r[dynamic-sized]
# Dynamically Sized Types

r[dynamic-sized.intro]
Most types have a fixed size that is known at compile time and implement the
trait [`Sized`][sized]. A type with a size that is known only at run-time is
called a _dynamically sized type_ (_DST_) or, informally, an unsized type.
[Slices] and [trait objects] are two examples of DSTs.

r[dynamic-sized.restriction]
Such types can only be used in certain cases:

r[dynamic-sized.pointer-types]
* [Pointer types] to DSTs are
  sized but have twice the size of pointers to sized types
    * Pointers to slices also store the number of elements of the slice.
    * Pointers to trait objects also store a pointer to a vtable.

r[dynamic-sized.question-sized]
* DSTs can be provided as
  type arguments to generic type parameters having the special `?Sized` bound.
  They can also be used for associated type definitions when the corresponding associated type declaration has a `?Sized` bound.
  By default, any type parameter or associated type has a `Sized` bound, unless it is relaxed using `?Sized`.

r[dynamic-sized.trait-impl]
* Traits may be implemented for DSTs.
  Unlike with generic type parameters, `Self: ?Sized` is the default in trait definitions.

r[dynamic-sized.struct-field]
* Structs may contain a DST as the
  last field; this makes the struct itself a
  DST.

> [!NOTE]
> [Variables], function parameters, [const] items, and [static] items must be `Sized`.

[sized]: special-types-and-traits.md#sized
[Slices]: types/slice.md
[trait objects]: types/trait-object.md
[Pointer types]: types/pointer.md
[Variables]: variables.md
[const]: items/constant-items.md
[static]: items/static-items.md
