const char * const versus const char *?

Question

The latter prevents you from modifying the_string inside print_string. It would actually be appropriate here, but perhaps the verbosity put off the developer.

char* the_string : I can change which char the_string points to, and I can modify the char to which it points.

const char* the_string : I can change which char the_string points to, but I cannot modify the char to which it points.

char* const the_string : I cannot change which char the_string points to, but I can modify the char to which it points.

const char* const the_string : I cannot change which char the_string points to, nor can I modify the char to which it points.

1. Mutable pointer to a mutable character

   char *p;
   

2. Mutable pointer to a constant character

   const char *p;
   

3. Constant pointer to a mutable character

   char * const p; 
   

4. Constant pointer to a constant character

   const char * const p;
   

const char * const means pointer as well as the data the pointer pointed to, are both const!

const char * means only the data the pointer pointed to, is const. pointer itself however is not const.

Example.

const char *p = "Nawaz";
p[2] = 'S'; //error, changing the const data!
p="Sarfaraz"; //okay, changing the non-const pointer. 

const char * const p = "Nawaz";
p[2] = 'S'; //error, changing the const data!
p="Sarfaraz"; //error, changing the const pointer. 

(I know this is old but I wanted to share anyway.)

Just wanted to elaborate on Thomas Matthews' answer. The Right-Left Rule of C type declarations pretty much says: when reading a C type declaration start at the identifier and go right when you can and left when you can't.

This is best explained with a couple examples:

Example 1

• Start at the identifier, we can't go right so we go left

  const char* const foo
              ^^^^^
  

  foo is a constant...

• Continue left

  const char* const foo
            ^
  

  foo is a constant pointer to...

• Continue left

  const char* const foo
        ^^^^
  

  foo is a constant pointer to char...

• Continue left

  const char* const foo
  ^^^^^
  

  foo is a constant pointer to char constant (Complete!)

Example 2

• Start at the identifier, we can't go right so we go left

  char* const foo
        ^^^^^
  

  foo is a constant...

• Continue left

  char* const foo
      ^
  

  foo is a constant pointer to...

• Continue left

  char* const foo
  ^^^^
  

  foo is a constant pointer to char (Complete!)

Example 1337

• Start at the identifier, but now we can go right!

  const char* const* (*foo[8])()
                          ^^^
  

  foo is an array of 8...

• Hit parenthesis so can't go right anymore, go left

  const char* const* (*foo[8])()
                      ^
  

  foo is an array of 8 pointer to...

• Finished inside parenthesis, can now go right

  const char* const* (*foo[8])()
                              ^^
  

  foo is an array of 8 pointer to function that returns...

• Nothing more to the right, go left

  const char* const* (*foo[8])()
                   ^
  

  foo is an array of 8 pointer to function that returns a pointer to a...

• Continue left

  const char* const* (*foo[8])()
              ^^^^^
  

  foo is an array of 8 pointer to functions that returns a pointer to a constant...

• Continue left

  const char* const* (*foo[8])()
            ^
  

  foo is an array of 8 pointer to functions that returns a pointer to a constant pointer to a...

• Continue left

  const char* const* (*foo[8])()
        ^^^^
  

  foo is an array of 8 pointer to functions that returns a pointer to a constant pointer to a char...

• Continue left

  const char* const* (*foo[8])()
  ^^^^^
  

  foo is an array of 8 pointer to functions that returns a pointer to a constant pointer to a char constant (Complete!)

Further explanation: http://www.unixwiz.net/techtips/reading-cdecl.html

Many people suggest reading the type specifier from right to left.

const char * // Pointer to a `char` that is constant, it can't be changed.
const char * const // A const pointer to const data.

In both forms, the pointer is pointing to constant or read-only data.

In the second form, the pointer cannot be changed; the pointer will always point to the same place.