指针的声明一般要加*，在类型名之后加*，来声明指针变量，指针变量存储的是地址。

• A pointer is declared like a variable, but with * after the type.
• What stored in a pointer variable is an address.
• Operator & can take the address of an object or a variable of fundamental types.
  • &: 取地址符号
• Operator can *take the content that the pointer points to
  • *: 对该指针取内容 ```cpp int num = 10; int p1 = NULL, p2 = NULL; // declaration two pointers, initialized to 0， NULL == 0 p1 = # // take the address of num, assign to p1 p2 = # // take the address of num, assign to p2

p1 = 20; // assign 20 to num p2 = 30; // assign 20 to num

How pointers work?
| ![image.png](https://cdn.nlark.com/yuque/0/2021/png/353587/1636874906986-ef59a227-03eb-47db-b2ca-fbcdce4e9787.png#clientId=u068b3ef7-73ab-4&from=paste&height=508&id=nPByx&margin=%5Bobject%20Object%5D&name=image.png&originHeight=508&originWidth=369&originalType=binary&ratio=1&size=19857&status=done&style=none&taskId=ua2b49780-0bb1-4cb4-b324-5c3697be1c4&width=369) | ```cpp
#include <iostream>
using namespace std;
int main()
{
    int num = 10;
    int * p1 = NULL, * p2 = NULL; // declaration, initialize to 0
    p1 = &num; // take the address of num, assign to p1
    p2 = &num; // take the address of num, assign to p2
    cout << "num = " << num << endl;
    *p1 = 20; // assign to num
    cout << "num = " << num << endl;
    *p2 = 30; // assign to num
    cout << "num = " << num << endl;
    cout << "p1 = " << *p1 << endl;  // p1's value has been modified
    return 0;
}
// results
// num = 10
// num = 20
// num = 30
// p1 = 30

| | —- | —- |

num是个整数，要占用四个字节，

Structure member accessing ，结构体成员读取
取成员的两种方法，等价：

• p->member
• (*p).member | | ```cpp

  include

  include

  using namespace std;

struct Student { char name[4]; int born; bool male; };

int main() { Student stu = {“Yu”, 2000, true}; Student *pStu = &stu;

cout << stu.name << " was born in " << stu.born
     << ". Gender: " << (stu.male ? "male" : "female") << endl;

strncpy(pStu->name, "Li", 4);
pStu->born = 2001;
(*pStu).born = 2002;
pStu->male = false;

cout << stu.name << " was born in " << stu.born
     << ". Gender: " << (stu.male ? "male" : "female") << endl;

printf("Address of stu: %p\n", pStu);       //C style
cout << "Address of stu: " << pStu << endl; //C++ style
cout << "Address of stu: " << &stu << endl;
cout << "Address of member name: " << &(pStu->name) << endl;
cout << "Address of member born: " << &(pStu->born) << endl;
cout << "Address of member male: " << &(pStu->male) << endl;

cout << "sizeof(pStu) = " << sizeof(pStu) << endl;

return 0;

}

// results // Yu was born in 2000. Gender: male // Li was born in 2002. Gender: female // Address of stu: 0x7ffda985df3c // Address of stu: 0x7ffda985df3c // Address of stu: 0x7ffda985df3c // Address of member name: 0x7ffda985df3c // Address of member born: 0x7ffda985df40 // Address of member male: 0x7ffda985df44 // sizeof(pStu) = 8

 |
| --- | --- |

Print out the addresses

- Since the value of a pointer is an address, we can print it out
- The address should be an unsigned 32-bit or 64-bit integer.
```cpp
cout << "sizeof(pStu) = " << sizeof(pStu) << endl;  // 取决于系统是32位（4），还是64位（8）

指针就是存储地址的特殊变量。

Pointers are variables, they also have addresses。
指针是一个变量，所以指针也有地址

| | ```cpp

include

using namespace std;

int main() { int num = 10; int p = # int **pp = &p; // 指针的指针 (*pp) = 20;

cout << "num = " << num << endl;

return 0;

} // results // num = 20

 |
| --- | --- |

Constant pointers 常数指针
```cpp
#include <iostream>
using namespace std;

int foo(const char *p)
{
    // the value that p points to cannot be changed
    // play a trick?
    char *p2 = p; //syntax error；const指针赋值给普通指针是不允许的
    //...
    return 0;
}

int main()
{
    int num = 1;
    int another = 2;

    // You cannot change the value that p1 points to through p1
    // 指针指向的内容不可通过该指针去修改
    const int *p1 = &num;
    *p1 = 3;  //error
    num = 3;  //okay

    // You cannot change value of p2 (address)
    // 可以修改指针指向的内容，但是不可以修改指针指向的地址
    int *const p2 = &num;
    *p2 = 3;       //okay
    p2 = &another; //error，始终指向num的地址，不可修改地址

    //You can change neither
    const int *const p3 = &num;
    *p3 = 3;       //error
    p3 = &another; // error

    return 0;
}