c++ virtual function call for object

Question

Learning C++ with help of Bruce Eckel "Thinking in C++". Stuck in exercise 30. Here it is:

If function calls to an object passed by value weren’t early-bound, a virtual call might access parts that didn’t exist. Is this possible? Write some code to force a virtual call, and see if this causes a crash. To explain the behavior, examine what happens when you pass an object by value.

I can understand result of calling virtual function for object, but I can not understand how to force compiler to do it, without proper constructors called.

Is there a way to treat one object as another without calling proper constructors or operators (for type conversion)?

Answer 1

Bruce is trying to illustrate object slicing, a situation when a polymorphic object is passed by value.

Here is how you can do it:

#include <iostream>
using namespace std;
struct hello {
    virtual void say() { cout << "hello" << endl; }
};
struct world : public hello {
    virtual void say() { cout << "world" << endl; }
};
void force(hello h) {
    h.say();
}
int main() {
    world w;
    w.say();
    force(w);
    return 0;
}

This code outputs (link to ideone)

world
hello

even though you'd expect an object of type world to "say" world, not hello. C++ compiler is smart in noticing that w is passed to hello by value, so it adjusts the vtable to avoid calls of methods in the derived class.

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Bonus exercise to test if you understand passing by reference: can you modify my code so that it prints world world? You are allowed to insert a single character.