const*和*const

References:

• const*和*const

The data types of const char * and char * const in C language

In C, const char * and char * const are two different pointer types with different meanings regarding the mutability of the pointer and the data it points to. Let's break them down:

1. const char * (Pointer to constant character)

• This means the pointer can be changed, but the data it points to cannot be modified.
• The data is constant (read-only), but the pointer itself can be reassigned to point to different data.

Example:

const char *ptr = "Hello";
ptr = "World";  // OK: You can change the pointer to point to another string.
*ptr = 'A';     // Error: You cannot change the content of what the pointer points to.

• const char * is often used for string literals and other data that should not be modified, but the pointer can still be reassigned.

2. char * const (Constant pointer to character)

• This means the pointer cannot be changed (it’s constant), but the data it points to can be modified.
• The pointer is constant, but the data it points to is mutable.

Example:

char str[] = "Hello";
char * const ptr = str;
ptr = "World";  // Error: You cannot change the pointer itself (it's constant).
*ptr = 'A';     // OK: You can modify the content of the data pointed to.

• char * const is useful when you need a pointer to always point to the same memory location, but you want to modify the data at that location.

Summary:

• const char *: The data is constant, but the pointer can change.
• char * const: The pointer is constant, but the data can change.

Combined Example:

You can combine both, creating a constant pointer to constant data:

const char * const ptr = "Hello";
ptr = "World";  // Error: You cannot change the pointer.
*ptr = 'A';     // Error: You cannot change the content.

This means neither the pointer nor the data can be changed.

In summary:

• const char *: Pointer to a constant character (modifiable pointer, constant data).
• char * const: Constant pointer to a character (constant pointer, modifiable data).

Based on chatGPT

An example of how these two data types work in the aspect of C complier

// Online C compiler to run C program online
#include <stdio.h>

int main() {

    const char *pc;
    char const* ppc;
    
    char *const pppc;
    
    const char *const pppc;
    
    char c = pc;
    char cc = ppc;
    char ccc = pppc;
    
    pc = NULL;
    ppc = NULL;
    pppc = NULL;
    pppc = NULL;
    
    return 0;
}

/*
ERROR!
/tmp/gFNT6SlhJE.c: In function 'main':
/tmp/gFNT6SlhJE.c:11:23: error: conflicting types for 'pppc'; have 'const char * const'
   11 |     const char *const pppc;
      |                       ^~~~
ERROR!
/tmp/gFNT6SlhJE.c:9:17: note: previous declaration of 'pppc' with type 'char * const'
    9 |     char *const pppc;
      |                 ^~~~
/tmp/gFNT6SlhJE.c:13:14: warning: initialization of 'char' from 'const char *' makes integer from pointer without a cast [-Wint-conversion]
   13 |     char c = pc;
      |              ^~
/tmp/gFNT6SlhJE.c:14:15: warning: initialization of 'char' from 'const char *' makes integer from pointer without a cast [-Wint-conversion]
   14 |     char cc = ppc;
      |               ^~~
/tmp/gFNT6SlhJE.c:15:16: warning: initialization of 'char' from 'const char *' makes integer from pointer without a cast [-Wint-conversion]
   15 |     char ccc = pppc;
      |                ^~~~
/tmp/gFNT6SlhJE.c:19:10: error: assignment of read-only variable 'pppc'
   19 |     pppc = NULL;
      |          ^
/tmp/gFNT6SlhJE.c:20:10: error: assignment of read-only variable 'pppc'
   20 |     pppc = NULL;
      |          ^


=== Code Exited With Errors ===

*/