constant pointer vs pointer on a constant value [duplicate]

Question

char * const a;

means that the pointer is constant and immutable but the pointed data is not.
You could use const_cast(in C++) or c-style cast to cast away the constness in this case as data itself is not constant.

const char * a;

means that the pointed data cannot be written to using the pointer a. Using a const_cast(C++) or c-style cast to cast away the constness in this case causes Undefined Behavior.

To parse complicated types, you start at the variable, go left, and spiral outwards. If there aren't any arrays or functions to worry about (because these sit to the right of the variable name) this becomes a case of reading from right-to-left.

So with char *const a; you have a, which is a const pointer (*) to a char. In other words you can change the char which a is pointing at, but you can't make a point at anything different.

Conversely with const char* b; you have b, which is a pointer (*) to a char which is const. You can make b point at any char you like, but you cannot change the value of that char using *b = ...;.

You can also of course have both flavours of const-ness at one time: const char *const c;.

char * const a;

*a is writable, but a is not; in other words, you can modify the value pointed to by a, but you cannot modify a itself. a is a constant pointer to char.

const char * a; 

a is writable, but *a is not; in other words, you can modify a (pointing it to a new location), but you cannot modify the value pointed to by a.

Note that this is identical to

char const * a;

In this case, a is a pointer to a const char.

Now that you know the difference between char * const a and const char * a. Many times we get confused if its a constant pointer or pointer to a constant variable.

How to read it? Follow the below simple step to identify between upper two.

Lets see how to read below declaration

char * const a;

read from Right to Left

Now start with a,

1 . adjacent to a there is const.

char * (const a);

---> So a is a constant (????).

2 . Now go along you get *

char (* (const a));

---> So a is a constant pointer to (????).

3 . Go along and there is char

(char (* (const a)));

---> a is a constant pointer to character variable

a is constant pointer to character variable. 

Isn't it easy to read?

Similarily for second declaration

const char * a;

Now again start with a,

1 . Adjacent to a there is *

---> So a is a pointer to (????)

2 . Now there is char

---> so a is pointer character,

Well that doesn't make any sense!!! So shuffle pointer and character

---> so a is character pointer to (?????)

3 . Now you have constant

---> so a is character pointer to constant variable

But though you can make out what declaration means, lets make it sound more sensible.

a is pointer to constant character variable

The easiest way to understand the difference is to think of the different possibilities. There are two objects to consider, the pointer and the object pointed to (in this case 'a' is the name of the pointer, the object pointed to is unnamed, of type char). The possibilities are:

1. nothing is const
2. the pointer is const
3. the object pointed to is const
4. both the pointer and the pointed to object are const.

These different possibilities can be expressed in C as follows:

1. char * a;
2. char * const a;
3. const char * a;
4. const char * const a;

I hope this illustrates the possible differences

The first is a constant pointer to a char and the second is a pointer to a constant char. You didn't touch all the cases in your code:

char * const pc1 = &a; /* You can't make pc1 point to anything else */
const char * pc2 = &a; /* You can't dereference pc2 to write. */

*pc1 = 'c' /* Legal. */
*pc2 = 'c' /* Illegal. */

pc1 = &b; /* Illegal, pc1 is a constant pointer. */
pc2 = &b; /* Legal, pc2 itself is not constant. */