Primitive Type fn

Function pointers, like fn(usize) -> bool.

See also the traits Fn, FnMut, and FnOnce.

Function pointers are pointers that point to code, not data. They can be called just like functions. Like references, function pointers are, among other things, assumed to not be null, so if you want to pass a function pointer over FFI and be able to accommodate null pointers, make your type Option<fn()> with your required signature.

Safety

Plain function pointers are obtained by casting either plain functions, or closures that don’t capture an environment:

fn add_one(x: usize) -> usize {
    x + 1
}

let ptr: fn(usize) -> usize = add_one;
assert_eq!(ptr(5), 6);

let clos: fn(usize) -> usize = |x| x + 5;
assert_eq!(clos(5), 10);

In addition to varying based on their signature, function pointers come in two flavors: safe and unsafe. Plain fn() function pointers can only point to safe functions, while unsafe fn() function pointers can point to safe or unsafe functions.

fn add_one(x: usize) -> usize {
    x + 1
}

unsafe fn add_one_unsafely(x: usize) -> usize {
    x + 1
}

let safe_ptr: fn(usize) -> usize = add_one;

//ERROR: mismatched types: expected normal fn, found unsafe fn
//let bad_ptr: fn(usize) -> usize = add_one_unsafely;

let unsafe_ptr: unsafe fn(usize) -> usize = add_one_unsafely;
let really_safe_ptr: unsafe fn(usize) -> usize = add_one;

ABI

On top of that, function pointers can vary based on what ABI they use. This is achieved by adding the extern keyword before the type, followed by the ABI in question. The default ABI is “Rust”, i.e., fn() is the exact same type as extern "Rust" fn(). A pointer to a function with C ABI would have type extern "C" fn().

extern "ABI" { ... } blocks declare functions with ABI “ABI”. The default here is “C”, i.e., functions declared in an extern {...} block have “C” ABI.

For more information and a list of supported ABIs, see the nomicon’s section on foreign calling conventions.

Variadic functions

Extern function declarations with the “C” or “cdecl” ABIs can also be variadic, allowing them to be called with a variable number of arguments. Normal Rust functions, even those with an extern "ABI", cannot be variadic. For more information, see the nomicon’s section on variadic functions.

Creating function pointers

When bar is the name of a function, then the expression bar is not a function pointer. Rather, it denotes a value of an unnameable type that uniquely identifies the function bar. The value is zero-sized because the type already identifies the function. This has the advantage that “calling” the value (it implements the Fn* traits) does not require dynamic dispatch.

This zero-sized type coerces to a regular function pointer. For example:

use std::mem;

fn bar(x: i32) {}

let not_bar_ptr = bar; // `not_bar_ptr` is zero-sized, uniquely identifying `bar`
assert_eq!(mem::size_of_val(&not_bar_ptr), 0);

let bar_ptr: fn(i32) = not_bar_ptr; // force coercion to function pointer
assert_eq!(mem::size_of_val(&bar_ptr), mem::size_of::<usize>());

let footgun = &bar; // this is a shared reference to the zero-sized type identifying `bar`

The last line shows that &bar is not a function pointer either. Rather, it is a reference to the function-specific ZST. &bar is basically never what you want when bar is a function.

Casting to and from integers

You cast function pointers directly to integers:

let fnptr: fn(i32) -> i32 = |x| x+2;
let fnptr_addr = fnptr as usize;

However, a direct cast back is not possible. You need to use transmute:

let fnptr = fnptr_addr as *const ();
let fnptr: fn(i32) -> i32 = unsafe { std::mem::transmute(fnptr) };
assert_eq!(fnptr(40), 42);

Crucially, we as-cast to a raw pointer before transmuteing to a function pointer. This avoids an integer-to-pointer transmute, which can be problematic. Transmuting between raw pointers and function pointers (i.e., two pointer types) is fine.

Note that all of this is not portable to platforms where function pointers and data pointers have different sizes.

Trait implementations

In this documentation the shorthand fn (T₁, T₂, …, Tₙ) is used to represent non-variadic function pointers of varying length. Note that this is a convenience notation to avoid repetitive documentation, not valid Rust syntax.

Due to a temporary restriction in Rust’s type system, these traits are only implemented on functions that take 12 arguments or less, with the "Rust" and "C" ABIs. In the future, this may change:

• PartialEq
• Eq
• PartialOrd
• Ord
• Hash
• Pointer
• Debug

The following traits are implemented for function pointers with any number of arguments and any ABI. These traits have implementations that are automatically generated by the compiler, so are not limited by missing language features:

• Clone
• Copy
• Send
• Sync
• Unpin
• UnwindSafe
• RefUnwindSafe

In addition, all safe function pointers implement Fn, FnMut, and FnOnce, because these traits are specially known to the compiler.

