Re: C#'s 'is' equivalent in D

[jmh530 via Digitalmars-d-learn]

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
    }
}