Pointer summary

Interesting pointer

c The problem of pointers in language is very interesting , Today, I'll make a simple summary of the pointer class I've seen .

1. The definition and use of pointers ：

c Pointer in language is a variable specially used to store memory address , Each pointer has a data type associated with it , The general form we define

char   *ch;
int    *a;
double *pd = NULL;

Each pointer variable stores the address of memory

int    a = 1;
int    *p = &a;

2. Pointers and arrays

(1) stay c in , The array name is actually a pointer to the first element of the array

int    a[] = {
    2,4,6,8,10};
int    *p = a;// Address assignment method 1 of the first element of the array 
int    *q = &a[0];// Address assignment method 2 of the first element of the array 

(2) Of course, there are pointer arrays and secondary pointers

int     a[5] = {
    1,3,5,7,9};
int     *b[5],i;
int      **c = b;

So because of “[ ]” Has a higher priority than “*” , So at the moment *b[5] Equivalent to *(b[5]).

for(i = 0;i < 5;i++)
b[i] = &a[i];

Or maybe

int *p = a;
for(i = 0;i < 5;i++)
      b[i] = p++;

At this time, the a The addresses of the elements of this array are assigned to b Array

Next is the secondary pointer , That is, the address where the memory address is stored （ It's like dolls ）.

int   **c = b;

Here will be b The address of the first element of is assigned to c .

for(i = 0;i < 5;i++,c++)
printf("%d ",**c);

Then output the data .

(3) Pointer array

int    q[2][3];

First of all, we know that one dimension of a two-dimensional array is the address where two dimensions are stored , Then we can define ：

int   *p[2];

Due to priority ,p The first and “[ ]” Union and recombination “*” combination , So it's a pointer to an array . It can be understood according to the assignment relationship of relatively equal quantities

for(int i = 0;i < 2;i++){
    
p[i] = q[i];
}

(4) Array pointer

It is generally defined as :

int    (*p)[4];

For its understanding, I think it is very similar to two-dimensional array ,p The first address of four consecutive one-dimensional arrays is stored in memory .

int    a[2][3]={
    1,2,3,4,5,6};
int    (*p)[3];
        p = a;
        p++;//a[1][];
        for(int i=0;i<3;i++){
    
        printf("%d ",(*p)[i]);
        }

The result is 4 5 6

3. Pointers and functions :

(1) Pointer type variables can also be used as parameters of functions

void func(int *a,int *b){
    
...
}

(2) The most interesting thing is that pointers can also point to functions , Call it a function pointer .
Generally defined as :

int (*p)(int,int);

For example, the following example ：

int get_max(int i,int j){
    
    return i>j?i:j;
}
int get_min(int i,int j){
    
    return i > j?j : i;
}
int compare(int i,int j,int flag){
    
    int ret;
    int (*p)(int,int);// Pointer to function , The return value of this function is int type 
    if(flag == GET_MAX)
    p = get_max;// Assignment of function name 
    else
    p = get_min;// Assignment of function name 
    ret = p(i,j);// Receive return value 
    return ret;
}

And here we need to distinguish

int   *p(int i,int j);// Functions that return pointers 
int   (*p)(int i,int j);// Defines a pointer to a function 

(3) Function pointer as formal parameter

int get_big(int i,int j){
    
return i > j?i : j;
}

int get_max(int i,int j,int k,int (*p))(int,int)){
    
int ret;
ret = p(i,j);
ret = p(ret,k);
return ret;
}

int main(void){
    
int i = 5,j = 10,k = 15,ret;
ret = get_max(i,j,k,get_big);// Pass this function as an argument 
printf("%d",ret);
return 0;
}

4. Strings and pointers

And array pointer

char ch[] = "hello world";
printf("%s",ch);
char *p = "hello world";// Define a string pointer 
printf("%s",p);

The pointer of the string can be used to copy the string array , You can use this example to understand .

char a[] = "hello world";
char b[20],c[20];
char *p1 = a,*p2 = c;

for(int i=0;i<sizeof(a);i++)
b[i]=a[i];
while(*p1 != '\0') *(p2++)=*(p1++);

printf("%s\n",a);
printf("%s\n",b);
printf("%s\n",c);

Of course, the results are the same

The above is my summary of pointer , Thank you for browsing , I hope I can put forward valuable suggestions .