviour be more useful as a default?

    abstract class Foo {
        void bar1() { }     // non-abstract, obviously
        void bar2();        // abstract, because it's in an 
abstract class
                            // (different from now)
        extern void bar3(); // non-abstract, but defined 
externally
    }