C ++继承和多态

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

Answer 1

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();