const char *p，char *const p和const char * const p之间的区别

先决条件：指针

当char, const, *, p都用在不同的排列中时, 含义会根据放置在何处而变化, 这会引起很多混乱。下一篇文章重点介绍所有这些的区别和用法。

预选赛const可以应用于任何变量的声明以指定其值不会更改。 const关键字适用于左侧的任何内容。如果左侧没有任何内容, 则适用于右侧的任何内容。

const char * ptr：

这是指向常量字符的指针。

你不能更改ptr所指向的值, 但是可以更改指针本身。 ” const char *”是指向const char的(非const)指针。

//C program to illustrate 
//char const *p
#include<stdio.h>
#include<stdlib.h>
  
int main()
{
     char a = 'A' , b = 'B' ;
     const char *ptr = &a;
      
     //*ptr = b; illegal statement (assignment of read-only location *ptr)
      
     //ptr can be changed
     printf ( "value pointed to by ptr: %c\n" , *ptr);
     ptr = &b;
     printf ( "value pointed to by ptr: %c\n" , *ptr);
}

输出如下：

value pointed to by ptr:A
value pointed to by ptr:B

注意：两者之间没有区别const char * p和char const * p因为两者都是指向const char的指针, 并且’*'(asterik)的位置也相同。

char * const ptr：这是指向非恒定字符的恒定指针。你不能更改指针p, 但可以更改ptr指向的值。

//C program to illustrate 
//char* const p
#include<stdio.h>
#include<stdlib.h>
  
int main()
{
     char a = 'A' , b = 'B' ;
     char * const ptr = &a;
     printf ( "Value pointed to by ptr: %c\n" , *ptr);
     printf ( "Address ptr is pointing to: %d\n\n" , ptr);
  
     //ptr = &b; illegal statement (assignment of read-only variable ptr)
  
     //changing the value at the address ptr is pointing to
     *ptr = b; 
     printf ( "Value pointed to by ptr: %c\n" , *ptr);
     printf ( "Address ptr is pointing to: %d\n" , ptr);
}

输出如下：

Value pointed to by ptr: A
Address ptr is pointing to: -1443150762

Value pointed to by ptr: B
Address ptr is pointing to: -1443150762

注意：指针始终指向相同的地址, 仅更改该位置的值。

const char * const ptr：这是指向常量字符的常量指针。你既不能更改ptr所指向的值, 也不能更改指针ptr。

//C program to illustrate 
//const char * const ptr 
#include<stdio.h>
#include<stdlib.h>
  
int main()
{
     char a = 'A' , b = 'B' ;
     const char * const ptr = &a;
      
     printf ( "Value pointed to by ptr: %c\n" , *ptr);
     printf ( "Address ptr is pointing to: %d\n\n" , ptr);
  
     //ptr = &b; illegal statement (assignment of read-only variable ptr)
     //*ptr = b; illegal statement (assignment of read-only location *ptr)
  
}

输出如下：

Value pointed to by ptr: A
Address ptr is pointing to: -255095482

注意：char const * const ptr与…相同const char * const ptr.

const关键字测验

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