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What Are Rust Raw Pointers and static variables?

What Are Rust Raw Pointers and static variables?

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Raw Pointers

Unsafe Rust has two new kinds called raw pointers, which are similar to references. They may be immutable or mutable and can be written as:

  • *const T (Immutable)
  • *mut T (Mutable)

Note: Asterisk “*” is not a dereferencing operator, its part of type name.

Let’s know how to make an immutable and mutable raw pointer from reference.

let mut = 5;

let r1 = &num as *const i32;

let r2 = &mut num as *i32;

Is something wrong! We didn’t include an unsafe keyword here? Well, it’s okay to create raw pointers outside unsafe but it’s must to include while dereferencing them.

Now, let’s generate a raw pointer whose validity can’t be sure.

fn main ( ) {

let address = 0x012345usize;

let r = address * const i32;

}

The overhead method to access memory is not optional; however, we might see or write this kind of code.

How raw pointers different from references?

  • Remain permissible to ignore the borrowing rules by having both immutable and mutable pointers or multiple mutable pointers to the same location
  • Aren’t guaranteed to point to valid memory
  • Are allowed to be null
  • Don’t implement any automatic cleanup

Static variables

  • In Rust, global variables are also called static variables
  • Static variables are similar to Constants
  • Identification style of static variables is in SCREAMING_SNAKE_CASE
  • Type footnote for static variables is a must (like constants).
  • Rust permits access to read-only static variables (immutable static variables), however doesn’t allow access to immutable static variables.
  • Static variables continuously have fixed addresses in memory.

Limitations​

  • Static variables must be threading safe.
  • This means that their type must implement the SyncThis links to the official Rust documentation trait.
  • For example, this links to official Rust documentation or RefCellThis links to official Rust documentation can’t be used in a static variable.
  • They are not thread-safe as having multiple threads access them at once causes a data race.
  • Static variables may mention to other static variables, but only through a reference, not by value.
  • Constants can’t refer to a static variable.

Example

fn get(n: usize) -> &'static str {

static STRS: &[&str] = &[

"zero", "one", "two", "three", "four",

"five", "six", "seven", "eight", "nine",

];

STRS[n]

}

Using statics or constants​

It may be confusing whether or not we should use a constant item or a static item., In general, Constants should be preferred over statics unless one of the following is true:

  • Big amounts of data are being stored
  • Required the single-address property of statics.
  • Inner mutability is obligatory

Mutable static variables