Pointer

Review the knowledge of pointers

Pointer Category

• Pointer point the constant:

  const char *name = "chen"    //statement a pointer point a constant

  because using const,so the Pointer can't change variable in the address which it point ,so the statement as follows is incorrect :

  name[3]='a'					//incorrect,pointer "name" can't change constant 

  but name is a normal pointer ,so it could change the items it point,statement as follows are correct:

  name = 'zhang'				//change the address the pointer point ,correct

  Also,Even you have changed your string you point ,you still can't change the string, Please somebody tell me why ,Thank you !

  name[3]='y'					//incorrect,but I don't know why!

• Constant Pointer

  A pointer can't change the address it point ,but it still can change the content it point,example:

  char *const name ="chen";     //define a constant pointer
  name[3] = 'a';					//correct ,it can change the constent of pointer
  name = "zhang";					//incorrect ,it can't change the address it points

• Constant Pointer points to constant

  A constant pointer points a constant ,the address pointer point is unchangeable and the content of address is unchangeable,example :

  const char *const name="chen";	//define a constant pointer point the constant
  name[3] = 'q';					//incorrect ,the constant is unchangeable
  name ='ninglang';				//incorrect ,the address pointed is unchangeable 

• Const

  Using a const to define a integer variable ,the keyword omitted is acceptable the definition as following is same:

  const int LIMITS = 100;
  const LIMITS = 100;

  formal parameters also can be describe by const,for example:

  int MAX(const int*ptr)

  the method promise the array can't be changed ,only be read.

Pointer array

#include<stdio.h>

int main(void){
    char *arr[3]={'abc','def','ghi'};
    char *pArr = arr[0];
    printf("The string array arr's element :");
    for(int index=0;index<3;index++){
        printf("%s",arr[index]);
    }
    printf("\n");
    printf("The string arr's first element's every element:");
    for(int index = 0;index<3;index++){
        printf("%c",*(pArr+index));
    }
    printf("\n");
    return 0;
}

in fact , you see the definition of Pointer array ,It is like as follows:

char *arr[3]={'abc','def','ghi'};

char *pChar1 = 'abc',*pChar2 = 'def',*pChar3='ghi'
char *arr[3] = {pChar1,pChar2,pChar3};

At the same time :

arr[0] = pChar;   //the arr first element is the pointer pChar

and the pChar is pointing the 'abc''s first element 'a', so we can use the code to print 'a'

printf("%c",pChar[0]);   //print 'a'
// print'a','b','c'
for(int i = 0;i<3;i++){
    printf("%c ",pChar[i]);
}

Pointer Function

int fun(int x,int y);			//normal function return integers

This function declaration is normal ,but There are some difference in next function declaration

int *fun(int x,int y)

This function declaration is pointer function ,the return is a pointer to int ,This is an address

Pointer To Function

To state a pointer to function ,which is a Pointer pointing function .declaration form:

int (*fun)(int x,int y)

There are two ways to assign values to pointer variables

fun = &function;
fun = function;

There are also two ways to call pointer to function

x=(*fun)();
x=fun();

Example:

#include<stdio.h>
#include<iostream>
using namespace std;
int add(int x,int y){
	return x+y;
}

int sub(int x,int y){
	return x-y;
}

int (*fun)(int x,int y);
int main(){
	//first kind 
	fun = add;
	cout<<"(*fun)(1,2)="<<(*fun)(1,2)<<endl;
    //second kind
	fun = &sub;
	cout<<"(*fun)(5,3)="<<(*fun)(5,3)<<" "<<fun(5,3);
	
	
	return 0;
	
}

New and Delete

operation new can get a space from heap and return the pointer to point the first address of the memory,and delete can free the space

int *p;
p = new int ; //new dynamically allocate an integer memory space,assign the first address to pointer variable


delete p;   	//free the space allocated by new

new assign space for multidimensional array:

int i = 3;
int *pi = new int[3][4];

new assign space with initial value:

#include<iostream>
using namespace std;
int main(){
	int *p;
	p = new int(99);      //initial value
	cout<<*p;
	delete p;
	return 0;
}

Malloc && Free get space