Struct std::marker::PhantomData
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pub struct PhantomData<T> where T: ?Sized;PhantomData<T> allows you to describe that a type acts as if it stores a value of type T,
even though it does not. This allows you to inform the compiler about certain safety properties
of your code.
For a more in-depth explanation of how to use PhantomData<T>, please see the Nomicon.
A ghastly note 👻👻👻
Though they both have scary names, PhantomData<T> and 'phantom types' are related, but not
identical. Phantom types are a more general concept that don't require PhantomData<T> to
implement, but PhantomData<T> is the most common way to implement them in a correct manner.
Examples
Unused lifetime parameter
Perhaps the most common time that PhantomData is required is
with a struct that has an unused lifetime parameter, typically as
part of some unsafe code. For example, here is a struct Slice
that has two pointers of type *const T, presumably pointing into
an array somewhere:
struct Slice<'a, T> { start: *const T, end: *const T, }
The intention is that the underlying data is only valid for the
lifetime 'a, so Slice should not outlive 'a. However, this
intent is not expressed in the code, since there are no uses of
the lifetime 'a and hence it is not clear what data it applies
to. We can correct this by telling the compiler to act as if the
Slice struct contained a borrowed reference &'a T:
use std::marker::PhantomData; struct Slice<'a, T: 'a> { start: *const T, end: *const T, phantom: PhantomData<&'a T> }
This also in turn requires that we annotate T:'a, indicating
that T is a type that can be borrowed for the lifetime 'a.
Unused type parameters
It sometimes happens that there are unused type parameters that
indicate what type of data a struct is "tied" to, even though that
data is not actually found in the struct itself. Here is an
example where this arises when handling external resources over a
foreign function interface. PhantomData<T> can prevent
mismatches by enforcing types in the method implementations:
use std::marker::PhantomData; use std::mem; struct ExternalResource<R> { resource_handle: *mut (), resource_type: PhantomData<R>, } impl<R: ResType> ExternalResource<R> { fn new() -> ExternalResource<R> { let size_of_res = mem::size_of::<R>(); ExternalResource { resource_handle: foreign_lib::new(size_of_res), resource_type: PhantomData, } } fn do_stuff(&self, param: ParamType) { let foreign_params = convert_params(param); foreign_lib::do_stuff(self.resource_handle, foreign_params); } }
Indicating ownership
Adding a field of type PhantomData<T> also indicates that your
struct owns data of type T. This in turn implies that when your
struct is dropped, it may in turn drop one or more instances of
the type T, though that may not be apparent from the other
structure of the type itself. This is commonly necessary if the
structure is using a raw pointer like *mut T whose referent
may be dropped when the type is dropped, as a *mut T is
otherwise not treated as owned.
If your struct does not in fact own the data of type T, it is
better to use a reference type, like PhantomData<&'a T>
(ideally) or PhantomData<*const T> (if no lifetime applies), so
as not to indicate ownership.