Pointer

[vlad61]

Hey guys/gals

I Pointers are confusing... mostly because i just dont see why they are such a big deal.. so i have a question.

#include<iostream>

using namespace std;

int main()
{
    int num = 5;
    int& num1=num;
	cout<<num1;
	return 0;
}

#include <iostream>
using namespace std;

int main ()
{
  int num1 = 5;
  int * num;

  num=&num1;

  cout<<*num;

  cin.ignore();
  return 0;
}

Why Does the tutorial on this site teach about that second piece of code when these two do exactly the same thing dont they?

My question is mostly - What is the need for (*) since there are two of them, the de-reference and the pointer.

When the &reference operator would do the same exact thing?

[genesys]

Pointers are used in data structures, linked lists, so they are pretty big deal.

2nd code just teaches you basic operations with pointers

[PanGalactic]

In the first example, num1 is a reference. A reference is an alias. So num and num1 refer to the exact same thing. Inside the compiler, num and num1 are just different names for the same thing. If you took the address of each, they would be equivalent.

assert(&num == &num1);

The second example creates a pointer, which is an entirely new variable with its own storage location. It stores the memory location of num. You can change num separately from num1; num can point to any integer variable. If you had a num2, you could say

num = &num2;

num now points to the location of num2 and knows nothing about num1. You cannot do that with the first one. num and num1 are inextricably linked. num = num2 would assign the value of num2 to num1 since num and num1 are just different names for the same thing.

In the first example, 4 bytes are allocated on the stack (for num). And num1 has no distinct storage. In the second, 8 bytes are allocated (assuming a 32-bit architecture).

Make sense?

[oldnewbie]

int num1 = 5;  //num1 is an int variable,has value 5,and a certain address in memory
  int * ptr;    // read right-to-left  : create a pointer named ptr, pointing to ( * ) an integer 
  //ptr can hold adress of variables of type int
  ptr=&num1;  // take address of num1 using &,put this adress in ptr,now ptr points to num1

 cout<<ptr;   //if you open ptr and look what is in you will see an adress value e.g 0X123456
 cout<<endl;
  cout<<*ptr; // if you go to that adress and look what is in ( using * ) you will see value of 5.

[vlad61]

Thanks guys, its just a tad more confusing than loops and if statements i guess..