Acessing subclass variable from base class object - c++

Question

Let's say i'm having the following classes in c++
class Animal{}
class Dog : public Animal {
int barkingVolume;
}
However, i don't have the header file for Dog class. But i have the object of Dog of type Animal at runtime.
The question is how I can access the variable barkingVolume?
Animal animalButDogObject;//someone has set the value at runtime
I need to access barkingVolume from animalButDogObject.
Actual scenario from COM/Directshow: I'm having IBaseFilter object which is of type IVendorFilter(custom filter from 3rd party vendor which extended IBaseFilter). While debugging using Visual studio i can see the type is IVendorFilter and it has variables which i need to change. However i cannot figure out how to do it. I cannot find anything like a reflection/evalutation in CPP

Answer 1

I'd rather comment than post an answer, but can't due to lack of reputation, so here we go.
This is pretty bad, but if you know the exact layout of the class you must access, you could just forward declare the whole thing and reinterpret_cast the object you need.
// FooBar.cpp or something
namespace FooBar
{
class Foo {};
class Bar : public Foo
{
public:
Bar(int ival, float fval) : ival(ival), fval(fval) {}
int ival = 0;
float fval = 0.0f;
};
}
// OtherFile.cpp
class ForwardDeclaredBar
{
public:
int ival;
float fval;
};
#include <iostream>
int main()
{
FooBar::Foo* foo = new FooBar::Bar(3, 2.7f);
auto bar = reinterpret_cast<ForwardDeclaredBar*>(foo);
std::cout << "ival = " << bar->ival << ", fval = " << bar->fval << std::endl; // shows expected values
return 0;
}
Again, this is pretty bad since any changes to the "real" class will mess up your result (reinterpret_cast will just shove whatever data it finds into the format you specified).
There are probably many other reasons which I've no idea about. I'm also unsure how well (if at all) this plays with more complex objects.

Answer 2

declare method on Animal (base) class and overwrite it on Dog (derived) class.
// Base class
class Animal
{
public:
virtual int getBarkingVolume() = 0;
};
// Derived class
class Dog : public Animal
{
private:
int barkingVolume = 8;
public:
int getBarkingVolume()
{
return barkingVolume;
}
};
In main method has Animal (base) type and each derived class that implements the appropriate method (getBarkingVolume) will be compatible with that type.
int main()
{
Animal* animal = new Dog();
std::cout<<"barking: "<< animal->getBarkingVolume();
}