std::ptr::Shared
Struct std::ptr::Shared
pub struct Shared<T> where T: ?Sized, { /* fields omitted */ }
A wrapper around a raw non-null *mut T
that indicates that the possessor of this wrapper has shared ownership of the referent. Useful for building abstractions like Rc<T>
or Arc<T>
, which internally use raw pointers to manage the memory that they own.
Methods
impl<T> Shared<T> where
T: ?Sized,
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T: ?Sized,
unsafe fn new(ptr: *const T) -> Shared<T>
Creates a new Shared
.
Safety
ptr
must be non-null.
impl<T> Shared<T> where
T: ?Sized,
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T: ?Sized,
unsafe fn as_mut_ptr(&self) -> *mut T
Acquires the underlying pointer as a *mut
pointer.
Methods from Deref<Target = *const T>
fn is_null(self) -> bool
1.0.0
Returns true
if the pointer is null.
Examples
Basic usage:
let s: &str = "Follow the rabbit"; let ptr: *const u8 = s.as_ptr(); assert!(!ptr.is_null());
unsafe fn as_ref<'a>(self) -> Option<&'a T>
1.9.0
Returns None
if the pointer is null, or else returns a reference to the value wrapped in Some
.
Safety
While this method and its mutable counterpart are useful for null-safety, it is important to note that this is still an unsafe operation because the returned value could be pointing to invalid memory.
Additionally, the lifetime 'a
returned is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
Examples
Basic usage:
let val: *const u8 = &10u8 as *const u8; unsafe { if let Some(val_back) = val.as_ref() { println!("We got back the value: {}!", val_back); } }
unsafe fn offset(self, count: isize) -> *const T
1.0.0
Calculates the offset from a pointer. count
is in units of T; e.g. a count
of 3 represents a pointer offset of 3 * size_of::<T>()
bytes.
Safety
Both the starting and resulting pointer must be either in bounds or one byte past the end of an allocated object. If either pointer is out of bounds or arithmetic overflow occurs then any further use of the returned value will result in undefined behavior.
Examples
Basic usage:
let s: &str = "123"; let ptr: *const u8 = s.as_ptr(); unsafe { println!("{}", *ptr.offset(1) as char); println!("{}", *ptr.offset(2) as char); }
fn wrapping_offset(self, count: isize) -> *const T
1.16.0
Calculates the offset from a pointer using wrapping arithmetic. count
is in units of T; e.g. a count
of 3 represents a pointer offset of 3 * size_of::<T>()
bytes.
Safety
The resulting pointer does not need to be in bounds, but it is potentially hazardous to dereference (which requires unsafe
).
Always use .offset(count)
instead when possible, because offset
allows the compiler to optimize better.
Examples
Basic usage:
// Iterate using a raw pointer in increments of two elements let data = [1u8, 2, 3, 4, 5]; let mut ptr: *const u8 = data.as_ptr(); let step = 2; let end_rounded_up = ptr.wrapping_offset(6); // This loop prints "1, 3, 5, " while ptr != end_rounded_up { unsafe { print!("{}, ", *ptr); } ptr = ptr.wrapping_offset(step); }
fn offset_to(self, other: *const T) -> Option<isize>
Calculates the distance between two pointers. The returned value is in units of T: the distance in bytes is divided by mem::size_of::<T>()
.
If the address different between the two pointers ia not a multiple of mem::size_of::<T>()
then the result of the division is rounded towards zero.
This function returns None
if T
is a zero-sized typed.
Examples
Basic usage:
#![feature(offset_to)] fn main() { let a = [0; 5]; let ptr1: *const i32 = &a[1]; let ptr2: *const i32 = &a[3]; assert_eq!(ptr1.offset_to(ptr2), Some(2)); assert_eq!(ptr2.offset_to(ptr1), Some(-2)); assert_eq!(unsafe { ptr1.offset(2) }, ptr2); assert_eq!(unsafe { ptr2.offset(-2) }, ptr1); }
fn is_null(self) -> bool
1.0.0
Returns true
if the pointer is null.
Examples
Basic usage:
let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr(); assert!(!ptr.is_null());
unsafe fn as_ref<'a>(self) -> Option<&'a T>
1.9.0
Returns None
if the pointer is null, or else returns a reference to the value wrapped in Some
.
Safety
While this method and its mutable counterpart are useful for null-safety, it is important to note that this is still an unsafe operation because the returned value could be pointing to invalid memory.