Trait Implementations

impl<Ret, T> Clone for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented on function pointers with any number of arguments.

fn clone(&self) -> Self

Returns a copy of the value.

const fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<Ret, T> Debug for extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Debug for extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Debug for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Debug for unsafe extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Debug for unsafe extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Debug for unsafe fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Hash for extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn hash<HH: Hasher>(&self, state: &mut HH)

Feeds this value into the given Hasher.

fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)where
    Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Ret, T> Hash for extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn hash<HH: Hasher>(&self, state: &mut HH)

Feeds this value into the given Hasher.

fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)where
    Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Ret, T> Hash for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn hash<HH: Hasher>(&self, state: &mut HH)

Feeds this value into the given Hasher.

fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)where
    Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Ret, T> Hash for unsafe extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn hash<HH: Hasher>(&self, state: &mut HH)

Feeds this value into the given Hasher.

fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)where
    Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Ret, T> Hash for unsafe extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn hash<HH: Hasher>(&self, state: &mut HH)

Feeds this value into the given Hasher.

fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)where
    Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Ret, T> Hash for unsafe fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn hash<HH: Hasher>(&self, state: &mut HH)

Feeds this value into the given Hasher.

fn hash_slice<H: Hasher>(data: &[Self], state: &mut H)where
    Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<Ret, T> Ord for extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

const fn max(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the maximum of two values.

const fn min(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the minimum of two values.

const fn clamp(self, min: Self, max: Self) -> Selfwhere
    Self: Sized,
    Self: PartialOrd,

Restrict a value to a certain interval.

impl<Ret, T> Ord for extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

const fn max(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the maximum of two values.

const fn min(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the minimum of two values.

const fn clamp(self, min: Self, max: Self) -> Selfwhere
    Self: Sized,
    Self: PartialOrd,

Restrict a value to a certain interval.

impl<Ret, T> Ord for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

const fn max(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the maximum of two values.

const fn min(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the minimum of two values.

const fn clamp(self, min: Self, max: Self) -> Selfwhere
    Self: Sized,
    Self: PartialOrd,

Restrict a value to a certain interval.

impl<Ret, T> Ord for unsafe extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

const fn max(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the maximum of two values.

const fn min(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the minimum of two values.

const fn clamp(self, min: Self, max: Self) -> Selfwhere
    Self: Sized,
    Self: PartialOrd,

Restrict a value to a certain interval.

impl<Ret, T> Ord for unsafe extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

const fn max(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the maximum of two values.

const fn min(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the minimum of two values.

const fn clamp(self, min: Self, max: Self) -> Selfwhere
    Self: Sized,
    Self: PartialOrd,

Restrict a value to a certain interval.

impl<Ret, T> Ord for unsafe fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn cmp(&self, other: &Self) -> Ordering

This method returns an Ordering between self and other.

const fn max(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the maximum of two values.

const fn min(self, other: Self) -> Selfwhere
    Self: Sized,

Compares and returns the minimum of two values.

const fn clamp(self, min: Self, max: Self) -> Selfwhere
    Self: Sized,
    Self: PartialOrd,

Restrict a value to a certain interval.

impl<Ret, T> PartialEq<extern "C" fn(T, ...) -> Ret> for extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

const fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<Ret, T> PartialEq<extern "C" fn(T) -> Ret> for extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

const fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<Ret, T> PartialEq<fn(T) -> Ret> for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

const fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<Ret, T> PartialEq<unsafe extern "C" fn(T, ...) -> Ret> for unsafe extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

const fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<Ret, T> PartialEq<unsafe extern "C" fn(T) -> Ret> for unsafe extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

const fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<Ret, T> PartialEq<unsafe fn(T) -> Ret> for unsafe fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn eq(&self, other: &Self) -> bool

This method tests for self and other values to be equal, and is used by ==.

const fn ne(&self, other: &Rhs) -> bool

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<Ret, T> PartialOrd<extern "C" fn(T, ...) -> Ret> for extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

const fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

const fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

const fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

const fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<Ret, T> PartialOrd<extern "C" fn(T) -> Ret> for extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

const fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

const fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

const fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

const fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<Ret, T> PartialOrd<fn(T) -> Ret> for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

const fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

const fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

const fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

const fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<Ret, T> PartialOrd<unsafe extern "C" fn(T, ...) -> Ret> for unsafe extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

const fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

const fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

const fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

const fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<Ret, T> PartialOrd<unsafe extern "C" fn(T) -> Ret> for unsafe extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

const fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

const fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

const fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

const fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<Ret, T> PartialOrd<unsafe fn(T) -> Ret> for unsafe fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn partial_cmp(&self, other: &Self) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

const fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

const fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

const fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

const fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<Ret, T> Pointer for extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Pointer for extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Pointer for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Pointer for unsafe extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Pointer for unsafe extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Pointer for unsafe fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Ret, T> Copy for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented on function pointers with any number of arguments.

impl<Ret, T> Eq for extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

impl<Ret, T> Eq for extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

impl<Ret, T> Eq for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

impl<Ret, T> Eq for unsafe extern "C" fn (T₁, T₂, …, Tₙ, ...) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

impl<Ret, T> Eq for unsafe extern "C" fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.

impl<Ret, T> Eq for unsafe fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented for function pointers with up to twelve arguments.