3 Apr 2023

Pointer

Cpp-Pointer

Keypoints

Basics
Define a pointer
How to use a pointer
Storage of pointer
Void pointer and wild pointer
Const in pointer
Pointer and array
Pointer and function
BubbleSort example

Basics

It’s used to get the storage indirectly. Pointer is to save address. Simply, pointer is an address.

Define a pointer

int a = 10;
int * p;
p = &a;         // & is to get the address. As 16 not decimal.

How to use a pointer

int a = 10;
int * p;
p = &a; 
*p = 1000; 
cout<< "a = "<< a << endl;          // cout 1000
cout<< "*p = "<< *p << endl;        // cout 1000

Using * to decode pointer: to get the data stored in the address which popinter refers to.

Storage of pointer

Taking 8 byte every pointer in 64 system; 4 byte in 32 system whatever the data type is.

int a = 10;
int * p = &a;
cout << "sizeof (int *) = " << sizeof(p) << endl;       // output is 8

Void pointer and wild pointer

Void pointer refers the 0 space. Widly used in pointer initialization.
Void pointer isn’t accessible.
Avoid wild pointer

int *p = Null;  // Void pointer
*p = 100;       // Output an error as 0~255 storage code is for system usage, can't be used for pointer.
int *x = (int *)0x1100; // Wild pointer: refers to a storage directly.

const in pointer

const pointer: pointer reference (address) isn’t adjustable while its refered value is.
pointer to const: pointer reference (address) is adjustable while its refered value isn’t adjustable.
const pointer to const: both not adjustable

int a = 10;
int b = 10;
const int *p = &a;  // pointer to const
*p = 20;            // ERROR
p = &b;             // reachable

int a = 10;
int b = 10;
int * const p = &a; // const pointer
*p = 20;            // reachable
p = &b;             // ERROR

int a = 10;
int b = 10;
const int * const p = &a; // const pointer to const
*p = 20;            // ERROR
p = &b;             // ERROR

Pointer and array

const pointer: pointer reference (address) isn’t adjustable while its refered value is.

int arr[5] = {1,2,3,4,5};
int *p = arr;       // arr is the 1st address of array
cout << *p << endl; // output 1
p++;                // add one int byte (8 in 64 system/ 4 in 32 sysem) to pointer 
cout << *p << endl; // output 2; since arr is also int type, p pointer now refers to the 2nd address

int *p2 = arr;
for (int i = 0; i<5; i++){
    cout<< *p2 << endl;
    p2++;
}

Pointer and function

void swap(int *p1, int *p2){
    int temp = *p1;
    *p1 = *p2;
    *p2 = temp;
}

int main(){
int a = 10;
int b = 20;
swap(&a, &b);   // value pass isn't OK while address is OK; 
                // Different address of parameter and argument though they share the same value;
                // So, address pass is OK.
cout << "a = " << a << endl;
cout << "b = " << b << endl;
return 0;
}

BubbleSort example

# include <iostream>
using namespace std;

void bubblesort(int *arr, int len){         // arr is a pointer, *arr is to get the value
    for (int i = 0; i<len; i++){
        for (int j = 0; j<len-i-1; j++){
            if (arr[j] > arr[j+1]){
                int temp = arr[j];
                arr[j] = arr[j+1];
                arr[j+1] = temp;
            }
        }
    }
}

void printarray(int *arr, int len){
    for (int i = 0; i < len; i++){
        cout<< arr[i] << endl;
    }
}

int main(){
    int arr[5] = {13,18,5,46,21};
    int len = sizeof(arr) / sizeof(arr[0]);
    bubblesort(arr, len);                   // arr is the variable name, also the pointer refers to the address.
    printarray(arr, len);
    return 0;
}

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