C++ inheritance and polymorphism

Question

Being a Java Programmer and a C++ noob, I'm having a tough time dealing with inheritance in C++. Right now, I have this:

class Parent {
public:
    Parent() {}
    virtual std::string overrideThis() { }
};

class Child : public Parent {
public:
    std::string attribute;
    Child(const std::string& attribute) : attribute(attribute) { }
    std::string overrideThis(){
    std::cout << "I'm in the child" << std::endl;
    return attribute.substr(1, attribute.size()-2);
    }
};

And this snippet somewhere else:

Child *child = new Child(value);
Child childObject = *(child);
std::cout << "I'm trying this: " << childObject.overrideThis() << endl;

The code above works as expected a the message is printed on screen. But if instead of that I try this:

Child *child = new Child(value);
Parent childObject = *(child);
std::cout << "I'm trying this: " << childObject.overrideThis() << endl;

I have a funny runtime error with lots of funny characters on my Screen. What's the proper way of using polymorphism with pointers? What I'm trying to do is invoke overrideThis() on a Child instance

Luchian Grigore · Accepted Answer · 2013-07-06 06:36:07Z

2

The program has undefined behavior because the function that is being called -Parent::overrideThis doesn't return, although it should.

The function in the Parent class is called because Parent childObject = *(child); slices the object the you attempt to copy - the new object, childObject is of type Parent, not Child.

For polymorphism to work, you need to use either pointers or references:

Parent* childObject1 = child;
Parent& childObject2 = *child;
childObject1->overrideThis();
childObject2.overrideThis();

edited Jul 6, 2013 at 6:36

answered Jul 6, 2013 at 5:58

Luchian Grigore

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4 Comments

Carlos Gavidia-Calderon Over a year ago

And how can I invoke overrideThis() on a reference? (Sorry if is a naive question xD)

Carlos Gavidia-Calderon Over a year ago

In which way I can invoke overrideThis()? Neither allow me to do childObject.overrideThis()

Ignacio Vazquez-Abrams Over a year ago

What about dynamic_cast?

Luchian Grigore Over a year ago

@IgnacioVazquez-Abrams what about it?

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