How to get child class Type and members from parent class?

[Matheus]

On Wednesday, 20 November 2019 at 13:46:07 UTC, Jacob Carlborg 
wrote:
> On Wednesday, 20 November 2019 at 10:05:11 UTC, zoujiaqing 
> wrote:
>> import std.stdio;
>>
>>
>> class A
>> {
>>     this(T)(T t)
>>     {
>>
>>     }
>>
>>     void write()
>>     {
>>         T _this = cast(T) this;
>>         writeln(this.v);
>>     }
>> }
>>
>> class B : A
>> {
>>     string v = "hello";
>> }
>>
>> void main()
>> {
>>     auto b = new B;
>>
>>     writeln(b.write()); // print hello
>> }
>
> You can use a template this parameter [1], like this:
>
> import std.stdio;
>
> class A
> {
>     void write(this T)()
>     {
>         T self = cast(T) this;
>         writeln(self.v);
>     }
> }
>
> class B : A
> {
>     string v = "hello";
> }
>
> void main()
> {
>     auto b = new B;
>     b.write();
> }
>
> [1] https://dlang.org/spec/template.html#template_this_parameter
>
> --
> /Jacob Carlborg

I'm not the OP but a lurker, and this is new to me, I mean in 
your example you're accessing a member "v" which wasn't defined 
in the Parent class.

So if someone creates something like this:

class C : A{
     string x = "world";  // x instead of v
}

Not the "x" instead of "v", of course it will only get an 
compiler error if that function is called in by "C" object.

I think this is a powerful and weird feature at the same time, 
because some could write a code like this:

import std.stdio;

class A{
     void write(this T)(){
         T self = cast(T) this;
         writeln(self.v);
     }

     void write2(this T)(){
         T self = cast(T) this;
         writeln(self.x);
     }
}

class B : A{
     string v = "hello";
}

class C : A{
     string x = "world";
}

void main(){
     auto b = new B;
     b.write();

     auto c = new C;
     c.write2();
}

This is a different way of designing things, do people use this 
often?

Matheus.