Additionally, the lifetime 'a
returned is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
Examples
Basic usage:
let val: *mut u8 = &mut 10u8 as *mut u8; unsafe { if let Some(val_back) = val.as_ref() { println!("We got back the value: {}!", val_back); } }
unsafe fn offset(self, count: isize) -> *mut T
1.0.0
Calculates the offset from a pointer. count
is in units of T; e.g. a count
of 3 represents a pointer offset of 3 * size_of::<T>()
bytes.
Safety
The offset must be in-bounds of the object, or one-byte-past-the-end. Otherwise offset
invokes Undefined Behavior, regardless of whether the pointer is used.
Examples
Basic usage:
let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr(); unsafe { println!("{}", *ptr.offset(1)); println!("{}", *ptr.offset(2)); }
fn wrapping_offset(self, count: isize) -> *mut T
1.16.0
Calculates the offset from a pointer using wrapping arithmetic. count
is in units of T; e.g. a count
of 3 represents a pointer offset of 3 * size_of::<T>()
bytes.
Safety
The resulting pointer does not need to be in bounds, but it is potentially hazardous to dereference (which requires unsafe
).
Always use .offset(count)
instead when possible, because offset
allows the compiler to optimize better.
Examples
Basic usage:
// Iterate using a raw pointer in increments of two elements let mut data = [1u8, 2, 3, 4, 5]; let mut ptr: *mut u8 = data.as_mut_ptr(); let step = 2; let end_rounded_up = ptr.wrapping_offset(6); while ptr != end_rounded_up { unsafe { *ptr = 0; } ptr = ptr.wrapping_offset(step); } assert_eq!(&data, &[0, 2, 0, 4, 0]);
unsafe fn as_mut<'a>(self) -> Option<&'a mut T>
1.9.0
Returns None
if the pointer is null, or else returns a mutable reference to the value wrapped in Some
.
Safety
As with as_ref
, this is unsafe because it cannot verify the validity of the returned pointer, nor can it ensure that the lifetime 'a
returned is indeed a valid lifetime for the contained data.
Examples
Basic usage:
let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr(); let first_value = unsafe { ptr.as_mut().unwrap() }; *first_value = 4; println!("{:?}", s); // It'll print: "[4, 2, 3]".
fn offset_to(self, other: *const T) -> Option<isize>
Calculates the distance between two pointers. The returned value is in units of T: the distance in bytes is divided by mem::size_of::<T>()
.
If the address different between the two pointers ia not a multiple of mem::size_of::<T>()
then the result of the division is rounded towards zero.
This function returns None
if T
is a zero-sized typed.
Examples
Basic usage:
#![feature(offset_to)] fn main() { let mut a = [0; 5]; let ptr1: *mut i32 = &mut a[1]; let ptr2: *mut i32 = &mut a[3]; assert_eq!(ptr1.offset_to(ptr2), Some(2)); assert_eq!(ptr2.offset_to(ptr1), Some(-2)); assert_eq!(unsafe { ptr1.offset(2) }, ptr2); assert_eq!(unsafe { ptr2.offset(-2) }, ptr1); }
Trait Implementations
impl<T, U> CoerceUnsized<Shared<U>> for Shared<T> where
T: Unsize<U> + ?Sized,
U: ?Sized,
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T: Unsize<U> + ?Sized,
U: ?Sized,
impl<T> Copy for Shared<T> where
T: ?Sized,
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T: ?Sized,
impl<T> !Send for Shared<T> where
T: ?Sized,
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T: ?Sized,
Shared
pointers are not Send
because the data they reference may be aliased.
impl<T> Pointer for Shared<T>
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fn fmt(&self, f: &mut Formatter) -> Result<(), Error>
Formats the value using the given formatter.
impl<T> !Sync for Shared<T> where
T: ?Sized,
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T: ?Sized,
Shared
pointers are not Sync
because the data they reference may be aliased.
impl<T> Clone for Shared<T> where
T: ?Sized,
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T: ?Sized,
fn clone(&self) -> Shared<T>
Returns a copy of the value. Read more
fn clone_from(&mut self, source: &Self)
1.0.0
Performs copy-assignment from source
. Read more
impl<T> Deref for Shared<T> where
T: ?Sized,
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T: ?Sized,
type Target = *const T
The resulting type after dereferencing
fn deref(&self) -> &*const T
The method called to dereference a value
impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Shared<T>
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© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/ptr/struct.Shared.html