Const Pointers

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Single Pointers

• Pointer to an int

The pointer can point to different integers and the int’s can be changed through the pointer. This sample of code assigns b to point to int b then changes b’s value to 100.

int b;
int* p;
p = &b;    /* OK */
*p = 100;  /* OK */
• Pointer to a const int
The pointer can point to different integers but the `int`'s value can't be changed through the pointer.

int b;
const int* p;
p = &b;    /* OK */
*p = 100;  /* Compiler Error */
• const pointer to int

The pointer can only point to one int but the int’s value can be changed through the pointer.

int a, b;
int* const p = &b; /* OK as initialisation, no assignment */
*p = 100;  /* OK */
p = &a;    /* Compiler Error */
• const pointer to const int

The pointer can only point to one int and the int can not be changed through the pointer.

int a, b;
const int* const p = &b; /* OK as initialisation, no assignment */
p = &a;   /* Compiler Error */
*p = 100; /* Compiler Error */

Pointer to Pointer

• Pointer to a pointer to an int

This code assigns the address of p1 to the to double pointer p (which then points to int* p1 (which points to int)).

Then changes p1 to point to int a. Then changes the value of a to be 100.

void f1(void)
{
int a, b;
int *p1;
int **p;
p1 = &b; /* OK */
p = &p1; /* OK */
*p = &a; /* OK */
**p = 100; /* OK */
}
• Pointer to pointer to a const int
void f2(void)
{
int b;
const int *p1;
const int **p;
p = &p1; /* OK */
*p = &b; /* OK */
**p = 100; /* error: assignment of read-only location ‘**p’ */
}
• Pointer to const pointer to an int
void f3(void)
{
int b;
int *p1;
int * const *p;
p = &p1; /* OK */
*p = &b; /* error: assignment of read-only location ‘*p’ */
**p = 100; /* OK */
}
• const pointer to pointer to int
void f4(void)
{
int b;
int *p1;
int ** const p = &p1; /* OK as initialisation, not assignment */
p = &p1; /* error: assignment of read-only variable ‘p’ */
*p = &b; /* OK */
**p = 100; /* OK */
}
• Pointer to const pointer to const int
void f5(void)
{
int b;
const int *p1;
const int * const *p;
p = &p1; /* OK */
*p = &b; /* error: assignment of read-only location ‘*p’ */
**p = 100; /* error: assignment of read-only location ‘**p’ */
}
• const pointer to pointer to const int
void f6(void)
{
int b;
const int *p1;
const int ** const p = &p1; /* OK as initialisation, not assignment */
p = &p1; /* error: assignment of read-only variable ‘p’ */
*p = &b; /* OK */
**p = 100; /* error: assignment of read-only location ‘**p’ */
}
• const pointer to const pointer to int
void f7(void)
{
int b;
int *p1;
int * const * const p = &p1; /* OK as initialisation, not assignment */
p = &p1; /* error: assignment of read-only variable ‘p’  */
*p = &b; /* error: assignment of read-only location ‘*p’ */
**p = 100; /* OK */
}