const int single pointer

In this section, you are going to learn

What is const int a = 5; ?

What is int const a = 5; ?

What is const int *p = &a; ?

What is int const *p = &a; ?

What is int *const p = &a; ?

What is const int *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 int a = 5;

• Example 2 : const int *p = &a;

• Example 3 : int *const p = &a;

• Example 4 : const int *const p = &a;

• Summary

Example 1 : const int a = 5;

• Step 1 : Consider the statement

const int a = 5;

• Step 2 : Remove all keywords after const

const a = 5;

• Step 3 : Remove everything before const

const a = 5;

• 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

const int a = 5;

a = 10; // --> This is invalid

Example 2 : const int *p = &a;

• Step 1 : Consider the statement

const int *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

int a = 5;

const int *p = &a;

*p = 10; // --> 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

int a = 5, b = 6;

const int *p = &a;

printf("*p is %d\n", *p); // prints 5

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

printf("*p is %d\n", *p); // prints 6

Example 3 : int *const p = &a;

• Step 1 : Consider the statement

int *const p = &a;

• Step 2 : Remove all keywords after const

int *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

int a = 5, b = 6;

int *const p = &a;

printf("*p is %d\n", *p); // prints 5

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

int a = 5, b = 6;

int *const p = &a;

printf("*p is %d\n", *p); // prints 5

*p = 100; // --> This is valid

printf("*p is %d\n", *p); // prints 100

Example 4 : const int *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 int *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 int *const p = &a;

• Step 2 : Remove all keywords after second const

const int *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

int a = 5, b = 6;

const int *const p = &a;

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

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

Summary

Statement	Meaning
const int a = 5;	a CAN NOT be changed again
int const a = 5;	a CAN NOT be changed again
const int *p = &a;	*p CAN NOT be changed p can be changed
int const *p = &a;	*p CAN NOT be changed p can be changed
int *const p = &a;	*p can be changed p CAN NOT be changed
const int *const p = &a;	*p CAN NOT be changed p CAN NOT be changed

See Also

• Current Module

  • Const Pointers

• Previous Module

  • Memcpy and Pointers

• Next Module

  • void Pointers

• Other Modules

  • Basic Equations

  • Basic Arrays

  • Malloc and Pointers

  • Type-Casting and Pointers

  • Functions and Pointers

  • Array of Pointers

  • Pointer to an Array

  • Function Pointers

  • Pre Increment with Pointers

  • Post Increment with Pointers

  • Pre decrement with Pointers

  • Post Decrement with Pointers