Last Minute Reviewing

I'm studying for an exam, and I had some quick questions. What does this code output?

Code:

---


class ExampleClass {
private: int i;
public:
    ExampleClass(int j) { i=j; }
    int get_i() { return i; }
};

int main() {
    ExampleClass objects[4] = {1,2,3,4};
    ExampleClass *p;
    p = &objects; // get address of objects
    cout << p->get_i() << endl; // use -> to call get_i()
    p = p +2;
    cout << p->get_i() << endl; // which object is output?
    return 0;
}

---

I'm really tempted to say ExampleClass 3, but the way the professor asked makes it seem like a trick question.

What makes you think it should NOT be 3?
--
Mats

ExampleClass 3. But like I said, that doesn't seem right.

Quote:

---

Originally Posted by arrgh

ExampleClass 3. But like I said, that doesn't seem right.

---

Sorry, edited my reply after re-reading your post and realizing you already said that.

--
Mats

There was a side note mentioning "be careful with pointer arithmetic" and 3 seems like the obvious answer, so...

Ok, so after fixing the pointer assignment (&objects is a pointer to an array, so p = objects; is the correct assignment), and including the relevant headers, using namespace std, it outputs 1 and 3.

However, as it stands it won't compile correctly, as it's got a funny pointer assignment [I sort of thought so]. Is it expected that you should spot stupid typo's by the teacher, or are programs (unless specifically noted) supposed to be correct code that can run?

--
Mats

Nope, it's just testing whether you can understand the code. Just know general output and ignore compile errors. If ExampleClass 3 is printed out, then what exactly do you have to watch out for when using pointer arithmetic with base class pointers?

Quote:

---

Originally Posted by arrgh

Nope, it's just testing whether you can understand the code. Just know general output and ignore compile errors. If ExampleClass 3 is printed out, then what exactly do you have to watch out for when using pointer arithmetic with base class pointers?

---

Not sure - pointer arithmetics is usually straightforward. But talking about base-classes makes me think that you are referring to inheritance, and I expect if we rewrite your example a bit:

Code:

---

#include <iostream>

using namespace std;

class Base
{
private:
  int x;
public:
  Base() { x = 0; }
  virtual int get_i() { return x; }
};

class ExampleClass: public Base {
private:
  int i;
public:
    ExampleClass(int j) { i=j; }
    virtual int get_i() { return i; }
};

int main() {
    ExampleClass objects[4] = {1,2,3,4};
    Base *p;
    p = objects; // get address of objects
    cout << p->get_i() << endl; // use -> to call get_i()
    p = p +2;
    cout << p->get_i() << endl; // which object is output?
    return 0;
}

---

then things go horribly wrong when we try to do the second get_i() call [NULL-pointer dereference, but that's just in this particular example].

--
Mats

The thing that you have to watch out here is probably that you see +2 in the code, so you might be tempted to think that you are now referring to the second item. But you didn't fall for it.

Quote:

---

then things go horribly wrong when we try to do the second get_i() call [NULL-pointer dereference, but that's just in this particular example].

---

Is there a way to fix it? Or is it better to not use base pointers in derived classes?

Quote:

---

Originally Posted by arrgh

Is there a way to fix it? Or is it better to not use base pointers in derived classes?

---

If you have an array of pointers, you'd be fine. But if your pointer is pointing to an array of objects, then you're stuffed... The only solution is to not use a base-pointer-to-array solution...

--
Mats

I see. Inheritance might have been the issue. I remember going over that in class, but that was after I got lost over the process of designing a software program to be used in the "real world", so nothing really sank in. Anyways, thanks for clearing that up!

Meh, matsp beat me to pointing out the assignment error.

But it would probably sneak through, because VC++ actually accepts this code. (Or used to, at least.)

Well, 2008 sure as heck doesn't (first code).
error C2440: '=' : cannot convert from 'ExampleClass (*)[4]' to 'ExampleClass *'
Second code, unfortunately, does compile, but produces an access violation, as mats seems to hint.

However, fixing the little typo problem:

Code:

---

#include <iostream>

using namespace std;

class ExampleClass {
private: int i;
public:
        ExampleClass(int j) { i=j; }
        int get_i() { return i; }
};

int main() {
        ExampleClass objects[4] = {1,2,3,4};
        ExampleClass *p;
        p = objects; // get address of objects (NOT &objects - don't try to take the address of an array!)
        cout << p->get_i() << endl; // use -> to call get_i()
        p = p +2;
        cout << p->get_i() << endl; // which object is output?
        return 0;
}

---

It runs fine and outputs 1 and 3. As it should.

Code:

---

ExampleClass objects[4] = {1,2,3,4};

---

Is initializing objects in that array only possible because the constructor only takes one argument? Is the [4] necessary?

In that way? Yes.
The [4] isn't strictly necessary... You can write objects[] too, but you can't omit it.

Quote:

---

Originally Posted by jw232

Code:

---

ExampleClass objects[4] = {1,2,3,4};

---

Is initializing objects in that array only possible because the constructor only takes one argument? Is the [4] necessary?

---

That and the constructor that takes one argument isn't labelled explicit. In such a case, initializing an array of objects won't compile.