Concept Subspace :: sus :: fn :: Fn

template <class F, class... S>
concept Fn
requires {
  // Receives and passes along the signature as a pack instead of a single
  // argument in order to consistently provide a static_assert() in `Sig` when
  // `S` is not a function signature.
    __private::InvokedFn<F, typename __private::Sig<S...>::Args>::returns()
  } -> __private::ValidReturnType<typename __private::Sig<S...>::Return>;
  requires FnMut<F, S...>;
  requires FnOnce<F, S...>;

The version of a callable object that may be called multiple times without mutating internal state.

A Fn is useful for a callable that is expected to be called one or more times and whose results do not change between calls. This is of course possible to violate with mutable or global state, but it is discouraged as it violates the Fn protocol expectations of the caller. FnMut should be used when the function will mutate anything and can return different values as a result.

A type that satisfies Fn will return a type that can be converted to R when called with the arguments Args.... Fn is satisfied by being callable as a const lvalue (which is done by providing an operator() that is const- or const&-qualified). Const lambdas will satisfy Fn but mutable ones will not.

The second argument of Fn<F, S> is a function signature with the format ReturnType(Args...), where Args... are the arguments that will be passed to the Fn and ReturnType is what is expected to be received back. It would appear as a matching concept as:

void function(Fn<ReturnType(Args...)> auto f) { ... }

Use of Fn

Fn should be received by value typically, but can also be received as a const reference.

A Fn should be called by passing it to call along with any arguments. This ensures the correct overload is called on the object and that method pointers are called correctly. A Fn may be called any number of times, unlike FnOnce, and should not be moved when called.

Type erasure

Using a concept like Fn in a function parameter requires the function to be a template. Template functions can not be virtual, they must appear in the header, and they can have a negative impact on binary size. So it can be desirable to work with a Fn without templates.

To do so, Fn supports being type-erased, on the heap or the stack, into a DynFn type. To receive ownership of a type-erased Fn, receive a Box<DynFn<R(Args...)>> instead. To receive a reference to a type-erased Fn, receive a DynFn<R(Args...)>&& instead.

See DynConcept for more on type erasure of concept-satisfying types.


Any callable type that satisfies Fn will also satisfy FnMut and FnOnce. A Fn may be called multiple times, or a single time, which is compatible with both FnMut and FnOnce. And while FnMut and FnOnce are able to mutate state when run, they are not required to and a constant Fn satisfies them.


A function that receives a Fn matching type and calls it:

// Accepts any type that can be called once with (Option<i32>) and returns
// i32.
static i32 do_stuff(sus::fn::Fn<i32(sus::Option<i32>)> auto f) {
  return sus::fn::call(f, sus::some(400)) +
         sus::fn::call(f, sus::some(100));

i32 x = do_stuff([i = 1_i32](sus::Option<i32> o) -> i32 {
  return sus::move(o).unwrap_or_default() + i;
sus_check(x == 401 + 101);

A Fn whose first parameter is a class can be matched with a method from that same class if the remaining parameters match the method's signature:

struct Class {
  Class(i32 value) : value_(value) {}
  i32 value() const { return value_; }

  i32 value_;

i32 map_class(const Class& c,
              sus::fn::Fn<i32(const Class&)> auto const& f) {
  return sus::fn::call(f, c);

// Map the class C to its value().
auto c = Class(42);
sus_check(map_class(c, &Class::value) == 42);

Using a method pointer as the parameter for Option::map() will call that method on the object inside the Option:

struct Class {
  Class(i32 value) : value_(value) {}
  i32 value() const { return value_; }

  i32 value_;

auto o = sus::Option<Class>(Class(42));
sus_check( == sus::some(42));