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digitalmars.D.learn - how can i use the derive-hierarchy at runtime for typechecking?

reply dennis luehring <dl.soluz gmx.net> writes:
is there a way to test at runtime if the type of an object is derived 
from another object type? maybe with the help of compiletime-reflection 
and additional information in the class?

class A{}
class B: A{}
class C: B{}

i need something like

B.type.is_derived_from( A.type ) ==> true
A.type.is_derived_from( C.type ) ==> false

B xx = new B;
B yy = new B;

xx.type == yy.type ==> true

is there an elegant way?

why do i need such stuff?
i use the normal D typechecking for the static part of my software
but i've got an GUI system with the ability to "construct" types
at runtime based on my static types (and i need a typesystem based on 
the class hierachy for it)

thx dennis
Sep 20 2007
parent reply Downs <default_357-line yahoo.de> writes:
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Just cast it.
If you have class A { } and class B : A { }
and A foo=new B;
then you can do cast(B) foo and it will be non-null.
On the other hand, if you do A bar=new A;
and you do cast(B) bar, it will be null.

Here's a little utility function for this
void ifIs(S, T)(S obj, void delegate(T) dg) {
  auto casted=cast(T) obj;
  if (casted) dg(casted);
}

Use it like thus

ifIs(foo, (B whee) { /* do stuff with whee */ });

Hope that answers your question.
 --downs/FeepingCreature
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Sep 20 2007
parent reply Chris Nicholson-Sauls <ibisbasenji gmail.com> writes:
Downs wrote:

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 Just cast it.
 If you have class A { } and class B : A { }
 and A foo=new B;
 then you can do cast(B) foo and it will be non-null.
 On the other hand, if you do A bar=new A;
 and you do cast(B) bar, it will be null.
 
 Here's a little utility function for this
 void ifIs(S, T)(S obj, void delegate(T) dg) {
   auto casted=cast(T) obj;
   if (casted) dg(casted);
 }
 
 Use it like thus
 
 ifIs(foo, (B whee) { /* do stuff with whee */ });
 



And thanks to an oft-forgotten feature, you could shrink that down a little
further to 
just a one-liner:

void ifIs (S, T) (S obj, void delegate(T) dg) {
   if (auto casted = cast(T) obj) dg(casted);
}

http://digitalmars.com/d/1.0/statement.html#IfStatement

-- Chris Nicholson-Sauls
Sep 20 2007
parent reply "Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:
"Chris Nicholson-Sauls" <ibisbasenji gmail.com> wrote in message 
news:fcupko$kh7$2 digitalmars.com...


 void ifIs (S, T) (S obj, void delegate(T) dg) {
   if (auto casted = cast(T) obj) dg(casted);
 }



Or you could skip Downs' confusing way of writing code entirely ( ;) ) and 
just use:

if(auto b = cast(B)foo)
{
    // do stuff with b
}
Sep 20 2007
next sibling parent reply Chris Nicholson-Sauls <ibisbasenji gmail.com> writes:
Jarrett Billingsley wrote:

 "Chris Nicholson-Sauls" <ibisbasenji gmail.com> wrote in message 
 news:fcupko$kh7$2 digitalmars.com...
 

 void ifIs (S, T) (S obj, void delegate(T) dg) {
   if (auto casted = cast(T) obj) dg(casted);
 }



 Or you could skip Downs' confusing way of writing code entirely ( ;) ) and 
 just use:
 
 if(auto b = cast(B)foo)
 {
     // do stuff with b
 } 
 
 



Or we could add a new '?.' form for member access that becomes a nop on null
references 
and just use:
(cast(B) foo)?.doStuff();

Er.......... okay, that's just insane.

-- Chris Nicholson-Sauls
Sep 20 2007
parent "Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:
"Chris Nicholson-Sauls" <ibisbasenji gmail.com> wrote in message 
news:fcv1d1$vp6$1 digitalmars.com...

 Or we could add a new '?.' form for member access that becomes a nop on 
 null references and just use:
 (cast(B) foo)?.doStuff();

 Er.......... okay, that's just insane.



