9. Pointer of C language (1) what is pointer and how to define pointer variables

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Pointer is C The soul of language

C Language can be used by pointer

         Represents some complex data structures

         Fast data transfer

         Make the function return more than one value

         Direct access to hardware

         It can easily handle strings

Before learning the pointer , We need to understand the concept and relationship between address and pointer

Address ： Refers to the number of memory unit , It's from 0 The starting nonnegative integer , In the computer , Byte is the basic unit of data storage , And it is also the smallest unit that the hardware can access , A byte is a number .

The pointer ： The pointer is the address , The address is the pointer , It's a value , It's the number

Pointer to the variable ： Is a variable for storing addresses , What we need to learn is pointer variables , Usually, when we describe, we will abbreviate pointer variables as pointers , Actually, they have different meanings

Let's learn how to define pointer variables through a simple example

#include <iostream>
#include <stdio.h>
using namespace std;

int main(int argc, char const *argv[])
{
    int *p; //p It's the name of the variable ,int*  Express p Variables are stored in int The address of a type variable 
    //int* p; Does not mean that a name is defined *p The variable of 
    //int* p; It should be understood that ：p Is a variable name. ,p The data type of the variable is int* type 
    // So-called int* type , It's actually storing int Type of variable address 
    int i = 3;
    p = &i;
    /* 
        p Save the i The address of , therefore p Yes i
        p No i,i Neither p, modify p The value of does not affect i Value , modify i The value of does not affect p
         If a pointer variable points to a variable , be * Pointer variables are exactly the same as ordinary variables 
         Example ：
             If p It's a pointer variable , also p Stored ordinary variables i The address of 
             be p It points to the normal variable i
            *p  Is exactly the same as  i
             Or say ： In all appearances *p All places can be replaced with i

            *p  That is to say p The content of the address is the variable 

         The pointer is the address , The address is the pointer , It's a value , It's the number  
         The address is the number of the memory unit 
         A pointer variable is a variable that holds an address 
         But be careful ： Usually, when we describe, we will abbreviate pointer variables as pointers , Actually, they have different meanings 
    */

    // p=i; error, Because of the different types ,p Only store int The address of a type variable , Cannot be stored int Type value 

    int j;
    j = *p; //*p Namely i
    printf("i=%d,j=%d\n", i, j);
    return 0;
}

The output is i=3,j=3

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summary ： What we need to know is

        1.int *p Its meaning is to define a pointer variable p,p It stores the address of the integer variable

                p No i,i Neither p, modify p The value of does not affect i Value , modify i The value of does not affect p

        2. Assignment statement p=&i; The meaning of &i Is to take i The address of , So this sentence expresses will i The address of is placed in the variable p in

        

        We wrote p=&i After this sentence ,P It points to i Variable , At this time ,P The storage address is 1000H

         

        3.*p Means to store in p The content of （1000H） Variable for address , In fact, it is 3

                here *P Is equivalent to i