p formatting.

The Pointer trait should format its output as a memory location. This is commonly presented as hexadecimal. For more information on formatters, see the module-level documentation.

Printing of pointers is not a reliable way to discover how Rust programs are implemented. The act of reading an address changes the program itself, and may change how the data is represented in memory, and may affect which optimizations are applied to the code.

The printed pointer values are not guaranteed to be stable nor unique identifiers of objects. Rust allows moving values to different memory locations, and may reuse the same memory locations for different purposes.

There is no guarantee that the printed value can be converted back to a pointer.

Examples

Basic usage with &i32:

let x = &42;

let address = format!("{x:p}"); // this produces something like '0x7f06092ac6d0'

Implementing Pointer on a type:

use std::fmt;

struct Length(i32);

impl fmt::Pointer for Length {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // use `as` to convert to a `*const T`, which implements Pointer, which we can use

        let ptr = self as *const Self;
        fmt::Pointer::fmt(&ptr, f)
    }
}

let l = Length(42);

println!("l is in memory here: {l:p}");

let l_ptr = format!("{l:018p}");
assert_eq!(l_ptr.len(), 18);
assert_eq!(&l_ptr[..2], "0x");