[GO] What is a pointer?

Overview

Recently, while writing Golang for part-time jobs and hobbies, and writing C language for school lectures and assignments, I thought that pointers are difficult to understand, and when I talk about that story with various engineer students, it is difficult for everyone to understand. I thought, so this time I tried to summarize the pointers in C language (Golang was made with reference to C language)! !! !!

What is a pointer?

A computer reserves a variable in a program in memory and stores an address to identify its location. Pointers are used when the program wants to handle the address. In other words, a pointer is a variable that handles an address.

syntax

Let's look at the syntax for using pointers in C.

Declaration of pointer

`Data type * pointer variable name;`

(Example) Declare a pointer to an int type variable 　int *p;

Substitute address

--You can get the address by adding "&" to the variable. &Variable name

(Example)

#include<stdio.h>
int main() {
    int x = 1;
    int *p;
    p = &x;
    printf("「&x "address: %p\n", &x);
    printf("Address indicated by "p": %p\n", p)

    return 0
}

(Execution result)

「&x "address: 00A2EE76
Address indicated by "p": 00A2EE76

Shows the value in the address indicated by the pointer

--If you add "*" to the pointer, you can get the value in the address indicated by the pointer.

*Pointer variable name;

(Example)

#include<stdio.h>

int main(){
    int x = 1;
    int *p = &x;
    printf("Value in the address indicated by "p": %d\n", *p);

    return 0;
}

(Execution result)

The value of the address indicated by "p": 1

Things to note

Attention 1

What I think is confusing and needs attention is how to interpret "". When assigning an address when declaring a pointer, it is "int * p = & x", so it looks as if "& x" is assigned to " p", but it is semantically "p = & x". It becomes. In other words, the following two are synonymous.

int x = 1;
int *p;
p = &x;

int x = 1;
int *p = &x;

Attention 2

The second point to note is about initialization. Just by declaring a pointer variable, the address indicated by that pointer variable becomes indefinite. Therefore, if you use the pointer variable without initializing it, you will access an unspecified address, which causes a bug.

#include<stdio.h>

int main() {
    int x = 1;
    int *p;
    //The following line causes a bug because p has not been initialized
    *p = x;
}

Arrays and pointers

You can work with arrays using pointer operations by assigning the start address of the array to a pointer variable. As you can see in the sample below, & num [0] and num are the same. Also, * (p + i) and num [i] are the same.

#include<stdio.h>

int main() {
    int *p;
    int i;
    int num[3] = {1, 5, 10};
    //The following line assigns the start address of the array to the pointer variable
    p = num;
    for(i = 0; i < 3; i++) {
        printf("%d\n", *(p + i));  //Advance the pointer by the number of elements to take the value
    }
    return 0;
}

(Execution result)

1
5
10

Summary

This time, I have summarized the basic points that are easy to misunderstand about pointers in C language. Next time, I'll summarize some of the misleading points about Golang pointers. !! !! !!