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digitalmars.D.learn - Casts that makes no sense (!)

reply Ary Borenszweig <ary esperanto.org.ar> writes:
The following compiles:

import std.stdio;

interface I {
}

class A : I {
}

class B {
}

int main() {
   I i = new A();
   A a = cast(A) i;
   B b = cast(B) i; // shouldn't compile
   B c = cast(B) a; // shouldn't compile

   writeln(a);
   writeln(b);
   writeln(c);

   return 0;
}

But two lines there doesn't make sense:

B b = cast(B) i;

An instance of I can never be a B, so why the cast is allowed?

B c = cast(B) a;

An instance of A can never be an A, so why the cast is allowed?

I think these casts should result in an error. This can prevent some bugs.

Java and C# work like that. You can't cast an object of instance of type 
A to type B if both types are classes and B isn't a supertype or subtype 
of A.
Mar 08 2010
next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 08 Mar 2010 09:38:08 -0500, Ary Borenszweig <ary esperanto.org.ar>  
wrote:


 The following compiles:

 import std.stdio;

 interface I {
 }

 class A : I {
 }

 class B {
 }

 int main() {
    I i = new A();
    A a = cast(A) i;
    B b = cast(B) i; // shouldn't compile
    B c = cast(B) a; // shouldn't compile

    writeln(a);
    writeln(b);
    writeln(c);

    return 0;
 }

 But two lines there doesn't make sense:

 B b = cast(B) i;

 An instance of I can never be a B, so why the cast is allowed?



class C: B, I {}

I i = new C;
B b = cast(B)i; // should work


 B c = cast(B) a;

 An instance of A can never be an A, so why the cast is allowed?



Aside from the typo, I agree with you there, this should never be  
possible, because a derived class can not inherit both B and A.


 I think these casts should result in an error. This can prevent some  
 bugs.

 Java and C# work like that. You can't cast an object of instance of type  
 A to type B if both types are classes and B isn't a supertype or subtype  
 of A.



This rule makes sense.  On the other hand, I wonder how the new ability to  
cast to any type works in terms of classes.  Can you define cast!(A)() in  
B for instance?  I suppose you could make a caveat that either the class  
must inherit A or define an opCast for A.

-Steve
Mar 08 2010
parent reply Ary Borenszweig <ary esperanto.org.ar> writes:
Steven Schveighoffer wrote:

 On Mon, 08 Mar 2010 09:38:08 -0500, Ary Borenszweig 
 <ary esperanto.org.ar> wrote:
 

 The following compiles:

 import std.stdio;

 interface I {
 }

 class A : I {
 }

 class B {
 }

 int main() {
    I i = new A();
    A a = cast(A) i;
    B b = cast(B) i; // shouldn't compile
    B c = cast(B) a; // shouldn't compile

    writeln(a);
    writeln(b);
    writeln(c);

    return 0;
 }

 But two lines there doesn't make sense:

 B b = cast(B) i;

 An instance of I can never be a B, so why the cast is allowed?



 class C: B, I {}
 
 I i = new C;
 B b = cast(B)i; // should work



Ah, good one! I just tested it in Java and it also compiles, so I was wrong.


 

 B c = cast(B) a;

 An instance of A can never be an A, so why the cast is allowed?



 Aside from the typo, I agree with you there, this should never be 
 possible, because a derived class can not inherit both B and A.



Where's the typo?


 

 I think these casts should result in an error. This can prevent some 
 bugs.

 Java and C# work like that. You can't cast an object of instance of 
 type A to type B if both types are classes and B isn't a supertype or 
 subtype of A.



 This rule makes sense.  On the other hand, I wonder how the new ability 
 to cast to any type works in terms of classes.  Can you define 
 cast!(A)() in B for instance?  I suppose you could make a caveat that 
 either the class must inherit A or define an opCast for A.



Yes, you'd always have to also check if an opCast is present.


 
 -Steve
Mar 08 2010
parent Ary Borenszweig <ary esperanto.org.ar> writes:
Steven Schveighoffer wrote:

 On Mon, 08 Mar 2010 10:12:53 -0500, Ary Borenszweig 
 <ary esperanto.org.ar> wrote:
 

 Steven Schveighoffer wrote:

 On Mon, 08 Mar 2010 09:38:08 -0500, Ary Borenszweig 
 <ary esperanto.org.ar> wrote:







 An instance of A can never be an A, so why the cast is allowed?



 possible, because a derived class can not inherit both B and A.



 Where's the typo?



 "An instance of A can never be an A"
 
 Obviously you meant an instance of B :)
 
 -Steve



Aaaah... I was looking for the typo in the code
Mar 08 2010
prev sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Mon, 08 Mar 2010 10:12:53 -0500, Ary Borenszweig <ary esperanto.org.ar>  
wrote:


 Steven Schveighoffer wrote:

 On Mon, 08 Mar 2010 09:38:08 -0500, Ary Borenszweig  
 <ary esperanto.org.ar> wrote:







 An instance of A can never be an A, so why the cast is allowed?



 possible, because a derived class can not inherit both B and A.



 Where's the typo?



"An instance of A can never be an A"

Obviously you meant an instance of B :)

-Steve
Mar 08 2010