C和C++中的指针|S1（简介，算术和数组）

本文概述

• C ++
• C

指针存储变量的地址或存储位置。

// General syntax
datatype *var_name; 

// An example pointer "ptr" that holds
// address of an integer variable or holds
// address of a memory whose value(s) can
// be accessed as integer values through "ptr"
int *ptr;

使用指针：

要在C中使用指针, 我们必须了解以下两个运算符。

要访问指向变量的地址, 我们使用一元运算符

&

(与号)返回该变量的地址。例如＆x给我们变量x的地址。

// The output of this program can be different
// in different runs. Note that the program
// prints address of a variable and a variable
// can be assigned different address in different
// runs.
#include <stdio.h>
  
int main()
{
     int x;
  
     // Prints address of x
     printf ( "%p" , &x);
  
     return 0;
}

还有一个运算符是

一元*

(星号)用于两件事：

声明指针变量：在C / C ++中声明指针变量时, 其名称前必须带有*。

// C program to demonstrate declaration of
// pointer variables.
#include <stdio.h>
int main()
{
     int x = 10;
  
     // 1) Since there is * in declaration, ptr
     // becomes a pointer varaible (a variable
     // that stores address of another variable)
     // 2) Since there is int before *, ptr is
     // pointer to an integer type variable
     int *ptr;
  
     // & operator before x is used to get address
     // of x. The address of x is assigned to ptr.
     ptr = &x;
  
     return 0;
}

要访问存储在地址中的值, 我们使用一元运算符(*), 该运算符返回位于其操作数指定地址处的变量的值。这也称为

取消引用

.

C ++

// C++ program to demonstrate use of * for pointers in C++
#include <iostream>
using namespace std;
  
int main()
{
     // A normal integer variable
     int Var = 10;
  
     // A pointer variable that holds address of var.
     int *ptr = &Var;
  
     // This line prints value at address stored in ptr.
     // Value stored is value of variable "var"
     cout << "Value of Var = " << *ptr << endl;
  
     // The output of this line may be different in different
     // runs even on same machine.
     cout << "Address of Var = " <<  ptr << endl;
  
     // We can also use ptr as lvalue (Left hand
     // side of assignment)
     *ptr = 20; // Value at address is now 20
  
     // This prints 20
     cout << "After doing *ptr = 20, *ptr is " << *ptr << endl;
  
     return 0;
}
  
// This code is contributed by
// shubhamsingh10

C

// C program to demonstrate use of * for pointers in C
#include <stdio.h>
  
int main()
{
     // A normal integer variable
     int Var = 10;
  
     // A pointer variable that holds address of var.
     int *ptr = &Var;
  
     // This line prints value at address stored in ptr.
     // Value stored is value of variable "var"
     printf ( "Value of Var = %d\n" , *ptr);
  
     // The output of this line may be different in different
     // runs even on same machine.
     printf ( "Address of Var = %p\n" , ptr);
  
     // We can also use ptr as lvalue (Left hand
     // side of assignment)
     *ptr = 20; // Value at address is now 20
  
     // This prints 20
     printf ( "After doing *ptr = 20, *ptr is %d\n" , *ptr);
  
     return 0;
}

输出：

Value of Var = 10
Address of Var = 0x7fffa057dd4
After doing *ptr = 20, *ptr is 20

下面是上述程序的图示：

指针表达式和指针算术

可以对指针执行一组有限的算术运算。指针可以是：

• 递增(++)
• 递减(—)
• 可以将整数添加到指针(+或+ =)
• 可以从指针中减去一个整数(–或-=)

指针算术是没有意义的, 除非在数组上执行。

注意：指针包含地址。添加两个地址是没有意义的, 因为不知道它将指向什么。减去两个地址可让你计算这两个地址之间的偏移量。

// C++ program to illustrate Pointer Arithmetic
// in C/C++
#include <bits/stdc++.h>
  
// Driver program
int main()
{
     // Declare an array
     int v[3] = {10, 100, 200};
  
     // Declare pointer variable
     int *ptr;
  
     // Assign the address of v[0] to ptr
     ptr = v;
  
     for ( int i = 0; i < 3; i++)
     {
         printf ( "Value of *ptr = %d\n" , *ptr);
         printf ( "Value of ptr = %p\n\n" , ptr);
  
         // Increment pointer ptr by 1
         ptr++;
     }
}

Output:Value of *ptr = 10
Value of ptr = 0x7ffcae30c710

Value of *ptr = 100
Value of ptr = 0x7ffcae30c714

Value of *ptr = 200
Value of ptr = 0x7ffcae30c718

数组名称作为指针

数组名称的行为类似于指针常量。该指针常量的值是第一个元素的地址。

例如, 如果我们有一个名为val的数组, 则

值

和

＆val [0]

可以互换使用。

// C++ program to illustrate Array Name as Pointers in C++
#include <bits/stdc++.h>
using namespace std;
  
void geeks()
{
     // Declare an array
     int val[3] = { 5, 10, 15};
  
     // Declare pointer variable
     int *ptr;
  
     // Assign address of val[0] to ptr.
     // We can use ptr=&val[0];(both are same)
     ptr = val ;
     cout << "Elements of the array are: " ;
     cout << ptr[0] << " " << ptr[1] << " " << ptr[2];
  
     return ;
}
  
// Driver program
int main()
{
     geeks();
     return 0;
}

Output:
Elements of the array are: 5 10 15

现在, 如果将此ptr作为参数发送给函数, 则可以以类似方式访问数组val。

指针和多维数组

考虑二维数字数组的指针符号。考虑以下声明

int nums[2][3]  =  { {16, 18, 20}, {25, 26, 27} };

通常, nums [i] [j]等于*(*(nums + i)+ j)

指针符号	数组符号	值
*(*数字)	nums [0] [0]	16
*(*数字+ 1)	nums [0] [1]	18
*(*数字+ 2)	nums [0] [2]	20
*(*(数字+1))	nums [1] [0]	25
*(*(数字+1)+1)	nums [1] [1]	26
*(*(数字+1)+ 2)	nums [1] [2]	27