const char double pointer

In this section, you are going to learn

What is const char **q; ?

What is const char *const *q; ?

What is const char *const *const q; ?

Inorder to answer above questions, let us remember a very simple rule

Anything after const keyword CAN NOT be changed

Is it that simple ?

Yes. Let us see how do we apply above rule to answer the questions

Topics in this section,

Example 1 : const char **q;

Example 2 : const char *const *q;

Example 3 : const char *const *const q;

Summary

Example 1 : const char **q;

Step 1 : Consider the statement

const char **q;

Step 2 : Remove all keywords after const

const **q;

Step 3 : Remove everything before const

const **q;

Step 4 : Remove assignment

const **q;

Example 1 : Snippet 1

Step 5 : Now apply the rule. Anything after const keyword CAN NOT be changed

We see **q after const
Means, **q CAN NOT be changed again in next line

char a = 5;
char *p;
const char **q;

p = &a;
q = &p;
**q = 100; // --> This is invalid

Example 1 : Snippet 2

Step 6 : Bonus point ! q, *q, **q are different

Means, q and *q can still be changed

char a = 5, b = 6;
char *x, *y;
const char **q;

x = &a;
q = &x;  // --> This is valid

y = &b;
*q = y; // --> This is valid

printf("**q = %d\n", **q); // --> prints 6

Example 2 : const char *const *q;

There are two occurences of const keyword
Hence, let us apply the same rules two times

2.1 : Rules w.r.t first const

Step 1 : Consider the statement

const char *const *q;

Step 2 : Remove all keywords after first const

const **q;

Step 3 : Remove everything before first const

const **q;

Step 4 : Remove assignment

const **q;

Step 5 : Now apply the rule. Anything after const keyword CAN NOT be changed

**q CAN NOT be changed

2.2 : Rules w.r.t second const

Step 1 : Consider the statement

const char *const *q;

Step 2 : Remove all keywords after second const

const char *const *q;

Step 3 : Remove everything before second const

const *q;

Step 4 : Remove assignment

const *q;

Step 5 : Now apply the rule. Anything after const keyword CAN NOT be changed

*q CAN NOT be changed

Summary of Example 2

**q, *q CAN NOT be changed
q can be changed

Example 3 : const char *const *const q;

There are three occurences of const keyword
Hence, let us apply the same rules three times

3.1 : Rules w.r.t first const

Step 1 : Consider the statement

const char *const *const q;

Step 2 : Remove all keywords after first const

const **q;

Step 3 : Remove everything before first const

const **q;

Step 4 : Remove assignment

const **q;

Step 5 : Now apply the rule. Anything after const keyword CAN NOT be changed

**q CAN NOT be changed

3.2 : Rules w.r.t second const

Step 1 : Consider the statement

const char *const *const q;

Step 2 : Remove all keywords after second const

const char *const *q;

Step 3 : Remove everything before second const

const *q;

Step 4 : Remove assignment

const *q;

Step 5 : Now apply the rule. Anything after const keyword CAN NOT be changed

*q CAN NOT be changed

3.3 : Rules w.r.t third const

Step 1 : Consider the statement

const char *const *const q;

Step 2 : Remove all keywords after third const

const char *const *const q;

Step 3 : Remove everything before third const

const q;

Step 4 : Remove assignment

const q;

Step 5 : Now apply the rule. Anything after const keyword CAN NOT be changed

q CAN NOT be changed

Summary of Example 3

**q, *q, q CAN NOT be changed

Summary

Statement	Meaning
const char **q;	`*q` CAN NOT be changed again `q` can be changed again `q` can be changed again
const char const q;	`*q` CAN NOT be changed again `q` CAN NOT be changed again `q` can be changed again
const char const const q;	`*q` CAN NOT be changed again `q` CAN NOT be changed again `q` CAN NOT be changed again