Raw pointers
- Rust course
- Advanced
- Lesson 31/81
- 4 min
Use *const T and *mut T raw pointers.
Raw pointers
*const T and *mut T, unlike references: can be null, can point at invalid memory, don't follow borrowing rules and aren't cleaned up automatically. Arithmetic can be done on them:
let v = [10, 20, 30];
let p: *const i32 = v.as_ptr();
unsafe {
println!("{}", *p.add(1)); // 20
}
println!("{}", p.is_null()); // false
let bos: *const i32 = std::ptr::null();add(n) advances the pointer by n elements (not bytes). Going outside the array is undefined behaviour.
From references to pointers
Going from a reference to a raw pointer is always safe; the reverse is unsafe because you must guarantee the pointer is valid:
let x = 42;
let p = &x as *const i32; // safe
let p2 = std::ptr::addr_of!(x); // the same, without an intermediate reference
let r: &i32 = unsafe { &*p }; // unsafe: p must be validRaw pointers are used most in FFI and when writing your own data structures (linked lists, trees).
Practice Pointer arithmetic
Get a pointer with as_ptr() for let v = [10, 20, 30];. Read the second and third elements with add in an unsafe block; print both and the result of is_null() separated by spaces.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let v = [10, 20, 30];
let p = v.as_ptr();
let (a, b) = unsafe { (*p.add(1), *p.add(2)) };
println!("{} {} {}", a, b, p.is_null());
}Expected output
20 30 false
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is undefined behaviour (UB)?
- A panic
- A warning
- A situation where the compiler gives no guarantees; the program may do anything Correct answer
- A compile error
Explanation: UB sometimes appears to "work"; it can break when the optimization level or compiler version changes. The Miri tool helps catch UB.
2 Advance the pointer
let p = v.as_ptr();
let ikinci = unsafe { *p.____(1) };let p = v.as_ptr();
let ikinci = unsafe { *p.add(1) };Explanation: add(n) advances the pointer by n elements.
3 Expression and safety
| Term | Meaning |
|---|---|
v.as_ptr() |
Safe |
p.is_null() |
Safe (check) |
*p.add(1) |
unsafe |
std::ptr::null() |
Null pointer |