const struct single pointer

In this section, you are going to learn

What is const struct ABC ?

What is struct ABC const ?

What is const struct ABC *p = &a; ?

What is struct ABC const *p = &a; ?

What is struct ABC *const p = &a; ?

What is const struct ABC *const p = &a; ?

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 a;

Example 2 : const struct ABC *p = &a;

Example 3 : struct ABC *const p = &a ;

Example 4 : const struct ABC *const p = &a;

Summary

Example 1 : const struct ABC a

Step 1 : Consider the statement

struct ABC
{
        int x;
        int y;
};

const struct ABC a = { .x = 10, .y = 20 };

Step 2 : Remove all keywords after const

const a = { .x = 10, .y = 20 };

Step 3 : Remove everything before const

const a = { .x = 10, .y = 20 };

Step 4 : Remove assignment

const a;

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

We see a after const
Means, variable a CAN NOT be changed again in next line

Example 1 : Snippet 1

struct ABC
{
        int x;
        int y;
};

const struct ABC a = { .x = 10, .y = 20 };

a.x = 100; // --> This is invalid

a.y = 200; // --> This is invalid

Example 1 : Snippet 2

struct ABC
{
        int x;
        int y;
};

const struct ABC a = { .x = 10, .y = 20 };
struct ABC b = { .x = 100, .y = 200 };

a = b; // --> This is invalid

Example 2 : const struct ABC *p = &a;

Step 1 : Consider the statement

const struct ABC *p = &a;

Step 2 : Remove all keywords after const

const *p = &a;

Step 3 : Remove everything before const

const *p = &a;

Step 4 : Remove assignment

const *p;

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

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

struct ABC
{
        int x;
        int y;
};

struct ABC a = { .x = 10, .y = 20 };

const struct ABC *p = &a;

p->x = 100; // --> This is invalid

p->y = 200; // --> This is invalid

Step 6 : Bonus point ! *p and p are different. Means you can change p
- From Step 4, we derived that “const *p”
- Means only *p is const and CAN NOT be changed
- But remember p is entirely different
- There is nothing which says p is constant
- Hence p can still be changed
- See below example

struct ABC a , b;

const struct ABC *p = &a;

p = &b; // --> This is valid

Example 3 : struct ABC *const p = &a;

Step 1 : Consider the statement

struct ABC *const p = &a;

Step 2 : Remove all keywords after const

struct ABC *const p = &a;

Step 3 : Remove everything before const

const p = &a;

Step 4 : Remove assignment

const p;

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

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

struct ABC a, b;

struct ABC *const p = &a;

p = &b; // --> This is invalid

Step 6 : Bonus point ! *p and p are different. Means you can change *p
- From Step 4, we derived that “const p”
- Means only p is const and CAN NOT be changed
- But remember *p is entirely different
- There is nothing which says *p is constant
- Hence *p can still be changed
- See below example

struct ABC a, b;

struct ABC *const p = &a;

p->x = 100; // --> This is valid
p->y = 200; // --> This is valid

Example 4 : const struct ABC *const p = &a;

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

4.1 : Rules w.r.t first const

Step 1 : Consider the statement

const struct ABC *const p = &a;

Step 2 : Remove all keywords after first const

const *p = &a;

Step 3 : Remove everything before first const

const *p = &a;

Step 4 : Remove assignment

const *p;

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

*p CAN NOT be changed

4.2 : Rules w.r.t second const

Step 1 : Consider the statement

const struct ABC *const p = &a;

Step 2 : Remove all keywords after second const

const struct ABC *const p = &a;

Step 3 : Remove everything before second const

const p = &a;

Step 4 : Remove assignment

const p;

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

p CAN NOT be changed

Summary of Example 4

Both p and *p CAN NOT be changed

struct ABC a, b;

const struct ABC *const p = &a;

p->x = 100; // --> This is invalid
p->y = 200; // --> This is invalid

p = &b;   // --> This is invalid

Summary

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