std::vector of references to derived class objects

[rsennat]

Hi All,
Here is the basic example of the using polymorphism. And then my question is below.

 typedef std::vector<Vehicle*>  VehicleList;
 
 void myCode(VehicleList& v)
 {
   for (VehicleList::iterator p = v.begin(); p != v.end(); ++p) {
     Vehicle& v = **p;  // just for shorthand
 
     // generic code that works for any vehicle...
     ...
 
     // perform the "foo-bar" operation.
     v.fooBar();
 
     // generic code that works for any vehicle...
     ...
   }
 }
===================
 class Car : public Vehicle {
 public:
   virtual void fooBar();
 };
 
 void Car::fooBar()
 {
   // car-specific code that does "foo-bar" on 'this'
   ...  ← this is the code that was in {...} of if (v is a Car)
 }
 
 class Truck : public Vehicle {
 public:
   virtual void fooBar();
 };
 
 void Truck::fooBar()
 {
   // truck-specific code that does "foo-bar" on 'this'
   ...  ← this is the code that was in {...} of if (v is a Truck)
 }

So, in this example, instead of std::vector<Veh icle*>, can I use std::vector<Veh icle> and still achieve the polymorphism, like calling the function of the derived class object reference it contains??

Can you give an example of the same.

Thanks
rsennat

[arnaudk]

C++ does not fully support the Liskov substitution principle; you can substitute the address of objects of derived classes in place of the base class pointer, but not derived objects themselves in place of a base class, so the answer to your question is: No.

Some minor comments:
line 6: Why not use a different local variable name to avoid confusion with the outer variable v?
line 12: I would avoid the shorthand altogether and just use (*p)->fooBar();

[weaknessforcats]

You will not be able to use anyt Truck methods if you have a vector<Vehicle> .

That's becuse the Truck-part of the object was sliced off when the the object was put in the vector.

Polymorphism in C++ only works if you have a pointer or reference to the object.

You should have a vector<Vehicle* >, or better, a vector of handles to Vehicle.

Read this: http://bytes.com/forum/thread651599.html.

[rsennat]

You will not be able to use anyt Truck methods if you have a vector<Vehicle> .

That's becuse the Truck-part of the object was sliced off when the the object was put in the vector.

Polymorphism in C++ only works if you have a pointer or reference to the object.

You should have a vector<Vehicle* >, or better, a vector of handles to Vehicle.

Read this: http://bytes.com/forum/thread651599.html.

Thanks for the information.
And I have one more related query, if polymorphism works with reference to an object, then can we use like this, vector<Vehicle& >?

[weaknessforcats]

Yes, you can.

The essential thing here is that a Vehicle* or a Vehicle& do not require a copy of the object. Once that Truck is copied as a Vehicle, the Truck portion is sawed off. Technically, this is called slicing and it's usually a bad sign.

Once you start using Vehicle* or Vehicle& in your vector, be sure those objects are always on the heap. That way you have control over deletion and not the compiler.

It won;t be long after this that you will start using Handles to a Vehicle.

And shortly after that, you will add a method to a Truck and you can't call it using a Vehicle* or Vehicle&. Therefore, you should design for a Visitor right up front. Read this: http://bytes.com/forum/thread674645.html.

[rsennat]

Yes, you can.

The essential thing here is that a Vehicle* or a Vehicle& do not require a copy of the object. Once that Truck is copied as a Vehicle, the Truck portion is sawed off. Technically, this is called slicing and it's usually a bad sign.

Once you start using Vehicle* or Vehicle& in your vector, be sure those objects are always on the heap. That way you have control over deletion and not the compiler.

It won;t be long after this that you will start using Handles to a Vehicle.

And shortly after that, you will add a method to a Truck and you can't call it using a Vehicle* or Vehicle&. Therefore, you should design for a Visitor right up front. Read this: http://bytes.com/forum/thread674645.html.

Oh ok. Thanks a lot for all the information.