c++ polymorphism and other function question

Question

i have got this code:

class father{
public:
    virtual void f() { cout<<1;}
};

class son:public father{
public:
    void f() {cout<<2;}
};

void test (father x){x.f();}

int main(){
    son s;
    test(s);
}

the question says:

1. the output is '1', what is the rule about polymorphism that the programmer forgot and how can i fix it so the output would be '2'?

2. there is another rule that the programmer forgot when he wrote the father class, and he need to add an empty function to avoid problems from other sons of the father class. what is the rule and what is the missing function?

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another question write the g function so the next code would run with no crashes

int x=11; g(x)=22;

Answer 1

What is it about the test function that makes you think the output would be 2?

Answer 2

You passed x by value. So the compiler will reduce the son object to a father object, including its virtual function table. If you want to call a virtual function of father in test, you need to pass x either by reference or by pointer.

Answer 3

In the function "test" the parameter "father x" is not a reference to class, it IS an instance of father that will be copy constructed from the passed parameter.

Answer 4

I tell my C++ students (in second year) that polymorphism will never sneak up on them; first you need an inheritance hierarchy, and second you need to be using pointers.

Answer 5

When you pass the son object to the test function by value, it will be converted to a father object because test takes a father, not a son. If you want to accept instances of subclass without conversion, you need to accept the argument by reference or pointer.

So if you change the signature of test to void test (father& x), it will work as you want.

Answer 6

1. Pass son s to test as a reference, rather than a copy. If s is passed as a copy, it will be cast to a father object in test. By using a reference of s instead, it will still behave, properly, as a son object.

2. I think this is referring to the recommendation that any base class should have a virtual destructor. That way, the object can be properly destroyed through a pointer or reference of its base class.

add to father:

virtual ~father() {}