A smart pointer is a pointer-like data structure that manages access to a value and provides behavior beyond storing a raw address. It can own data, carry metadata, expose methods, or support dereferencing with *. section 13.2.1 section 15.3.1
In Rust, smart pointers help with several practical problems:
- Use Box to place owned data on the heap while keeping a known-size pointer. section 13.3
- Use Box to give recursive structures a finite size, such as a Holder containing another Holder. section 13.2.1
- Use Box<dyn trait> to treat a concrete value as a trait object and use its trait methods. section 13.3
- Implement Deref when a custom wrapper should support dereferencing and behave more like the value it contains. section 15.3.1
Breaking a recursive type's infinite size
A direct recursive field such as Option<Holder> cannot compile because the compiler cannot determine the type's size. Wrapping the field as Option<Box<Holder>> adds indirection with a known size. Box::new then constructs each nested Holder, and *my_box accesses a boxed value just as dereferencing accesses a reference. section 13.2.1 section 13.3
Common smart pointers include Box, Rc, and RefCell; familiar types such as String and Vec also behave as smart pointers. The important idea is that the pointer supplies an interface or capability in addition to referring to data. section 15.3.1
