Unsized Types - The Rust Programming Language

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# Unsized Types

Most types have a particular size, in bytes, that is knowable at compile time. For example, an `i32` is thirty-two bits big, or four bytes. However, there are some types which are useful to express, but do not have a defined size. These are called ‘unsized’ or ‘dynamically sized’ types. One example is `[T]`. This type represents a certain number of `T` in sequence. But we don’t know how many there are, so the size is not known.

Rust understands a few of these types, but they have some restrictions. There are three:

1. We can only manipulate an instance of an unsized type via a pointer. An `&[T]` works fine, but a `[T]` does not.
2. Variables and arguments cannot have dynamically sized types.
3. Only the last field in a `struct` may have a dynamically sized type; the other fields must not. Enum variants must not have dynamically sized types as data.

So why bother? Well, because `[T]` can only be used behind a pointer, if we didn’t have language support for unsized types, it would be impossible to write this:

    impl Foo for str {

or

    impl<T> Foo for [T] {

Instead, you would have to write:

    impl Foo for &str {

Meaning, this implementation would only work for references, and not other types of pointers. With the `impl for str`, all pointers, including (at some point, there are some bugs to fix first) user-defined custom smart pointers, can use this `impl`.

# ?Sized

If you want to write a function that accepts a dynamically sized type, you can use the special bound syntax, `?Sized`:

    # #![allow(unused_variables)]
    #fn main() {
    struct Foo<T: ?Sized> {
        f: T,
    }
    #}

This `?Sized`, read as “T may or may not be `Sized`”, which allows us to match both sized and unsized types. All generic type parameters implicitly have the `Sized` bound, so the `?Sized` can be used to opt-out of the implicit bound.