[C Advance] 26. Analysis of the nature of pointers

Summary

1) The essence of the pointer is a variable. The special that the content stored in 161d0262cbdaf1 is a memory address.

2) When the pointer declared, * indicates that the declared variable is a pointer; when the pointer used, * indicates the value memory space pointed to by the pointer

3) Pointer as a function parameter:

• The pointer is a variable, so you can declare the pointer parameter
• When a function body needs change the value of the actual parameter
• You need to use pointer parameters; the will be copied to the formal parameter when the function is called
• Pointers are suitable for functions with complex data types as parameters

4) : 161d0262cbddf5 left and right-when const appears on the *, the data pointed to by is constant; when const appears on the *, the pointer of is constant.

const int* p;        // 指针p可变，指向的数据*p不可变
int const* p;        // 指针p可变，指向的数据*p不可变
int* const p;        // 指针p不可变，指向的数据*p可变
const int* const p;  // 指针p不可变，指向的数据*p也不可变

Analysis of the nature of pointers

variable in the program is just an alias for a section of storage space of , so do I have to use this alias to use this storage space?

1. The declaration and use of pointers

What does the following program output? Why

int i = 5;
int* p = &i;

printf("%d, %p\n", i, p);    // 5, 0XBFDB216C

*p = 10;

printf("%d, %p\n", i, p);    // 10, 0XBFDB216C

• When the pointer declared, * indicates that the declared variable is a pointer
• When the pointer used, * represents the value memory space pointed to by the pointer


2. Call by value and call by address

• The pointer is a variable, so you can declare the pointer parameter
• When a function body needs change the value of the actual parameter, you need to use the pointer parameter
• When the function is called, the will be copied to the formal parameter

• Pointers are suitable for complex data types of 161d0262cbe2be as parameters

  // 指针作为参数分析
  void swap1(int a, int b)
  {
    int t = a;
    a = b;
    b = a;
  }
  
  void swap2(int* pa, int* pb)
  {
    int t = *pa;
    *pa = *pb;
    *pb = t;
  }
  
  int main()
  {
    int a = 1, b = 2;
  
    swap1(a, b);                 // 将实参a和b的值，赋值给swap1参数a、b，函数内部实际修改的是函数的形参a和b，不会影响实参
    printf("%d, %d\n", a, b);    // 1, 2
    
    swap2(&a, &b);               // 将实参a和b的地址，赋值给参数指针pa和pb，函数内部使用*号打开了这两个地址，并修改了里面的值，所以外面的实参也会变
    printf("%d, %d\n", a, b);    // 2, 1
    return 0;
  }

3. const and pointer

Formula: left and right points - When const appears on the left side of *, the data pointed to is constant; when const appears on the right side of *, the pointer is constant.

const int* p;        // 指针p可变，指向的数据*p不可变
int const* p;        // 指针p可变，指向的数据*p不可变
int* const p;        // 指针p不可变，指向的数据*p可变
const int* const p;  // 指针p不可变，指向的数据*p也不可变

This article is summarized from "C Language Advanced Course" by Tang Zuolin from "Ditai Software Academy".
If there are any errors or omissions, please correct me.