Michael Otu

Posted on Nov 15, 2020

Do you know what a pointer is?

#cpp #c #pointers #address

Simply explaining a pointer

A pointer is a variable that contains the address of another variable. Basically saying that a pointer is a variable that points to the memory location of another variable and can change the value at that address indirectly.

Making sense of value and memory address

Two facts we know or have to know to set things straight:

& for the address and
* for the value at an address.

Let's say we have int num = 3; We assign the value 3 to the variable num.

So to print out the value of num:

printf("value of num: %d\n", num);
/* value of num: 3 */

How do we then get the address of num? Remember the &? We precede the variable with &.

printf("address of num: %p\n", &num);
/* address of num: 0x7ffe5faa05cc */

So now we have an address, how do we then get the value at that address? Remember the second fact, about *? Precede the memory address with a *. We got the address by &num and by preceding the address, &num, by *, as *(&num), we would get the value at that address.

printf("value at address of num: %d\n", *(&num));
/* value at address of num: 3 */

Takeaways

So what we have been able to deduce is:

int num = 3; means num has a value of 3.
&num returns the memory address of num.
*(&num) returns the value at the memory address, &num, of num.

Final script

#include <stdio.h>
int main() {
    int num = 3;
    printf("value of num: %d\n", num);
    printf("address of num: %p\n", &num);
    printf("value at address of num: %d\n", *(&num));

    return 0;
}

Delving into pointers

Creating a pointer is simple. It is basically as declaring or initializing a variable - well a pointer is a variable. Consider the snippet below as to how to create a pointer.

// NOTE: int is used for the sake of simplicity

// case 1 - initialize a pointer
int *ptr = &var;

// case 2 - declare and define later
// ptr should be initialized to NULL
// until it is assigned a valid location
// int *ptr1;
int *ptr1 = NULL;
ptr = &var;

Now let's get down to pointer business. Say we have num assigned the value of 3, just as we have done before. We shall create a pointer to num.

int num = 3;
int *ptr = &num;

I have a question for you? You better play with the code to understand better.

Question

Which is the pointer? *, ptr or *ptr?

My answer

Well, I hope you gave it some thought.

* is not the pointer. Remember the second fact. Given an address, * returns that value at that memory address.
*ptr is the value at the memory address, &num - num.As such is not the pointer.
ptr is the pointer. This is so because it has the memory address of num as value.

I will encourage you to take a break, clear your head and read again or proceed.

Pointer value and address

How do we get the value of a pointer? Don't get confused about the value of a pointer and what value the pointer points to. Try to understand the two before moving on. Something more or less like, the value of a pointer and the value at a pointer - what the pointer points to.

printf("value of ptr: %p\n", ptr);
/* value of ptr: 0x7ffe5faa05cc */

How do we get the value at the pointer? Now the pointer, ptr is the same as &num. As such the value at the pointer (memory address) - &num, is *(&num). *ptr == *(&num).

printf("value at ptr: %d\n", *ptr);
/* value at ptr: 3 */

The pointer, ptr, has its own address. How do we get the address of a pointer? This is a familiar question. Precede the variable with &.

printf("address of ptr: %p\n", &ptr);
/* address of ptr: 0x7ffe5faa05d0 */

Take aways

In int *ptr = &num, ptr is the pointer.
ptr should be assigned NULL until given a location.
ptr is the same as &num as such *ptr, *(&num) is the same as num.
The pointer has its own address, &ptr.

Final script

#include <stdio.h>

int main() {
    int num = 3;
    printf("value of num: %d\n", num);
    printf("address of num: %p\n", &num);
    printf("value at address of num: %d\n", *(&num));

    int *ptr = &num;
    printf("value of ptr: %p\n", ptr);
    printf("value at ptr: %d\n", *ptr);
    printf("address of ptr: %p\n", &ptr);

    return 0;
}

Source

Top comments (9)

Junxiao Shi • Nov 16 '20

A pointer is a magical being that causes your program to crash with a segmentation error.

ѕυηιℓ gαяg • Nov 19 '20

the one who knows it will perish and the one who ignores it will get segmentation fault :-p

Michael Otu • Nov 23 '20

I always get a segmentation fault in CPP rather than C, when using pointers. So I switched to python and there was nothing like a segmentation fault. I like C more though.

Junxiao Shi • Nov 23 '20

You can use Python until it's too slow and you need an extension - written in C.
I needed C extension only once in many years, for a custom hash function.