fn_addr_eq in std::ptr - Rust (original) (raw)

Function fn_addr_eq

1.85.0 · Source

pub fn fn_addr_eq<T, U>(f: T, g: U) -> bool

where
    T: FnPtr,
    U: FnPtr,

Expand description

Compares the addresses of the two function pointers for equality.

This is the same as f == g, but using this function makes clear that the potentially surprising semantics of function pointer comparison are involved.

There are very few guarantees about how functions are compiled and they have no intrinsic “identity”; in particular, this comparison:

• May return true unexpectedly, in cases where functions are equivalent.
  For example, the following program is likely (but not guaranteed) to print (true, true)when compiled with optimization:

let f: fn(i32) -> i32 = |x| x;  
let g: fn(i32) -> i32 = |x| x + 0;  // different closure, different body  
let h: fn(u32) -> u32 = |x| x + 0;  // different signature too  
dbg!(std::ptr::fn_addr_eq(f, g), std::ptr::fn_addr_eq(f, h)); // not guaranteed to be equal  

• May return false in any case.
  This is particularly likely with generic functions but may happen with any function. (From an implementation perspective, this is possible because functions may sometimes be processed more than once by the compiler, resulting in duplicate machine code.)

Despite these false positives and false negatives, this comparison can still be useful. Specifically, if

• T is the same type as U, T is a subtype of U, or U is a subtype of T, and
• ptr::fn_addr_eq(f, g) returns true,

then calling f and calling g will be equivalent.

§Examples

use std::ptr;

fn a() { println!("a"); }
fn b() { println!("b"); }
assert!(!ptr::fn_addr_eq(a as fn(), b as fn()));