Question

Why, in the following code, is sizeof(X) == 4 and sizeof(Y) == 8?

Moreover, in the class X, why do the member functions not take any memory space?

class X {
    int i;
public:
    X() { i = 0; }
    void set(int ii) { i = ii; }
    int read() const { return i; }
    int permute() { return i = i * 47; }
};

class Y : public X {
    int i; // Different from X's i
public:
    Y() { i = 0; }
    int change() {
        i = permute(); // Different name call
        return i;
    }
    void set(int ii) {
        i = ii;
        X::set(ii); // Same-name function call
    }
};


    cout << "sizeof(X) = " << sizeof(X) << endl;
    cout << "sizeof(Y) = " << sizeof(Y) << endl;

Answer 1

Objects of class Y have two integer members; objects of class X have one. Your comments express the fact that Y‘s i is different from X‘s i, so it looks like you already knew the answer.

See http://codepad.org/PZsiyFIk for an example of how an object of class Y actually has two i members.

Code repeated here:

#include <iostream>
using namespace std;

class X {
  int i;
public:
  X(): i(4) {}
  int getI() {return i;}
};

class Y: public X {
  int i;
public:
  Y(): i(10) {}
  int getMyI() {return i;}
};

int main() {
  Y y;
  cout << y.getI() << ' ' << y.getMyI() << '\n';
}

Outputs

4 10

There is only one object in main called y. Note that y must have two i fields for the output to make sense.