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digitalmars.D.learn - C#'s 'is' equivalent in D

reply Just Dave <abcdef 1234.com> writes:



     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 

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...
Oct 10 2019
next sibling parent Just Dave <abcdef 1234.com> writes:
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.
Oct 10 2019
prev sibling next sibling parent drug <drug2004 bk.ru> writes:
On 10/10/19 6:47 PM, Just Dave wrote:


 
 
      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 

 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...
       }
```
Oct 10 2019
prev sibling next sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
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
}
Oct 10 2019
parent Just Dave <abcdef 1234.com> writes:
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!
Oct 10 2019
prev sibling next sibling parent Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:
On Thursday, October 10, 2019 9:47:58 AM MDT Just Dave via Digitalmars-d-
learn wrote:




      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

 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
Oct 10 2019
prev sibling parent reply jmh530 <john.michael.hall gmail.com> writes:
On Thursday, 10 October 2019 at 15:47:58 UTC, Just Dave wrote:




     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 


     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);
     }
}
Oct 10 2019
parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
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:


 
 
     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.
Oct 10 2019
parent jmh530 <john.michael.hall gmail.com> writes:
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:


 
 
     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
     }
}
Oct 10 2019