Overload operator < in super class using pure virtual methods - c++

Question

I have a hierarchy of Figure as base class and several sub classes Circle, Square, ecc. I want to overload < operator in Figure to sort Figures by eg. Surface which is implemented in base classes.
This is my code:
//Figure.hpp
class Figure
{
public:
virtual float surface() const = 0;
virtual float perimeter() const = 0;
friend bool operator<(const Figure& lhs, const Figure& rhs);
friend bool operator>=(const Figure& lhs, const Figure& rhs);
}
//Figure.cpp
bool operator<(const Figure& lhs, const Figure& rhs)
{
return lhs.surface() < rhs.surface();
}
bool operator>=(const Figure& lhs, const Figure& rhs)
{
return lhs.surface() >= rhs.surface();
}
//Square.hpp
class Square : public Figure{
public:
Square(float size);
float surface() const{
return mSize * mSize;
};
float perimeter()const{
return mSize * 4;
}
private:
float mSize;
};
Problem is I get an error at runtime:
libc++abi.dylib: Pure virtual function called!
at lhs.surface().
I call the < operator in a LinkedList with Template:
template <typename T>
void List<T>::insertNewNode(T& dataIn)
{
if(isEmpty())
{
insertBegin(dataIn);
}else //otherwise
{
if(dataIn < *startPtr->data)
{
insertBegin(dataIn);
}
else if(dataIn >= *endPtr->data) /
{
insertEnd(dataIn);
}
else
{
//...
}
}
}
//main.cpp
List<Figure> *list = new List<Figure>();
Figure *square = new Square(46);
list->insertNewNode(*square);
EDIT
https://github.com/sanandrea/GenericLinkedList
Is this feasible or I am doing everything wrong?

Answer 1

The problem is both Object Slicing and undefined behavior.
You insert values via
void List<T>::insertNewNode(T& dataIn)
which is OK. Since you pass it by reference the polymorphism will be kept. However, when you later call insertBegin or insertEnd to actually create the node you pass it by T:
void List<T>::insertBegin(T dataIn)
Here dataIn have lost the polymorphism due to Object Slicing. You also have undefined behavior when you store a pointer to the parameter in the newly created node:
ListNode<T> * newPtr = new ListNode<T>(&dataIn); //creates new node
Note that &dataIn is a pointer to the parameter of insertBegin, not a pointer to the dataIn that was passed in insertNewNode(T& dataIn).
You also have Object Slicing in your node: T ListNode<T>::getData() //returns data stored in node.