Virtual functions and inheritance

[H. S. Teoh via Digitalmars-d-learn]

On Tue, Jan 27, 2015 at 04:38:57AM +0000, David Monagle via Digitalmars-d-learn wrote:
> Hi guys,
> 
> I'm a former C++ developer and really enjoying working with D now. I
> have a question that I hope some of you may be able to answer.
> 
> class Parent {
>   @property string typeName() {
>     return typeof(this).stringof;
>   }
> }

Try this:

	class Parent {
	    @property string typeName() {
		return typeid(this).toString();
	    }
	}


> class Child : Parent {
> }
> 
> void main() {
>   auto p = new Parent;
>   auto c = new Child;
>   assert(p.typeName == "Parent");
>   assert(p.typeName == "Child");

Bug: the second assert should have c.typeName. :-) But I got what you
mean.


> }
> 
> 
> I'm looking for an explanation as to why this doesn't work, then a
> suggestion for how I may achieve child classes being able to generate
> a string description of their own type, without redefining the
> typeName property on each child. (I'm currently solving this with a
> mixin, but I was hoping for a better solution.
> 
> I'm assuming it doesn't work because either typeof(this) or .stringof
> is evaluated at compile time?

typeof(this) is evaluated at compile-time. To get runtime information,
you need to use typeid().

Of course, with my suggested change your code still won't work as-is,
because typeid().toString() returns a fully-qualified name, so if your
source file is test.d, it would return "test.Parent" and "test.Child".
But you should be able to work with that. :-)


T

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The volume of a pizza of thickness a and radius z can be described by
the following formula: pi zz a. -- Wouter Verhelst