const structure triple pointer

In this section, you are going to learn

What is const struct ABC ***r; ?

What is const struct ABC *const **r; ?

What is const struct ABC *const *const *r; ?

What is const struct ABC *const *const *const r; ?

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

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 struct ABC ***r;

Example 2 : const struct ABC *const **r;

Example 3 : const struct ABC *const *const *r;

Example 4 : const struct ABC *const *const *const r;

Summary

Example 1 : const struct ABC ***r;

Step 1 : Consider the statement

const struct ABC ***r;

Step 2 : Remove all keywords after const

const ***r;

Step 3 : Remove everything before const

const ***r;

Step 4 : Remove assignment

const ***r;

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

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

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

Means, r, *r, **r can still be changed

Example 2 : const struct ABC *const **r;

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 struct ABC *const **r

Step 2 : Remove all keywords after first const

const ***r

Step 3 : Remove everything before first const

const ***r

Step 4 : Remove assignment

const ***r

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

***r CAN NOT be changed

2.2 : Rules w.r.t second const

Step 1 : Consider the statement

const struct ABC *const **r

Step 2 : Remove all keywords after second const

const struct ABC *const **r

Step 3 : Remove everything before second const

const **r

Step 4 : Remove assignment

const **r

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

**r CAN NOT be changed

Summary of Example 2

***r, **r CAN NOT be changed
*r, r can be changed

Example 3 : const struct ABC *const *const *r;

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 struct ABC *const *const *r;

Step 2 : Remove all keywords after first const

const ***r;

Step 3 : Remove everything before first const

const ***r;

Step 4 : Remove assignment

const ***r;

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

***r CAN NOT be changed

3.2 : Rules w.r.t second const

Step 1 : Consider the statement

const struct ABC *const *const *r;

Step 2 : Remove all keywords after second const

const struct ABC *const **r;

Step 3 : Remove everything before second const

const **r;

Step 4 : Remove assignment

const **r;

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

**r CAN NOT be changed

3.3 : Rules w.r.t third const

Step 1 : Consider the statement

const struct ABC *const *const *r;

Step 2 : Remove all keywords after third const

const struct ABC *const *const *r;

Step 3 : Remove everything before third const

const *r;

Step 4 : Remove assignment

const *r;

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

*r CAN NOT be changed

Summary of Example 3

***r, **r, *r, CAN NOT be changed
r can be changed

Example 4 : const struct ABC *const *const *const r;

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

4.1 : Rules w.r.t first const

Step 1 : Consider the statement

const struct ABC *const *const *const r;

Step 2 : Remove all keywords after first const

const ***r;

Step 3 : Remove everything before first const

const ***r;

Step 4 : Remove assignment

const ***r;

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

***r CAN NOT be changed

4.2 : Rules w.r.t second const

Step 1 : Consider the statement

const struct ABC *const *const *const r;

Step 2 : Remove all keywords after second const

const struct ABC *const **r;

Step 3 : Remove everything before second const

const **r;

Step 4 : Remove assignment

const **r;

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

**r CAN NOT be changed

4.3 : Rules w.r.t third const

Step 1 : Consider the statement

const struct ABC *const *const *const r;

Step 2 : Remove all keywords after third const

const struct ABC *const *const *r;

Step 3 : Remove everything before third const

const *r;

Step 4 : Remove assignment

const *r;

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

*r CAN NOT be changed

4.4 : Rules w.r.t fourth const

Step 1 : Consider the statement

const struct ABC *const *const *const r;

Step 2 : Remove all keywords after fourth const

const struct ABC *const *const *const r;

Step 3 : Remove everything before fourth const

const r;

Step 4 : Remove assignment

const r;

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

r CAN NOT be changed

Summary of Example 4

***r, **r, *r, r CAN NOT be changed

Summary

Statement	Meaning
const struct ABC ***r;	`*r` CAN NOT be changed again `r` can be changed again `*r` can be changed again `r` can be changed again
const struct ABC const *r;	`*r` CAN NOT be changed again `r` CAN NOT be changed again `*r` can be changed again `r` can be changed again
const struct ABC const const *r;	`*r` CAN NOT be changed again `r` CAN NOT be changed again `*r` CAN NOT be changed again `r` can be changed again
const struct ABC const const *const r;	`*r` CAN NOT be changed again `r` CAN NOT be changed again `*r` CAN NOT be changed again `r` can be changed again