Table of Contents

• Box
• Arc and Rc:
• Cell, RefCell, and Mutex
• Cow
• *const T and *mut T

Box

• Used for explicitly allocating values on the heap.
• Provides exclusive ownership.
• Use Box when you want to opt-in to storing something on the heap, which otherwise wouldn't go there.
• Generally, use Box when:
  • Writing a recursive type.
  • Optimizing the code (e.g., a large struct that is expensive to move around).
  • Want to use dyn SomeTrait and have ownership of it.

Arc and Rc:

• Both Arc and Rc are reference counting types that provide shared ownership.
• Anything shared with Arc or Rc is immutable.
• Arc is more expensive, but is thread-safe.
• Rc is less expensive, but is not thread-safe.

Cell, RefCell, and Mutex

• All three Cell, RefCell, and Mutex provide shared ownership.
• Anything shared with Cell, RefCell, or Mutex is mutable.
• Cell and RefCell are not thread-safe.
• Mutex is thread-safe.
• Use Cell if the type is Copy, RefCell otherwise.

Cow

• Cow is a clone-on-write smart pointer.
• Usually used with strings.
• Generally, use Cow for optional ownership (i.e., operations that may or may not modify input).
• Cow is also a smart pointer.

*const T and *mut T

• Both *const T and *mut T represent a generic raw pointer.
• Dereferencing *const T yields an immutable place expression, *mut T yields a mutable one.
  • A place expression is an expression that represents a memory location.
• In terms of variance, *const T is covariant, while *mut T is invariant.