C++ - Call Child Method Overwrites in List of Parents [duplicate] - c++

Question

This question already has answers here:
What is object slicing?
(18 answers)
Closed 5 years ago.
Here's a sample program for what I'm talking about:
#include <iostream>
#include <list>
using namespace std;
class Thing {
public:
virtual void jump() { cout << "Called from Thing class" << endl; }
};
class Car: public Thing {
public:
virtual void jump() { cout << "Vroom vroom; imma car biotch" << endl; }
};
int main(int argc, char* argv[]) {
Car myCar;
list<Thing> myList;
myList.push_back(myCar);
std::list<Thing>::iterator iterator;
for (iterator = myList.begin(); iterator != myList.end(); ++iterator) {
iterator->jump();
}
return 0;
}
Output:
Called from Thing class
What I want to do is make a list of "Things". I want to be able to add instances of the child class from the Thing class; then I want to be able to call their overwritten function from an iterator.
The only issue is, even while using the "virtual" keyword, the iterator uses the Thing::jump function as opposed to the Car::jump function.
What can I do to make it not only work with Car, but all potential child classes from Thing, Car, etc. that overwrite the "jump" function?

Answer 1

In C++ values are what they say they are. You have a list of Thing not a list of Thing or derived types.
When you insert something into the list, a copy of it is created. In the case of inserting a derived type instance, this copies the Thing portion of the object, something known as object slicing (the Thing portion is sliced off).
There are a number of ways to store polymorphic data, but an instance of the base type is not one of them.
You could store std::unique_ptr<Base> in your list, for example. If you do so, ensure your base type has a virtual destructor to avoid undefined behaviour when theynare deleted by the default deleter.
#include <iostream>
#include <list>
#include <memory>
class Thing {
public:
virtual void jump() { std::cout << "Called from Thing class\n"; }
~Thing(){}
};
class Car: public Thing {
public:
virtual void jump() { std::cout << "Vroom vroom; I am a car\n"; }
};
then in main:
std::list<std::unique_ptr<Thing>> myList;
myList.push_back(std::make_unique<Car>());
for(auto&& ptr:myList){
ptr->jump();
}