C#'s 'is' equivalent in D

[jmh530]

On Thursday, 10 October 2019 at 16:33:47 UTC, H. S. Teoh wrote:
> On Thu, Oct 10, 2019 at 03:58:02PM +0000, jmh530 via 
> Digitalmars-d-learn wrote:
>> On Thursday, 10 October 2019 at 15:47:58 UTC, Just Dave wrote:
>> > In C# you can do something like:
>> > 
>> > 
>> >     if (obj is Person)
>> >     {
>> >         var person = obj as Person;
>> >         // do stuff with person...
>> >     }
> [...]
>> You mean something like below:
>> 
>> class Person {
>>     int id;
>>     this(int x) {
>>         id = x;
>>     }
>> }
>> 
>> void main() {
>>     auto joe = new Person(1);
>>     if (is(typeof(joe) == Person)) {
>>         assert(joe.id == 1);
>>     }
>> }
>
> Unfortunately, typeof is a compile-time construct, so this will 
> not work if you're receiving a Person object via a base class 
> reference. The correct solution is to cast the base class to 
> the derived type, which will yield null if it's not an instance 
> of the derived type.
>
>
> T

Ah, you mean something like below:

class Person {
     int id;
     this(int x) {
         id = x;
     }
}

class Employee : Person {
     int job_id;
     this(int x, int y) {
         super(x);
         job_id = y;
     }
}

void main() {
     import std.stdio : writeln;

     Person joe = new Employee(1, 2);

     if (is(typeof(joe) == Employee)) {
         writeln("here"); //not called in this case
     }
}