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Understanding pointer indirection

Posted on 2008-06-22
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Hi,

Can somebody explain what indirection really means?

1) I don't understand why they call it indirection... what is the source of this name?
2) as far as i can understand, indirecting a pointer is changing the value of the variable it points to. Is this true, or is there more to indirection?
3) supose I create a pointer using "int *ptr;"... is it possible to indirect the pointer, even if it does not point to a variable?

Thank!
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Question by:codeQuantum
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Infinity08 earned 1000 total points
ID: 21842348
>> 1) I don't understand why they call it indirection... what is the source of this name?

Basically, because it provides a way to indirectly access data.


>> 2) as far as i can understand, indirecting a pointer is changing the value of the variable it points to. Is this true, or is there more to indirection?

Indirection of a pointer is getting access to the data it points to. For retrieving and/or modifying that data.


>> 3) supose I create a pointer using "int *ptr;"... is it possible to indirect the pointer, even if it does not point to a variable?

As long as the pointer points to a valid memory location, you can get the data at that memory location by indirection of the pointer.

A pointer can point to valid memory by explicitly making it point to valid memory, for example :

        int *p = new int(5);
        int value = 4;
        int *p2 = &value;

But it could also be "by accident". Uninitialized pointers can have any value, so they could point to a valid memory location, in which case you can get the data at that location by indirection of the pointer.
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by:codeQuantum
ID: 21842433
Thanks, point 1) and 2) are clear to me now.

I was asking that last question because I am doing an exercise right now that is related.

It says : >> create 2 pointers... A and B. Then make it so each of those pointers is assigned a memory zone that can hold an int. <<

So if I understand your answer correctly, I have to create two int variable and associate the pointers to them, or use new int(123) in order to solve the problem? or is there another way to do this?

(in other words, is there an elegant solution that does not involve creating unnecessary int variables, or using an arbitrary number inside the new int(x)?)
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by:Infinity08
Infinity08 earned 1000 total points
ID: 21842473
>> or use new int(123) in order to solve the problem?

I think the question meant to allocate dynamic memory, so you'll have to use new, yes.


>> (in other words, is there an elegant solution that does not involve creating unnecessary int variables, or using an arbitrary number inside the new int(x)?)

The new int(5) was just an example. You can use new in a variety of ways (to allocate memory that can contain one or more objects of a given type). For example :

        int *p = new int;             // <--- allocate memory for one int
        int *p2 = new int[10];     // <--- allocate memory for 10 ints
        int *p3 = new int(5);       // <--- allocate memory for one int, and initialize that int value to 5

Note that you need to explicitly release the allocated memory once you don't need it any more. Use either delete or delete [] for that (the latter when you used new []) :

        delete p;
        delete [] p2;
        delete p3;


Here's a nice tutorial about dynamic memory :

        http://www.cplusplus.com/doc/tutorial/dynamic.html
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by:codeQuantum
ID: 21842530
So I guess I need to use

int *p = new int;

Thanks infinity08, you have been very helpful. :)
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by:Infinity08
ID: 21842539
>> So I guess I need to use
>> 
>> int *p = new int;

Yep. Twice - once for A and once for B ;)
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by:codeQuantum
ID: 21842543
One last detail, if I wanted to break that operation on two lines, would that work :

int *p;
*p = new int;

?

I am sometimes confused by the indirection operator * that seem to be used in 2 different ways between declaration and indirection.
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by:Infinity08
ID: 21842569
>> int *p;
>> *p = new int;

Make that :

        int *p;
        p = new int;

and it will work. The type of p is int* (pointer-to-int). *p is the indirection of p, so it actually refers to the int value.


>> that seem to be used in 2 different ways between declaration and indirection.

This is a declaration :

        int *p;

The * here is NOT the indirection operator. It is part of the type of p (int* or pointer-to-int)
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by:codeQuantum
ID: 21842603
One last thing (sorry, I promess this is the last one!)

I read that arrays are basically pointers (to their first element). If it is so, why can't I do this :

      int untab[] = {5, 19, 14, 8, -10, 99};
      int *unptr = new int;
      untab = unptr; // returns an error

Both are int pointers, so what's the problem?

I understand that I would lose information doing that. But since C++ is not as protective as Java, I don't see why it does not compile...
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by:Infinity08
ID: 21842621
>> I read that arrays are basically pointers

That's not true. Arrays will degenerate to a pointer in certain contexts, so you can do this for example :

        int untab[] = {5, 19, 14, 8, -10, 99};
        int *unptr = untab;                                   // <--- now unptr will point to the first element in the untab array (5)

However, that doesn't work the other way around. You cannot re-assign an array identifier, and let it point to different memory.
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