digitalmars.D.learn - testing if an instance is a subclass of a given class
- Paul Dufresne (32/32) Sep 16 2008 Well, I have decided that translating Python code to D would be a nice w...
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
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
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
downs wrote:Paul Dufresne wrote: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 */ }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