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digitalmars.D.learn - testing if an instance is a subclass of a given class

reply Paul Dufresne <dufresnep gmail.com> writes:
Well, I have decided that translating Python code to D would be a nice way to
learn D.
It clearly shows me how D is a statically-typed language, and Python a
dynamically-typed language.

I'd like to translate to D:
  if issubclass(type(item), Base):
    item.on_value_changed(True)
  elif issubclass(type(item), Group):
    item.on_last_change()

My problem is to translate issubclass.

using typeid(item) I would get some info about the type... but it is unclear
how to test for subclass from there.

item is(item:Base) is a compile-time test, so it does not seems to do what I
want.

I am thinking about something like:
if (cast (Base) item)!=null then
     item.on_value_changed(True);
else if (cast(Group) item)!=null then
        item.on_last_change();

oh... as I am writing I see the suggestion in spec_DMD_1.00.pdf to do:
In order to determine if an object o is an instance of a class B use a cast:
if (cast(B) o)
{
     // o is an instance of B
}
else
{
     // o is not an instance of B
}

My guess is that it is really a subclassing test... is it?

And would it be possible to write an isSubclass function in D based on this
test...
I don't see how I would pass the Class for the test... what come in mind
naturally would be:
bool isSubclass (Class C, Object o){
  if (cast (C o) then return true else return false;
}

but the Class type does probably not exist ... does it?
Sep 16 2008
next sibling parent BCS <ao pathlink.com> writes:
reading only the subject


class C {}
class B : C {}
class A : C {}

C c = somthing();

if(auto b = cast(B)c)
{
  // is a B, use b
}
Sep 16 2008
prev sibling parent reply downs <default_357-line yahoo.de> writes:
Paul Dufresne wrote:
 And would it be possible to write an isSubclass function in D based on this
test...
 I don't see how I would pass the Class for the test... what come in mind
naturally would be:
 bool isSubclass (Class C, Object o){
   if (cast (C o) then return true else return false;
 }
 
 but the Class type does probably not exist ... does it?

Close. bool isA(T)(Object obj) { return (cast(T) obj)?true:false; } .. class A { } class B : A { } A x = new B; if (isA!(B)(x)) { } else { } Note however, that this is an exercise in redundancy, as it is almost exactly the same as "if (cast(B) x)". I really recommend you use that instead.
Sep 16 2008
parent downs <default_357-line yahoo.de> writes:
downs wrote:
 Paul Dufresne wrote:
 And would it be possible to write an isSubclass function in D based on this
test...
 I don't see how I would pass the Class for the test... what come in mind
naturally would be:
 bool isSubclass (Class C, Object o){
   if (cast (C o) then return true else return false;
 }

 but the Class type does probably not exist ... does it?

Close. bool isA(T)(Object obj) { return (cast(T) obj)?true:false; } .. class A { } class B : A { } A x = new B; if (isA!(B)(x)) { } else { } Note however, that this is an exercise in redundancy, as it is almost exactly the same as "if (cast(B) x)". I really recommend you use that instead.

In fact, there's a nifty idiom that exploits the fact that you can create a variable in an if statement .. A x = new B; if (auto xAsB = cast(B) x) { /* do op on xAsB */ }
Sep 16 2008