Pointers in C/C++

Pointers store addresses. In C, they are core to arrays, strings, DMA, and callbacks.

Example:

int a = 5;
int *p = &a;

printf("%d\n", a);    // 5
printf("%p\n", (void*)&a);
printf("%p\n", (void*)p);
printf("%d\n", *p);   // 5

Pointer size

Depends on architecture, not type:

So int*, char*, double* all same size on same machine.

Pointer type decides: 1) how many bytes to read on dereference
2) pointer arithmetic step size

Example:

int a = 1025;
int *p = &a;
char *c = (char*)p;

printf("%d\n", *p); // reads 4 bytes
printf("%d\n", *c); // reads 1 byte

This also relates to endianness.

int a = 10;
int *p = &a;
printf("%d", *p);

Pointer

Explains the pointer mental model—how an int* points to an integer variable and how an int** (pointer-to-pointer) points to the int* pointer.

int *ptr = NULL;

Always init pointers to avoid wild pointers.

Generic pointer:

void *vp;

Example:

int a=10;
void *vp=&a;
printf("%d\n", *(int*)vp);

Wild:

int *p; // uninitialized

Dangling:

int *p = malloc(4);
free(p);   // p now dangling
p = NULL;

If p is int* and p=200:

Formula:

p + n == p + n*sizeof(*p)

Allowed if inside same array:

ptr2 - ptr1

returns element distance.

❌ Adding pointers is invalid.

int x = 5;
int *p = &x;
int **q = &p;
int ***r = &q;

Used in:

C is call-by-value, but we emulate reference using pointers.

void inc(int a) { a++; }

void inc(int *p) { (*p)++; }