Core CS Concept
Memory addressing explained through the lens of CS50, CS61C, and CMU 15-213 work. Dereference, address-of, pointer arithmetic, NULL and dangling pointers, smart pointers in modern C++, with real gdb sessions and Valgrind output for double-free bugs. Verified CS graduates with Memory Systems specialization, starting at $20 per task.
What it means
A pointer is a variable that stores a memory address. Dereferencing the pointer accesses the value at that address. C and C++ programs use pointers for pass-by-reference, dynamic allocation, and linked data structures.
C and C++ expose pointers explicitly with & and *; Java and Python wrap the same machinery as implicit references with null and None as the empty sentinel. Rust adds a borrow checker that statically prevents the dangling-pointer and double-free bugs that haunt C.
Primary example
#include <stdio.h>
int main(void) {
int x = 42;
int *p = &x; // p stores the address of x
printf("x = %d\n", x); // 42
printf("p = %p\n", (void*)p); // 0x7ffeefbff5ac (example)
printf("*p = %d\n", *p); // 42 (dereference)
*p = 100; // modify x through p
printf("x = %d\n", x); // 100
return 0;
}
Three pointer patterns
The & operator takes a variable and returns its address; the * operator takes a pointer and returns the value at that address. They are inverses: *(&x) is x, and &(*p) is p. Reading int *p inside-out: "p is a pointer such that *p is an int".
Adding 1 to a pointer advances it by sizeof(pointed-to type), not by 1 byte. For int *p, p + 1 advances by 4 bytes; for char *p, by 1 byte. This is why arr[i] is identical to *(arr + i): the compiler scales i by the element size before dereferencing.
unique_ptr owns the pointee exclusively, with automatic delete at scope exit. shared_ptr uses reference counting for joint ownership. weak_ptr breaks shared_ptr cycles. Default to unique_ptr; raw new is a code smell in modern C++.
Wrong way vs right way
#include <stdlib.h>
#include <stdio.h>
int main(void) {
int *p = malloc(sizeof(int));
// p could be NULL here under memory pressure
*p = 42; // segfault if malloc failed
printf("%d\n", *p);
free(p);
// p still holds the freed address; later code
// can dereference it and corrupt the heap
return 0;
}#include <stdlib.h>
#include <stdio.h>
int main(void) {
int *p = malloc(sizeof(int));
if (p == NULL) { // ALWAYS check malloc return
fprintf(stderr, "out of memory\n");
return 1;
}
*p = 42; // safe now
printf("%d\n", *p);
free(p);
p = NULL; // prevent dangling pointer
return 0;
}Cross-language
#include <memory>
#include <iostream>
struct Node {
int value;
std::unique_ptr<Node> next;
Node(int v) : value(v), next(nullptr) {}
};
int main() {
// unique_ptr: exclusive ownership, no leak
auto head = std::make_unique<Node>(1);
head->next = std::make_unique<Node>(2);
head->next->next = std::make_unique<Node>(3);
// No need to delete; unique_ptr destructors
// cascade and free all 3 nodes when head goes out of scope.
// shared_ptr: reference-counted ownership
auto sp1 = std::make_shared<int>(42);
auto sp2 = sp1; // ref count = 2
std::cout << sp1.use_count() << std::endl; // 2
return 0;
}FAQ
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