Last Minute Reviewing

This is a discussion on Last Minute Reviewing within the C++ Programming forums, part of the General Programming Boards category; I'm studying for an exam, and I had some quick questions. What does this code output? Code: class ExampleClass { ...

  1. #1
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    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.

  2. #2
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    What makes you think it should NOT be 3?
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    Mats
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    ExampleClass 3. But like I said, that doesn't seem right.

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    Quote Originally Posted by arrgh View Post
    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.

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

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    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?

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    Please don't PM me for help - and no, I don't do help over instant messengers.

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    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?

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    Quote Originally Posted by arrgh View Post
    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].

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    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  9. #9
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    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.
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

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    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?

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    Quote Originally Posted by arrgh View Post
    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...

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    Please don't PM me for help - and no, I don't do help over instant messengers.

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    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!

  13. #13
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    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.)
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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  14. #14
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    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.
    Last edited by Elysia; 05-07-2008 at 06:08 AM.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    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?

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