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digitalmars.D.learn - Implicit conversion from class in parent class fails?

reply "Namespace" <rswhite4 googlemail.com> writes:
Why work this:

[code]
class Foo { }
class Bar : Foo { }
class Quatz : Bar { }

void foo(Foo f) {

}

void main() {
	Foo f = new Foo();
	Foo f2;
	
	foo(f);
	foo(f2);
	
	Bar b = new Bar();
	Bar b2;
	
	foo(b);
	foo(b2);
	
	Quatz q = new Quatz();
	Quatz q2;
	
	foo(q);
	foo(q2);
}
[/code]

but not:

[code]
import std.stdio;

class Foo { }
class Bar : Foo { }
class Quatz : Bar { }

void bar(Foo[] fs) {

}

void main() {
	Foo[] fs = [new Foo()];
	Foo[] fs2;
	
	bar(fs);
	bar(fs2);
	
	Bar[] bs = [new Bar()];
	Bar[] bs2;
	
	bar(bs);
	bar(bs2);
	
	Quatz[] qs = [new Quatz()];
	Quatz[] qs2;
	
	bar(qs);
	bar(qs2);
}
[/code]

I think that should work also.
Jun 16 2012
parent reply Andrew Wiley <wiley.andrew.j gmail.com> writes:
On Sat, Jun 16, 2012 at 11:52 AM, Namespace <rswhite4 googlemail.com> wrote=
:

 Why work this:

 [code]
 class Foo { }
 class Bar : Foo { }
 class Quatz : Bar { }

 void foo(Foo f) {

 }

 void main() {
 =A0 =A0 =A0 =A0Foo f =3D new Foo();
 =A0 =A0 =A0 =A0Foo f2;

 =A0 =A0 =A0 =A0foo(f);
 =A0 =A0 =A0 =A0foo(f2);

 =A0 =A0 =A0 =A0Bar b =3D new Bar();
 =A0 =A0 =A0 =A0Bar b2;

 =A0 =A0 =A0 =A0foo(b);
 =A0 =A0 =A0 =A0foo(b2);

 =A0 =A0 =A0 =A0Quatz q =3D new Quatz();
 =A0 =A0 =A0 =A0Quatz q2;

 =A0 =A0 =A0 =A0foo(q);
 =A0 =A0 =A0 =A0foo(q2);
 }
 [/code]

 but not:

 [code]
 import std.stdio;

 class Foo { }
 class Bar : Foo { }
 class Quatz : Bar { }

 void bar(Foo[] fs) {

 }

 void main() {
 =A0 =A0 =A0 =A0Foo[] fs =3D [new Foo()];
 =A0 =A0 =A0 =A0Foo[] fs2;

 =A0 =A0 =A0 =A0bar(fs);
 =A0 =A0 =A0 =A0bar(fs2);

 =A0 =A0 =A0 =A0Bar[] bs =3D [new Bar()];
 =A0 =A0 =A0 =A0Bar[] bs2;

 =A0 =A0 =A0 =A0bar(bs);
 =A0 =A0 =A0 =A0bar(bs2);

 =A0 =A0 =A0 =A0Quatz[] qs =3D [new Quatz()];
 =A0 =A0 =A0 =A0Quatz[] qs2;

 =A0 =A0 =A0 =A0bar(qs);
 =A0 =A0 =A0 =A0bar(qs2);
 }
 [/code]

 I think that should work also.


The problem is that this would also work:
 [code]
 import std.stdio;

 class Foo { }
 class Bar : Foo { }
 class Quatz : Bar { }

 void bar(Foo[] fs) {
    fs[0] =3D new Foo(); // <-- OH NOES
 }

void main() {
        Foo[] fs =3D [new Foo()];
        Foo[] fs2;

        bar(fs);
        bar(fs2);

        Bar[] bs =3D [new Bar()];
        Bar[] bs2;

        bar(bs);
        bar(bs2);

        Quatz[] qs =3D [new Quatz()];
        Quatz[] qs2;

        bar(qs);
        bar(qs2);
}
 [/code]
Jun 16 2012
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 06/16/2012 11:55 AM, Andrew Wiley wrote:

 On Sat, Jun 16, 2012 at 11:52 AM, Namespace<rswhite4 googlemail.com>  wrote:

 Why work this:

 [code]
 class Foo { }
 class Bar : Foo { }
 class Quatz : Bar { }

 void foo(Foo f) {

 }

 void main() {
         Foo f = new Foo();
         Foo f2;

         foo(f);
         foo(f2);

         Bar b = new Bar();
         Bar b2;

         foo(b);
         foo(b2);

         Quatz q = new Quatz();
         Quatz q2;

         foo(q);
         foo(q2);
 }
 [/code]

 but not:

 [code]
 import std.stdio;

 class Foo { }
 class Bar : Foo { }
 class Quatz : Bar { }

 void bar(Foo[] fs) {

 }

 void main() {
         Foo[] fs = [new Foo()];
         Foo[] fs2;

         bar(fs);
         bar(fs2);

         Bar[] bs = [new Bar()];
         Bar[] bs2;

         bar(bs);
         bar(bs2);

         Quatz[] qs = [new Quatz()];
         Quatz[] qs2;

         bar(qs);
         bar(qs2);
 }
 [/code]

 I think that should work also.


 The problem is that this would also work:
   [code]
   import std.stdio;

   class Foo { }
   class Bar : Foo { }
   class Quatz : Bar { }

   void bar(Foo[] fs) {
      fs[0] = new Foo(); //<-- OH NOES
   }

 void main() {
          Foo[] fs = [new Foo()];
          Foo[] fs2;

          bar(fs);
          bar(fs2);

          Bar[] bs = [new Bar()];
          Bar[] bs2;

          bar(bs);
          bar(bs2);

          Quatz[] qs = [new Quatz()];
          Quatz[] qs2;

          bar(qs);
          bar(qs2);
 }
   [/code]


The following is a shorter example, which happens to also produce a more 
educating error message:

import std.stdio;

class Foo { }
class Bar : Foo { }
class Quatz : Bar { }

void zar(ref Foo fs) {// <-- ref
     fs = new Quatz(); // <-- OH NOES
}

void main()
{
     auto b = new Bar();
     zar(b);    // <-- Error: cast(Foo)b is not an lvalue
}

After all of the above, a const reference works as Namespace expects:

import std.stdio;

class Foo { }
class Bar : Foo { }
class Quatz : Bar { }

void zar(const Foo[] fs) {    // <-- const makes it work
     // ...
}

void main()
{
     zar([ new Bar() ]);
}

Ali

-- 
D Programming Language Tutorial: http://ddili.org/ders/d.en/index.html
Jun 16 2012