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。

限定符const可以应用于任何变量的声明，以指定其值不会更改。 const关键字适用于紧靠其左侧的任何内容。如果左侧没有任何内容，则适用于右侧的任何内容。

1. const char * ptr：这是指向常量字符的指针。您不能更改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
   #include
     
   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：

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

   注意： const char * p和char const * p之间没有区别，因为它们都是指向const char的指针，并且’*’(asterik)的位置也相同。

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

   // C program to illustrate 
   // char* const p
   #include
   #include
     
   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：

   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：指针总是指向相同的地址，只有该位置的值被更改。

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

   // C program to illustrate 
   //const char * const ptr 
   #include
   #include
     
   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：

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

   注意：char const * const ptr与const char * const ptr相同。

const关键字测验

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