PhantomData in std::marker - Rust (original) (raw)
Struct PhantomData
1.0.0 · Source
pub struct PhantomData<T>
where
T: ?Sized;Expand description
Zero-sized type used to mark things that “act like” they own a T.
Adding a PhantomData<T> field to your type tells the compiler that your type acts as though it stores a value of type T, even though it doesn’t really. This information is used when computing certain safety properties.
For a more in-depth explanation of how to use PhantomData<T>, please seethe Nomicon.
§A ghastly note 👻👻👻
Though they both have scary names, PhantomData and ‘phantom types’ are related, but not identical. A phantom type parameter is simply a type parameter which is never used. In Rust, this often causes the compiler to complain, and the solution is to add a “dummy” use by way of PhantomData.
§Examples
§Unused lifetime parameters
Perhaps the most common use case for PhantomData is 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 theSlice struct contained a reference &'a T:
use std:📑:PhantomData;
struct Slice<'a, T> {
start: *const T,
end: *const T,
phantom: PhantomData<&'a T>,
}This also in turn infers the lifetime bound T: 'a, indicating that any references in T are valid over the lifetime 'a.
When initializing a Slice you simply provide the valuePhantomData for the field phantom:
fn borrow_vec<T>(vec: &Vec<T>) -> Slice<'_, T> {
let ptr = vec.as_ptr();
Slice {
start: ptr,
end: unsafe { ptr.add(vec.len()) },
phantom: PhantomData,
}
}§Unused type parameters
It sometimes happens that you have unused type parameters which 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 with FFI. The foreign interface uses handles of type *mut () to refer to Rust values of different types. We track the Rust type using a phantom type parameter on the struct ExternalResource which wraps a handle.
use std:📑:PhantomData;
use std::mem;
struct ExternalResource<R> {
resource_handle: *mut (),
resource_type: PhantomData<R>,
}
impl<R: ResType> ExternalResource<R> {
fn new() -> Self {
let size_of_res = mem::size_of::<R>();
Self {
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);
}
}§Ownership and the drop check
The exact interaction of PhantomData with drop check may change in the future.
Currently, adding a field of type PhantomData<T> indicates that your type owns data of typeT in very rare circumstances. This in turn has effects on the Rust compiler’s drop checkanalysis. For the exact rules, see the drop check documentation.
§Layout
For all T, the following are guaranteed:
size_of::<PhantomData<T>>() == 0align_of::<PhantomData<T>>() == 1
Tests for self and other values to be equal, and is used by ==.
Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.
This method returns an ordering between self and other values if one exists. Read more
Tests less than (for self and other) and is used by the < operator. Read more
Tests less than or equal to (for self and other) and is used by the<= operator. Read more
Tests greater than (for self and other) and is used by the >operator. Read more
Tests greater than or equal to (for self and other) and is used by the >= operator. Read more