Basic Qn: How to identify sub-class object

[abhishekm71]

I have one base Class A and 4 sub Classes B, C, D, E that inherit Class A.

std::vector<A* > myVec;
A* ptr = new B();  // new objects of subclasses created in RANDOM ORDER.
myVec.pushback( ptr );
ptr = new C();
myVec.pushback( ptr );
ptr = new D();
myVec.pushback( ptr );
ptr = new E();
myVec.pushback( ptr );

Assuming we do not know the order in which the objects were inserted in the vector, how can we determine the class of the object say myVec[0].

[Cubbi]

If the types of your objects are polymorphic, you can print their type names:

#include <iostream>
#include <vector>
#include <algorithm>
#include <typeinfo>

struct A { virtual ~A() {} };
struct B : A {};
struct C : A {};
struct D : A {};
struct E : A {};

int main()
{
    std::vector<A*> myVec{new B, new C, new D, new E};
    std::random_shuffle(myVec.begin(), myVec.end());
    for(std::size_t n = 0; n < myVec.size(); ++n)
    {
        std::cout << "myVec[" << n << "] points to "
                  << typeid(*myVec[n]).name() << '\n';
        delete myVec[n];
    }
}

$ ./tests  | c++filt -t
myVec[0] points to B
myVec[1] points to E
myVec[2] points to C
myVec[3] points to D

But why would you want that kind of information?

[abhishekm71]

Ok thanks cubbi.

In my application, the user adds different objects to the vector at his will. Each object has very different properties and methods. I want to perform some operations on each element depending on the type of that object.

[Cubbi]

Each object has very different properties and methods.

Doesn't sound like they are suitable candidates for inheritance, then. If you really need a single vector holding them, and their order is decided at runtime, then a vector of variant types might be more sensible. Or you could describe the requirements in more detail (it's hardly something like "i want you to write a program with std::vector, in which I will put a house, a multiplication, a string, and a type trait")

[abhishekm71]

Sorry for being so vague -).

The proposed application is a simplified electrical control drawing cum simulation software.
Class A is control element class that itself inherits from QGraphicsItem (I'm using Qt)
Class B, C, D etc are different control elements like push button, relay coil, relay NO contact, NC contact etc. Now each of these elements has a distinct set of properties and some inter relations like the coil, NO, and NC are parts of a relay. These are totally different from push buttons.

The idea is that the user be able to construct / modify the control drawing in an easy drag and drop manner. Also if possible, give him an opportunity to simulate it.
Should I divide them further into similar groups of classes?

[Cubbi]

I would look into loosening the coupling - when I think of a relay coil, it don't see it as something that has a bounding rectangle and a shape, knows how to collide with things or is able to paint itself. It's a physical object with properties useful for simulation, not a graphics item with properties useful for rendering a scene.
Someone who is familiar with Qt may have better ideas, but from a cursory glance, looks like QT lets you store a custom QVariant in a QGraphicsItem, why don't you use those?

[abhishekm71]

I guess you are right. Having a bounding rectangle for my elements was looking wierd to me as well. Will research on qvariant as you suggested.

Thanks for your help!

[abhishekm71]

And sorry I don't understand, what does 'loosening the coupling' mean?

EDIT: ok, I got what you meant. THANKS! Following link explains the phrase:
http://stackoverflow.com/questions/226977/what-is-loose-coupling-please-provide-examples