• klickverbot (71/71) Nov 25 2009 (This would probably fit better on bugzilla, but for whatever reason I

klickverbot <klickverbot gmail.com> writes:

(This would probably fit better on bugzilla, but for whatever reason I 
cannot access it at the moment. Please feel free to move this to the tracker 
and CC me if it is really a bug.)

As you might remember from #d and d.D.learn, I want to check if a given 
member function has been overridden (at runtime, of course). A simple test 
using delegates looks promising:
---
class A {
public:
  void foo() {
    Stdout( "I am in A." ).newline;
  }
}

class B : A {
public:
  override void foo() {
    Stdout( "I am in B." ).newline;
  }
}

void main() {
  auto baseMethod = &A.foo;

  A a = new A();
  a.foo();
  auto aMethod = &a.foo;
  Stdout.formatln( "foo overridden in a: {}",
    ( baseMethod !is aMethod.funcptr ) );

  A b = new B();
  b.foo();
  auto bMethod = &b.foo;
  Stdout.formatln( "foo overridden in b: {}",
    ( baseMethod !is bMethod.funcptr ) );
}
---

For use in my application, I tried to move the checking code to A, which 
should be no problem, at least according to the principle of least surprise:
---
class A {
public:
  void foo() {
    Stdout( "I am in A." ).newline;
  }

  bool isFooOverridden() {
    auto methodThis = &foo;
    auto methodBase = &A.foo;
    return ( methodThis.funcptr !is methodBase.funcptr );
  }
}

class B : A {
public:
  override void foo() {
    Stdout( "I am in B." ).newline;
  }
}

void main() {
  A a = new A();
  a.foo();
  Stdout.formatln( "foo overridden in a: {}", a.isFooOverridden() );

  A b = new B();
  b.foo();
  Stdout.formatln( "foo overridden in b: {}", b.isFooOverridden() );
}
---

However, this does not work because the address of A.foo is looked up at 
runtime via the vtable in this case, effectively defeating the intent behind 
the pointer comparison.

Have I hit a bug here or is it written somewhere in the spec? If this should 
really be the intended behavior, I would strongly suggest altering it 
because it seems highly counterintuitive to me. Moveover, I cannot really 
see that the current behavior would have any benefits.

Sorry if this has already been discussed, but as I mentioned before, I 
cannot access the D bugzilla at the moment…