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Pointer

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Existing English translations were reused. 1 current source blocks remain in Simplified Chinese, without triggering paid translation.

1.Pointer

1.1.Basic concepts of pointers

Role of pointers: Memory can be accessed indirectly through pointers

  • Memory addresses are numbered starting from 0 and are generally represented in hexadecimal.

  • Pointer variables can be used to store addresses.

    ​

1.2.Definition and use of pointer variables

Pointer variable definition syntax: 数据类型 * 变量名；

Example:

int main() {

	//1、指针的定义
	int a = 10; //定义整型变量a
	
	//指针定义语法： 数据类型 * 变量名 ;
	int * p;

	//指针变量赋值
	p = &a; //指针指向变量a的地址
	cout << &a << endl; //打印数据a的地址
	cout << p << endl;  //打印指针变量p

	//2、指针的使用
	//通过*操作指针变量指向的内存
	cout << "*p = " << *p << endl;


	return 0;
}

Running result (the address may differ on each run; denoted below by <地址>):

<地址>
<地址>
*p = 10

The difference between pointer variables and ordinary variables

  • An ordinary variable stores data, while a pointer variable stores an address.
  • A pointer variable can use the "*" operator to access the memory space that the pointer variable points to; this process is called dereferencing.

Summary 1: We can obtain the address of a variable using the & symbol

Summary 2: Pointers can be used to record addresses.

Summary 3: Dereferencing a pointer variable allows manipulation of the memory pointed to by the pointer

1.3.Memory space occupied by a pointer

Question: A pointer is also a data type, so how much memory space does this data type occupy?

Example:

int main() {

	int a = 10;

	int * p;
	p = &a; //指针指向数据a的地址

	cout << *p << endl; //* 解引用
	cout << sizeof(p) << endl;
	cout << sizeof(char *) << endl;
	cout << sizeof(float *) << endl;
	cout << sizeof(double *) << endl;


	return 0;
}

Run result (type size or type name may vary depending on compiler and platform):

10
8
8
8
8

Summary: All pointer types are 4 bytes under a 32-bit operating system.

1.4.Null pointers and wild pointers

Null pointer : a pointer variable points to the memory space with address 0

**Purpose:** Initialize a pointer variable

**Note:** The memory pointed to by a null pointer cannot be accessed.

Example 1: Null pointer

int main() {

	//指针变量p指向内存地址编号为0的空间
	int * p = NULL;

	//访问空指针报错 
	//内存编号0 ~255为系统占用内存，不允许用户访问
	cout << *p << endl;


	return 0;
}

Run result: The program triggered undefined behavior; this run received SIGSEGV and then terminated abnormally, with no reliable normal output.

Wild pointer: A pointer variable points to an invalid memory space

Example 2: Wild pointer

int main() {

	//指针变量p指向内存地址编号为0x1100的空间
	int * p = (int *)0x1100;

	//访问野指针报错 
	cout << *p << endl;


	return 0;
}

Run result: The program triggered undefined behavior; this run received SIGSEGV and then terminated abnormally, with no reliable normal output.

Summary: Neither null pointers nor wild pointers refer to space we have allocated, so do not access them.

1.5.const-qualified pointer

There are three cases of const modifying pointers.

  1. const modifying pointer --- pointer to constant
  2. const modifying constant --- constant pointer
  3. const modifying both pointer and constant

Example:

int main() {

	int a = 10;
	int b = 10;

	//const修饰的是指针，指针指向可以改，指针指向的值不可以更改
	const int * p1 = &a; 
	p1 = &b; //正确
	//*p1 = 100;  报错
	

	//const修饰的是常量，指针指向不可以改，指针指向的值可以更改
	int * const p2 = &a;
	//p2 = &b; //错误
	*p2 = 100; //正确

    //const既修饰指针又修饰常量
	const int * const p3 = &a;
	//p3 = &b; //错误
	//*p3 = 100; //错误


	return 0;
}

Run result: The program ended normally, with no output in the terminal.

Tip: Look at whether a pointer or a constant immediately follows the right side of `const`. If it is a pointer, it is a pointer to const; if it is a constant, it is a const pointer.

1.6.Pointers and arrays

**Purpose:** Use pointers to access elements in an array

Example:

int main() {

	int arr[] = { 1,2,3,4,5,6,7,8,9,10 };

	int * p = arr;  //指向数组的指针

	cout << "第一个元素： " << arr[0] << endl;
	cout << "指针访问第一个元素： " << *p << endl;

	for (int i = 0; i < 10; i++)
	{
		//利用指针遍历数组
		cout << *p << endl;
		p++;
	}


	return 0;
}

Operation result:

第一个元素： 1
指针访问第一个元素： 1
1
2
3
4
5
6
7
8
9
10

1.7.Pointers and Functions

**Purpose:** Using pointers as function parameters allows modifying the values of the actual arguments.

Example:

//值传递
void swap1(int a ,int b)
{
	int temp = a;
	a = b; 
	b = temp;
}
//地址传递
void swap2(int * p1, int *p2)
{
	int temp = *p1;
	*p1 = *p2;
	*p2 = temp;
}

int main() {

	int a = 10;
	int b = 20;
	swap1(a, b); // 值传递不会改变实参

	swap2(&a, &b); //地址传递会改变实参

	cout << "a = " << a << endl;

	cout << "b = " << b << endl;


	return 0;
}

Operation result:

a = 20
b = 10

Summary: If you do not want to modify the actual argument, use pass-by-value; if you want to modify the actual argument, use pass-by-address

1.8.Pointers, Arrays, Functions

**Case Description:** Encapsulate a function that uses bubble sort to sort an integer array in ascending order.

例如数组：int arr[10] = { 4,3,6,9,1,2,10,8,7,5 };

Example:

//冒泡排序函数
void bubbleSort(int * arr, int len)  //int * arr 也可以写为int arr[]
{
	for (int i = 0; i < len - 1; i++)
	{
		for (int j = 0; j < len - 1 - i; j++)
		{
			if (arr[j] > arr[j + 1])
			{
				int temp = arr[j];
				arr[j] = arr[j + 1];
				arr[j + 1] = temp;
			}
		}
	}
}

//打印数组函数
void printArray(int arr[], int len)
{
	for (int i = 0; i < len; i++)
	{
		cout << arr[i] << endl;
	}
}

int main() {

	int arr[10] = { 4,3,6,9,1,2,10,8,7,5 };
	int len = sizeof(arr) / sizeof(int);

	bubbleSort(arr, len);

	printArray(arr, len);


	return 0;
}

Run result (type size or type name may vary depending on compiler and platform):

1
2
3
4
5
6
7
8
9
10

Summary: When an array name is passed to a function as an argument, it decays to a pointer to its first element