What do you think this is, Io?
Sep 20 2007
prev sibling next sibling parent reply dennis luehring <dl.soluz gmx.net> writes:
Jarrett Billingsley schrieb:

 "Chris Nicholson-Sauls" <ibisbasenji gmail.com> wrote in message 
 news:fcupko$kh7$2 digitalmars.com...
 

 void ifIs (S, T) (S obj, void delegate(T) dg) {
   if (auto casted = cast(T) obj) dg(casted);
 }



 Or you could skip Downs' confusing way of writing code entirely ( ;) ) and 
 just use:
 
 if(auto b = cast(B)foo)
 {
     // do stuff with b
 } 
 
 



and how can i insert this test in my base-class to hide the type stuff

class base
{
   int base_bla;

   bool is_derived_from( base other )
   {
     auto check = cast(typeof(this))other;
     return !( check is null );

     // why can't i compile this
     // return ( auto check = cast(typeof(this))other );
   }
}

class test: base
{
   int test_bla;
}

class blub: test
{
   int blub_bla;
}

the is_derived_from test is always true
maybe because of the base type parameter in is_derived_from

and the other thing i need to know(show) the derive-hierachy

"the hole story"

in my c++ project i use objects which represents the type of an object
for example

class car // real implementation
{
}

class car_class // object-type of class car
{
   car create_instance(); ...
   bool is_derived_from
   base_class[] derived_from_me();
   base_class[] base_class_of();
}

and many [whatever]_class objects in my known-types-pool

for example:
i can ask which "types" i know
i can create real types of it
i can ask for the derives/bases of an "type"
i can test if an "type" is hierachical "compatible"=derived from with 
another "type"

and just need to know my base-class interface for doing this
...

all this is done by using my own decription.language (with derive 
ability) and an c++ code generator...

i hope to find a better (more compiletime based) way to get the same 
features - i want to get rid of this generator stuff

ciao dennis
Sep 20 2007
parent Regan Heath <regan netmail.co.nz> writes:
dennis luehring wrote:

 Jarrett Billingsley schrieb:

 "Chris Nicholson-Sauls" <ibisbasenji gmail.com> wrote in message 
 news:fcupko$kh7$2 digitalmars.com...


 void ifIs (S, T) (S obj, void delegate(T) dg) {
   if (auto casted = cast(T) obj) dg(casted);
 }



 Or you could skip Downs' confusing way of writing code entirely ( ;) ) 
 and just use:

 if(auto b = cast(B)foo)
 {
     // do stuff with b
 }



 and how can i insert this test in my base-class to hide the type stuff
 
 class base
 {
   int base_bla;
 
   bool is_derived_from( base other )
   {
     auto check = cast(typeof(this))other;
     return !( check is null );
 
     // why can't i compile this



Because using 'auto' like this only works in an "if" statement. eg.

if ( auto check = cast(typeof(this))other ) return true;
return false;


     // return ( auto check = cast(typeof(this))other );
   }
 }
 
 class test: base
 {
   int test_bla;
 }
 
 class blub: test
 {
   int blub_bla;
 }
 
 the is_derived_from test is always true
 maybe because of the base type parameter in is_derived_from



writefln(typeof(this).stringof) shows "base" even when called on an 
object of type "test" or "blub".  So, even if you say:

test t = new test;
blub b = new blub;

if (t.is_derived_from(b)) {} //is 'test' derived from 'blub'

you're really just asking if t is derived from "base", which it is (has 
to be to have the is_derived_from method - in this case)

Regan
Sep 21 2007
prev sibling parent Downs <default_357-line yahoo.de> writes:
Jarrett Billingsley wrote:

 "Chris Nicholson-Sauls" <ibisbasenji gmail.com> wrote in message 
 news:fcupko$kh7$2 digitalmars.com...
 

 void ifIs (S, T) (S obj, void delegate(T) dg) {
   if (auto casted = cast(T) obj) dg(casted);
 }



 Or you could skip Downs' confusing way of writing code entirely ( ;) ) and 
 just use:
 
 if(auto b = cast(B)foo)
 {
     // do stuff with b
 } 
 
 



:surprised:
 --downs, learning new stuff every day! :)
Sep 23 2007