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digitalmars.D.learn - How do you return a subclass instance from a base class method?

reply Daniel Donnelly, Jr. <enjoysmath gmail.com> writes:
I have SubclassOf derived from PosetRelation.  For any poset 
relation, the transitivity law applies, however, I'd like to 
return the correct type:

```
    PosetRelation transitivity(PosetRelation R, PosetRelation S)
    {
       if (R.op == S.op)
       {
          if (R.right is S.left)
             return new SubclassOf(R.left, S.right);
       }

       return null;
    }

```

How does one accomplish this in D?  Because PosetRelation doesn't 
know about SubclassOf, in general.
Nov 16 2022
next sibling parent reply MorteFeuille123 <MorteFeuille123 mrt.rt> writes:
On Thursday, 17 November 2022 at 04:25:13 UTC, Daniel Donnelly, 
Jr. wrote:

 I have SubclassOf derived from PosetRelation.  For any poset 
 relation, the transitivity law applies, however, I'd like to 
 return the correct type:


 How does one accomplish this in D?  Because PosetRelation 
 doesn't know about SubclassOf, in general.


You can use TypeInfoClass:

```d
class Base
{
}

class Derived : Base
{
}

Object newDerivedFromTi(Base b)
{
     return typeid(b).create();
}

void main(string[] args)
{
     Base b = new Base;
     Base d = new Derived;
     assert(cast(Derived)newDerivedFromTi(d));
}
```

But that only works with default constructors, i.e no parameters.
Another way is to define a virtual function in the Base:

```d
class Base
{
     Object createMostDerived(Base b1, Base b2)
     {
         return new typeof(this);
     }
}

class Derived : Base
{
     override Object createMostDerived(Base b1, Base b2)
     {
         return new typeof(this);
     }
}

Object newDerivedFromTi(Base b)
{
     return b.createMostDerived(b, b);
}

void main(string[] args)
{
     Base b = new Base;
     Base d = new Derived;
     assert(cast(Derived)newDerivedFromTi(d));
}
```

assuming the PosetRelation (here called Base) actually cary the 
SubclassOf type (here called Derived).
Nov 16 2022
parent reply Daniel Donnelly, Jr. <enjoysmath gmail.com> writes:
On Thursday, 17 November 2022 at 05:21:05 UTC, MorteFeuille123 
wrote:

 On Thursday, 17 November 2022 at 04:25:13 UTC, Daniel Donnelly, 
 Jr. wrote:

 [...]


 You can use TypeInfoClass:

 [...]


I don't get it - you never made use of b1 or b2...
Nov 16 2022
parent MorteFeuille123 <MorteFeuille123 mrt.rt> writes:
On Thursday, 17 November 2022 at 06:48:13 UTC, Daniel Donnelly, 
Jr. wrote:

 On Thursday, 17 November 2022 at 05:21:05 UTC, MorteFeuille123 
 wrote:

 On Thursday, 17 November 2022 at 04:25:13 UTC, Daniel 
 Donnelly, Jr. wrote:

 [...]


 You can use TypeInfoClass:

 [...]


 I don't get it - you never made use of b1 or b2...


yeah this was oversimplified on purpose, I did not realize that 
this coulmd be confusing. I dont know what you do with your 
constructor parameter either, would I say, as a second 
explanation.
Nov 17 2022
prev sibling next sibling parent Daniel Donnelly, Jr. <enjoysmath gmail.com> writes:
```
PosetRelation transitivity(PosetRelation R, PosetRelation S)
{
    // These if conditions are typically ordered from easiest to
    // most involved-to-check.
    if (R.op == S.op &&
        is(typeof(R) == typeof(S)) &&
        R.right == S.left)
    {
       return new typeof(R)(
          R.left, S.right,
          by("transitivity of " ~ R.op));     // Proof by this 
axiom
    }

    return null;
}
```

Not sure if that would work yet.  Have to test it.
Nov 16 2022
prev sibling next sibling parent reply zjh <fqbqrr 163.com> writes:
On Thursday, 17 November 2022 at 04:25:13 UTC, Daniel Donnelly, 
Jr. wrote:

 ...


`crtp`, will it work?
Nov 16 2022
parent Daniel Donnelly, Jr. <enjoysmath gmail.com> writes:
On Thursday, 17 November 2022 at 05:34:49 UTC, zjh wrote:

 On Thursday, 17 November 2022 at 04:25:13 UTC, Daniel Donnelly, 
 Jr. wrote:

 ...


 `crtp`, will it work?


Can't use CRTP, because once you choose a derived class to pass 
into the template system, how do you pass in subclasses of that 
class and so on...
Nov 16 2022
prev sibling parent Steven Schveighoffer <schveiguy gmail.com> writes:
On 11/16/22 11:25 PM, Daniel Donnelly wrote:

 I have SubclassOf derived from PosetRelation.  For any poset relation, 
 the transitivity law applies, however, I'd like to return the correct type:
 
 ```
     PosetRelation transitivity(PosetRelation R, PosetRelation S)
     {
        if (R.op == S.op)
        {
           if (R.right is S.left)
              return new SubclassOf(R.left, S.right);
        }
 
        return null;
     }
 
 ```
 
 How does one accomplish this in D?  Because PosetRelation doesn't know 
 about SubclassOf, in general.


I'd use a template:

```d
T transitivity(T : PosetRelation)(T R, T S)
{
       if (R.op == S.op)
       {
          if (R.right is S.left)
             return new T(R.left, S.right);
       }

       return null;
}
```

-Steve
Nov 17 2022