Trait Deref

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1.0.0 · Source

pub trait Deref {
    type Target: ?Sized;

    // Required method
    fn deref(&self) -> &Self::Target;
}

Expand description

Used for immutable dereferencing operations, like *v.

In addition to being used for explicit dereferencing operations with the (unary) * operator in immutable contexts, Deref is also used implicitly by the compiler in many circumstances. This mechanism is called “Deref coercion”. In mutable contexts, DerefMut is used and mutable deref coercion similarly occurs.

Warning: Deref coercion is a powerful language feature which has far-reaching implications for every type that implements Deref. The compiler will silently insert calls to Deref::deref. For this reason, one should be careful about implementing Deref and only do so when deref coercion is desirable. See below for advice on when this is typically desirable or undesirable.

Types that implement Deref or DerefMut are often called “smart pointers” and the mechanism of deref coercion has been specifically designed to facilitate the pointer-like behavior that name suggests. Often, the purpose of a “smart pointer” type is to change the ownership semantics of a contained value (for example, Rc or Cow) or the storage semantics of a contained value (for example, Box).

§Deref coercion

If T implements Deref<Target = U>, and v is a value of type T, then:

In immutable contexts, *v (where T is neither a reference nor a raw pointer) is equivalent to *Deref::deref(&v).
Values of type &T are coerced to values of type &U
T implicitly implements all the methods of the type U which take the &self receiver.

For more details, visit the chapter in The Rust Programming Language as well as the reference sections on the dereference operator, method resolution, and type coercions.

§When to implement `Deref` or `DerefMut`

The same advice applies to both deref traits. In general, deref traits should be implemented if:

a value of the type transparently behaves like a value of the target type;
the implementation of the deref function is cheap; and
users of the type will not be surprised by any deref coercion behavior.

In general, deref traits should not be implemented if:

the deref implementations could fail unexpectedly; or
the type has methods that are likely to collide with methods on the target type; or
committing to deref coercion as part of the public API is not desirable.

Note that there’s a large difference between implementing deref traits generically over many target types, and doing so only for specific target types.

Generic implementations, such as for Box<T> (which is generic over every type and dereferences to T) should be careful to provide few or no methods, since the target type is unknown and therefore every method could collide with one on the target type, causing confusion for users. impl<T> Box<T> has no methods (though several associated functions), partly for this reason.

Specific implementations, such as for String (whose Deref implementation has Target = str) can have many methods, since avoiding collision is much easier. String and str both have many methods, and String additionally behaves as if it has every method of str because of deref coercion. The implementing type may also be generic while the implementation is still specific in this sense; for example, Vec<T> dereferences to [T], so methods of T are not applicable.

Consider also that deref coercion means that deref traits are a much larger part of a type’s public API than any other trait as it is implicitly called by the compiler. Therefore, it is advisable to consider whether this is something you are comfortable supporting as a public API.

The AsRef and Borrow traits have very similar signatures to Deref. It may be desirable to implement either or both of these, whether in addition to or rather than deref traits. See their documentation for details.

§Fallibility

This trait’s method should never unexpectedly fail. Deref coercion means the compiler will often insert calls to Deref::deref implicitly. Failure during dereferencing can be extremely confusing when Deref is invoked implicitly. In the majority of uses it should be infallible, though it may be acceptable to panic if the type is misused through programmer error, for example.

However, infallibility is not enforced and therefore not guaranteed. As such, unsafe code should not rely on infallibility in general for soundness.

§Examples

A struct with a single field which is accessible by dereferencing the struct.

use std::ops::Deref;

struct DerefExample<T> {
    value: T
}

impl<T> Deref for DerefExample<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        &self.value
    }
}

let x = DerefExample { value: 'a' };
assert_eq!('a', *x);