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Pointers

Carry around where a value lives (its address) and change the caller's variable.

This lesson is for C++ · C.
Python, JavaScript, TypeScript, Java and Craykon have no pointers that hold raw addresses; objects are reached through references (see passing lists in the functions lesson).
C# allows pointers only inside unsafe code.

A pointer is a variable that holds the memory address of a value instead of the value itself.
The example below swaps two variables by passing their addresses, then takes an array through a pointer to find its sum and its largest element.

#include <stdio.h> void swap(int *a, int *b) {    int t = *a;    *a = *b;    *b = t;} int sum(const int *p, int n) {  /* 배열은 첫 칸의 포인터로 넘어와요 */    int s = 0;    for (const int *end = p + n; p < end; p++) s += *p;    return s;} int *find_max(int *p, int n) {    if (n == 0) return NULL;    int *best = p;    for (int i = 1; i < n; i++)        if (p[i] > *best) best = p + i;    return best;} int main(void) {    int a, b, n, arr[100];    scanf("%d %d", &a, &b);    swap(&a, &b);    printf("%d %d\n", a, b);    scanf("%d", &n);    for (int i = 0; i < n; i++) scanf("%d", arr + i);    printf("%d\n", sum(arr, n));    int *m = find_max(arr, n);    if (m != NULL) printf("%d %td\n", *m, m - arr);    return 0;}
stdin
3 8
4
5 1 7 2
stdout
8 3
15
7 2

Key points

  • &x is the address of x, and *p (dereferencing) is the value at the place p points to.
    After int *p = &x;, *p = 5; means x = 5;.
    You have used addresses ever since passing &n to scanf.
  • Passing a number copies it, so changes inside the function don't reach the caller.
    To change the caller's variable, pass its address: swap(&a, &b).
  • An array passed to a function or used in an expression turns into a pointer to its first element (decay).
    That's why p[i] is exactly *(p + i).
  • Pointer arithmetic: p + 1 points one element ahead (sizeof(int), usually 4 bytes), not one byte.
    Subtracting two pointers into the same array (m - arr) gives a count of elements, i.e. the index.
  • A pointer may point just past the end of an array (p + n), but you must not read that spot.
  • A pointer to nothing is NULL.
    The example's find_max returns NULL for zero elements, and the caller checks before reading.

How each language writes it

C++Pointers work as in C; the null pointer is nullptr (since C++11) — to change the caller, a reference void swap(int& a, int& b) is simpler, and in practice you use std::swap(a, b).
A reference is an alias that can't be null or be re-pointed, so use pointers only when you really need them
Cint *p = &x;, read and write through *p — the null pointer is NULL.
A function that changes the caller is void swap(int *a, int *b); one that takes an array also takes its length, like int sum(const int *p, int n)

Common mistakes

Dereferencing NULL

int *m = find_max(arr, 0);
printf("%d\n", *m);

→ Undefined behaviour, usually a crash with Segmentation fault. Nothing tells you where it went wrong, unlike Java's NullPointerException

Check if (m != NULL) before reading *m.

Returning a local variable's address

int *f(void) {
    int x = 3;
    return &x;
}

→ x disappears when the function returns, so the pointer points at a place that no longer exists (a dangling pointer). GCC warns function returns address of local variable; running it prints garbage or crashes

Put the result in an array the caller provides, or in dynamic memory (next lesson).

Measuring a parameter array with sizeof

int count(int arr[]) {
    return sizeof(arr) / sizeof(arr[0]);
}

→ 8 / 4 = 2 on a 64-bit machine. The parameter int arr[] is really int *arr, so the length is lost (inside main, int arr[5] gives 5)

In C, take the length n separately; in C++, pass a std::vector or std::span (C++20), which carry their length.

How languages differ
Before dereferencing a pointer, always ask whether it points at something that is still alive.
Dereferencing NULL, a local variable of a function that has returned, or a spot outside an array makes C and C++ print garbage or crash, with no error message.

Practice

  1. A function that reverses ranges8P0062
  2. Min, max and sum through pointers2P0073
  3. Walking to the longest run with pointers2P0074
  4. Name tags pointing at the same list8P0262
  5. A stock ledger passed by reference1P0085
  6. Detaching train cars with a pointer to a pointer2P0201
  7. Fix by reference, find with lower_bound1P0125
  8. Cloned adventurers and a shared vault3P0159