Question about pointers.

[monoceres]

I tested playing a bit with pointers today and I stumbled on something I can't get my head around.

As you can see in this example, the difference between int pointers is 3 which is I can't find the logic in, and I got even more confused when the difference between double pointers were 2.

#include <iostream>
int main ()
{
    int a=10;
    int b=20;
    int *ptra=&a,*ptrb=&b;
    int diff=ptrb-ptra;
    std::cout << "ptra: " << ptra << "\n" <<
        "ptrb: " << ptrb << "\n" << "Size of pointer: " << sizeof(ptra) << "\n" <<
        "Difference: " << diff << "\nData pointed by ptra: " << *ptra << "\nData pointed by ptrb: " << *ptrb << "\n" <<
        "Data pointed by ptra+difference: " << *(ptra+diff) << "\n";
    return 0;
}

It would be great if someone can explain

[Jon]

You are assuming that a and b are stored next to each other in memory - they might not be. Create an array of 2 ints and do the same things with the first and second element.

[monoceres]

You are assuming that a and b are stored next to each other in memory - they might not be. Create an array of 2 ints and do the same things with the first and second element.

Ah I see, now I get a more logical result, 1.

Thanks for the answer!

[Wus]

If you are just starting in on pointers I wish you the best of luck. When I took a data structures course based on c++ the only way I could really wrap my head around pointers was to sit at a white board and draw out what I thought my program was doing line by line (we had to implement our own LinkedList, BTree, etc).

Something ironic though is that using Java has taught me a lot about pointers. Yes I realize that Java doesn't expose pointers directly to the programmer, but while trying to understand the way Java handles method parameters, return values, and garbage collection you end up learning about pointers! And thats not to mention the weird ways Java deals with pointers when using JNI to interface with native code.

Edited May 14, 2008 by Wus

[monoceres]

Nah I have been using pointers before, it's just that I want to learn more about how the memory works. I also didn't see the use of them before, I was like "Why use pointers when you can just pass the data around in my program" but now I see that it's a real waste to copy the same data around hundreds of times when you can just pass a reference to it It becomes even more important when you work with bigger things like classes and stuff.

[Valik]

<pedantic>

...I see that it's a real waste to copy the same data around hundreds of times when you can just pass a reference to it...

You mean pointer. Given that "reference" has a specific and slightly different meaning, it's important to be explicit.

</pedantic>

[monoceres]

Speaking about reference and pointers, what is the difference between them?

I mean the memory address of the variable is used in both ways:

#include <iostream>
void withptr(int*);
void withref(int&);
int main ()
{
    int a=10;
    int *ptr=&a;
    withptr(ptr);
    std::cout << a << std::endl;
    withref(a);
    std::cout << a << std::endl;
    return 0;
}

void withptr(int *pointer){
    *pointer*=2;
}
void withref(int &ref)
{
    ref*=2;
}

[Valik]

http://www.parashift.com/c++-faq-lite/references.html