C和C++中的指针|集合1(简介，算术和数组)

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。

• 为了访问变量的指针地址，我们使用一元运算运算符＆ (＆)来返回该变量的地址。例如＆x给我们变量x的地址。

C

// 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 
 
int main()
{
    int x;
 
    // Prints address of x
    printf("%p", &x);
 
    return 0;
}

• 一元运算运算符是一元* (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

// C program to demonstrate declaration of
// pointer variables.
#include 
int main()
{
    int x = 10;
 
    // 1) Since there is * in declaration, ptr
    // becomes a pointer variable (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Dereferencing 。

C++

// C++ program to demonstrate use of * for pointers in C++
#include 
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 
 
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

• 以下是上述程序的图示：

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(—)
• 可以将一个整数添加到指针(+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，因为不知道它指向的是什么。减去两个地址可让您计算这两个地址之间的偏移量。

CPP

// C++ program to illustrate Pointer Arithmetic
// in C/C++
#include 
 
// 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val和＆val [0]可以互换使用。

CPP

// C++ program to illustrate Array Name as Pointers in C++
#include 
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val数组。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)

Pointer Notation	Array Notation	Value
*(*nums)	nums[0][0]	16
*(*nums + 1)	nums[0][1]	18
*(*nums + 2)	nums[0][2]	20
*(*(nums + 1))	nums[1][0]	25
*(*(nums + 1) + 1)	nums[1][1]	26
*(*(nums + 1) + 2)	nums[1][2]	27

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