In C# you can do something like:


    if (obj is Person)
    {
        var person = obj as Person;
        // do stuff with person...
    }

where you can check the type of an object prior to casting. Does D have a similar mechanism? It's so widely useful in the C# realm that they even added syntactic sugar to allow:

    if (obj is Person person)
    {
        // do stuff with person...
    }

I would presume since D has reference objects there must exist some mechanism for this...

Even though static solutions would be more performance minded, I'd actually prefer to see the runtime equivalent so I don't have to rethink how I think as performance isn't really my major concern right now.

On 10/10/19 6:47 PM, Just Dave wrote:
> In C# you can do something like:
> 
> 
>      if (obj is Person)
>      {
>          var person = obj as Person;
>          // do stuff with person...
>      }
> 
> where you can check the type of an object prior to casting. Does D have a similar mechanism? It's so widely useful in the C# realm that they even added syntactic sugar to allow:
> 
>      if (obj is Person person)
>      {
>          // do stuff with person...
>      }
> 
> I would presume since D has reference objects there must exist some mechanism for this...

```D
      if (auto person = cast(Person) obj)
      {
          // do stuff with person...
      }
```

On Thursday, 10 October 2019 at 15:47:58 UTC, Just Dave wrote:
>     if (obj is Person person)

Looks the same as D's

if(auto person = cast(Person) obj) {
  // use person in here
} else {
  // it was some other type
}

On Thursday, October 10, 2019 9:47:58 AM MDT Just Dave via Digitalmars-d- learn wrote:
> In C# you can do something like:
>
>
>      if (obj is Person)
>      {
>          var person = obj as Person;
>          // do stuff with person...
>      }
>
> where you can check the type of an object prior to casting. Does D have a similar mechanism? It's so widely useful in the C# realm that they even added syntactic sugar to allow:
>
>      if (obj is Person person)
>      {
>          // do stuff with person...
>      }
>
> I would presume since D has reference objects there must exist some mechanism for this...

D's solution is basically the same as C++'s solution. You cast and then check whether the result is null. So,

if(cast(Person)obj !is null)
{
}

or since using a pointer or reference in an if condition checks whether it's null or not

if(cast(Person)obj)
{
}

and you can even declare a variable that way if you want. e.g.

if(auto person = cast(Person)obj)
{
}

When D's is is used to compare two objects, it checks whether they're equal bitwise. So, it's typically used for comparing pointers or references for equality (whereas using == with references would compare the objects themselves for equality).

- Jonathan M Davis

On Thursday, 10 October 2019 at 15:53:20 UTC, Adam D. Ruppe wrote:
> On Thursday, 10 October 2019 at 15:47:58 UTC, Just Dave wrote:
>>     if (obj is Person person)
>
> Looks the same as D's
>
> if(auto person = cast(Person) obj) {
>   // use person in here
> } else {
>   // it was some other type
> }

Excellent!

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...
>     }
>
> where you can check the type of an object prior to casting. Does D have a similar mechanism? It's so widely useful in the C# realm that they even added syntactic sugar to allow:
>
>     if (obj is Person person)
>     {
>         // do stuff with person...
>     }
>
> I would presume since D has reference objects there must exist some mechanism for this...

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);
    }
}

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

-- 
LINUX = Lousy Interface for Nefarious Unix Xenophobes.

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
    }
}