Polymorphism: via base pointer getting the original object

Polymorphism: via base pointer getting the original object

[U.Mutlu]

Hi,
I have a base class Base_t and multiple derived classes from it (like Dog_t 
and Cat_t, and so on).
I collect the base addresses of these objects in a vector.
By using this vector I then need to access the original objects.
Not just some virtual functions of the objects, but the complete original object.
How best to do this?

The following solution is IMO not that satisfactory.

Is there a better way, maybe in newer C++ standards like c++20? Something like:
auto& X = helper(pBase);    // X will be of type Dog_t or Cat_t etc.

Thx


class Base_t { ...
class Dog_t : public Base_t { ...
class Cat_t : public Base_t { ...
...

enum Type_t { Unknown, Base, Dog, Cat };

struct Identity_t
   {
     Type_t Type;
     union
       {
         Base_t* pBase;
         Dog_t*  pDog;
         Cat_t*  pCat;
       };

     Identity_t() : Type(Unknown) { pBase = nullptr; }
   };

Identity_t Identity(Base_t* pBase)
   {
     Identity_t I;
     if ((I.pDog = dynamic_cast<Dog_t*>(pBase))) { I.Type = Dog; return I; }
     if ((I.pCat = dynamic_cast<Cat_t*>(pBase))) { I.Type = Cat; return I; }
     return I;
   }


// And using it like this:
// p is the address of a derived object, ie. Dog_t* or Cat_t*

auto I = Identity(p);
if (I.Type == Dog)
   { // use I.pDog to access it
     auto& X = *I.pDog;
     X.print();
   }
else if (I.Type == Cat)
   { // use I.pCat to access it
     auto& X = *I.pCat;
     X.print();
   }

...
// end of code

Re: Polymorphism: via base pointer getting the original object

[Jonathan Wakely]

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On Wed, 26 Apr 2023, 18:21 U.Mutlu, <um@mutluit.com> wrote:

> Hi,
> I have a base class Base_t and multiple derived classes from it (like
> Dog_t
> and Cat_t, and so on).
> I collect the base addresses of these objects in a vector.
> By using this vector I then need to access the original objects.
> Not just some virtual functions of the objects, but the complete original
> object.
> How best to do this?
>
> The following solution is IMO not that satisfactory.
>
> Is there a better way, maybe in newer C++ standards like c++20? Something
> like:
> auto& X = helper(pBase);    // X will be of type Dog_t or Cat_t etc.
>

How can helper return different types?

You could make it return std::variant<Cat*, Dog*> and then use std::visit
to access the active pointer stored in it.

So something like your Identity function that returns a variant, then do:

std::visit([](auto* x){ x->print(); }, Identity(p);



> Thx
>
>
> class Base_t { ...
> class Dog_t : public Base_t { ...
> class Cat_t : public Base_t { ...
> ...
>
> enum Type_t { Unknown, Base, Dog, Cat };
>
> struct Identity_t
>    {
>      Type_t Type;
>      union
>        {
>          Base_t* pBase;
>          Dog_t*  pDog;
>          Cat_t*  pCat;
>        };
>
>      Identity_t() : Type(Unknown) { pBase = nullptr; }
>    };
>
> Identity_t Identity(Base_t* pBase)
>    {
>      Identity_t I;
>      if ((I.pDog = dynamic_cast<Dog_t*>(pBase))) { I.Type = Dog; return I;
> }
>      if ((I.pCat = dynamic_cast<Cat_t*>(pBase))) { I.Type = Cat; return I;
> }
>      return I;
>    }
>
>
> // And using it like this:
> // p is the address of a derived object, ie. Dog_t* or Cat_t*
>
> auto I = Identity(p);
> if (I.Type == Dog)
>    { // use I.pDog to access it
>      auto& X = *I.pDog;
>      X.print();
>    }
> else if (I.Type == Cat)
>    { // use I.pCat to access it
>      auto& X = *I.pCat;
>      X.print();
>    }
>
> ...
> // end of code
>
>

Re: Polymorphism: via base pointer getting the original object

[Andrew Haley]

On 4/26/23 18:20, U.Mutlu wrote:
> How best to do this?

In a word or two, don't. This is basic OOP: all accesses to an object should
be via its (possibly virtual) member functions. There's nothing to stop you
from defining

   virtual Dog_t Base_t::as_dog() { return nullptr; }

in the base class, and overriding it as

   virtual Dog_t Dog_t::as_dog() { return this; }

in the derived class. If you really need to...

Can I remind you that this list is for specific GCC help, not C++ language issues?

Thanks.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671

Re: Polymorphism: via base pointer getting the original object

[U.Mutlu]

Andrew Haley via Gcc-help wrote on 04/26/23 19:35:
> On 4/26/23 18:20, U.Mutlu wrote:
>> How best to do this?
>
> In a word or two, don't. This is basic OOP: all accesses to an object should
> be via its (possibly virtual) member functions. There's nothing to stop you
> from defining
>
>    virtual Dog_t Base_t::as_dog() { return nullptr; }
>
> in the base class, and overriding it as
>
>    virtual Dog_t Dog_t::as_dog() { return this; }
>
> in the derived class. If you really need to...

Thx, just tried this out, but it's not of much help as one has to know the 
type in advance, ie. at compile time.

> Can I remind you that this list is for specific GCC help, not C++ language
> issues?
>
> Thanks.

Oh, sorry, I forgot. Can you recommend a mailing list where such issues are 
discussed? Thx.