Pointers
Author
Alright been messing around wiht pointers tutorials and all and got stuck on some code. Now the thing i cant seem to understand is at a certain point says that ptr += 2; mean it take ptr and adds 8 to it?? that totaly lost me.
#include <iostream>
using namespace std;
main()
{
int *ptr;
ptr = new int[5];
*ptr = 5; // this is ptr[0] = 5
ptr[1] = 10; // ptr[1] = 10
ptr += 2; // the tutorial says its ptr +8 but i cant seem to figure out why.
*ptr = 15; // now this one and next one really got me lost.?
ptr -= 2;
for (int i = 0; i < 5; i++)
{
cout << ptr << endl;
}
}
If you have an array of chars and have a pChar pointing to an element then pChar+2 gives you the character two ahead of the one you're on. In terms of memory this is 2 bytes since a char is a byte. If however you have an array of ints and a pInt, each int is 4 bytes typically and so when you do pInt+2 you are moving two ints along the array but 8 bytes.
Hope that helps,
Dave
*ptr = 5; // this is ptr[0] = 5ptr[1] = 10; // ptr[1] = 10ptr += 2;*ptr = 15;ptr -= 2;Hope that helps,
Dave
Author
tyvm bro that helps a alot its all clear accept the ptr -= 2;
i know it send me back to ptr[0] but dont know why i would do that hehe
i know it send me back to ptr[0] but dont know why i would do that hehe
With:
ptr -= 2;
The pointer holds an address to somewhere in memory where that array of integers is stored. In memory the integers are arranged in sequence, a contiguous block. Each integer is 4 bytes and so you have:
If the pointer is pointing at the 3rd integer and you move it back 2 integers, you are moving the pointer backwards in the memory by 8 bytes, as you can see in the diagram. So when you -= 2, 2 * the size of the item stored (which is 4) is subtracted from the address of the pointer.
Does that help,
Dave
ptr -= 2;
The pointer holds an address to somewhere in memory where that array of integers is stored. In memory the integers are arranged in sequence, a contiguous block. Each integer is 4 bytes and so you have:
[4bytes][4bytes][4bytes][4bytes][4bytes]^ ^| |ptr ptr-2 ints-8 bytesIf the pointer is pointing at the 3rd integer and you move it back 2 integers, you are moving the pointer backwards in the memory by 8 bytes, as you can see in the diagram. So when you -= 2, 2 * the size of the item stored (which is 4) is subtracted from the address of the pointer.
Does that help,
Dave
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