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484f75d May 3, 2018
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folly::Function is a polymorphic function wrapper that is not copyable and does not require the wrapped function to be copy constructible. It is similar to std::function, but different with respect to some interesting features.

There are some limitations in std::function that folly::Function tries to avoid. std::function is copy-constructible and requires that the callable that it wraps is copy-constructible as well, which is a constraint that is often inconvenient. In most cases when using a std::function you don't make use of its copy-constructibility, so you might sometimes feel like you get back very little in return for a noticeable restriction.

This restriction becomes apparent when trying to use a lambda capturing a unique_ptr (or any non-copyable type) as a callback for a folly::Future.

std::unique_ptr<Foo> foo_ptr = new Foo;

    [foo_ptr = std::move(foo_ptr)] mutable
    (int x)
    { foo_ptr->setX(x); }

This piece of code did not compile before folly::Future started using folly::Function instead of std::function to store the callback. Because the lambda captures something non-copyable (the unique_ptr), it is not copyable itself. And std::function can only store copyable callables.

The implementation of folly::Future did not make use of the copy-constructibility of std::function at any point. There was no benefit from the fact that the std::function is copy-constructible, but the fact that it can only wrap copy-constructible callables posed a restriction.

A workaround was available: folly::MoveWrapper, which wraps an object that may be non-copyable and implements copy operations by moving the embedded object. Using a folly::MoveWrapper, you can capture non-copyable objects in a lambda, and the lambda itself is still copyable and may be wrapped in a std::function. It is a pragmatic solution for the above problem, but you have to be a little careful. The problem is that you can’t use a MoveWrapper anywhere where copy operations are assumed to behave like actual copy operations. Also, a folly::MoveWrapper<std::unique_ptr<T>> essentially behaves like auto_ptr<T>. Ask yourself whether you’d want to use lots of auto_ptrs in your codebase. And the original question still persists: we very often don’t benefit from copy-constructibility of std::function, so why do we have to live with this restriction? I.e. why do we have to use MoveWrapper?

folly::Function is an actual solution to this problem, as it can wrap non-copyable callables, at the cost of not being copy-constructible, which more often than not is not a relevant restriction. folly::Future now uses folly::Function to store callbacks, so the good news is: the code example from the top of this note is becoming a perfectly valid way to use future callbacks. The code compiles and behaves as you would expect.

Here are more details about folly::Function: much like std::function, it wraps an arbitrary object that can be invoked like a given function type. E.g. a folly::Function<int(std::string, double)> can wrap any callable object that returns an int (or something that is convertible to an int) when invoked with a std::string and a double argument. The function type is a template parameter of folly::Function, but the specific type of the callable is not. Also, like most implementations of std::function, folly::Function will store small callable objects in-place whereas larger callables will be stored on the heap. (Unlike std::function, you can set the size of the in-place storage as a template parameter of folly::Function.)

Other than copyability, there is one more significant difference between std::function and folly::Function, and it concerns const-correctness. std::function does not enforce const-correctness: it allows you to store mutable callables (i.e. callables that may change their inner state when executed, such as a mutable lambda) and call them in a const context (i.e. when you only have access to a const reference to the std::function object). For example:

class FooBar {
  void call_func() const {
  std::function<void()> func_;

The call_func member function is declared const. However, when it calls func_(), it may change the inner state of func_, and thereby the inner state of the FooBar object. Inside the FooBar class, func_ is like a non-const method that is callable from const methods.

Some people consider std::function in the standard broken with respect to this. (Paper N4348 explains this problem in more detail.) It also lists possible ways to fix the problem. folly::Function, however, goes a different way: you have to declare whether you want to store a const function, in which case you can invoke any reference (const or non-const) of the folly::Function, or a non-const (mutable) function, in which case you need a non-const reference to the folly::Function to be able to invoke it. In the above example, let’s say that func_ stores a const function, which makes it okay that it gets invoked from call_func (a const method). Instead of std::function, you could use folly::Function<void() const> for the func_ member.

Const-ness is part of a function type. To illustrate:

class Foo {
  int operator()() { return 1; }
  int operator()(char const*) { return 2; }
  int operator()(int) { return 3; }
  int operator()(int) const { return 4; }
  int operator()(int, int) const { return 5; }

You can overload methods multiple times using different argument signatures. Const-ness is part of that signature, so even for the same set of argument types you can overload a const and a non-const version. It’s not even particularly unusual to do that. Take for instance the begin() method of STL container types: begin() returns an iterator, begin() const returns a const_iterator. folly::Function allows you to select a specific overload easily:

folly::Function<int()> uf1 = Foo();
// uf1() returns 1
folly::Function<int(char const*)> uf2 = Foo();
// uf2() returns 2
folly::Function<int(int)> uf3 = Foo();
// uf3() returns 3
folly::Function<int(int) const> uf4 = Foo();
// uf4() returns 4
folly::Function<int(int, int) const> uf5 = Foo();
// uf5() returns 5

If cfoo is a const-reference to a Foo object, cfoo(int) returns 4. If foo is a non-const reference to a Foo object, foo(int) returns 3. Normal const-to-non-const conversion behaviour applies: if you call foo(int, int) it will return 5: a non-const reference will invoke the const method if no non-const method is defined. Which leads to the following behaviour:

folly::Function<int(int, int)> uf5nc = Foo();
// uf5nc() returns 5

If you are wondering if the introduction of const function types means that you have to change lots of normal function types to const function types if you want to use folly::Function: not really, or at least not as much as you might think. There are only two reasons to use a folly::Function with a const function type:

  • a callable object defines both const and non-const operator() and you explicitly want to select the const one
  • you need to invoke a folly::Function from a const context (i.e. you only have a const reference to the folly::Function)

In practice, you will not need the const variant very often. Adding const to a function type adds a restriction for the callable: it must not change its inner state when invoked. If you don’t care whether it does or not, don’t worry about const!

A folly::Function<R(Args...) const> can be converted into a folly::Function<R(Args...)>: either way the stored callable will not change its inner state when invoked. The former type expresses and guarantees that, the latter does not. When you get rid of the const, the selected function stays the same:

folly::Function<int(int)> uf4nc = std::move(uf4);
// uf4nc() returns 4, not 3!

If you want to go the other way, you are talking about a (potentially dangerous) const cast: a callable that may or may not change its inner state is declared as one that guarantees not to do that. Proceed at your own risk! This conversion does not happen implicitly, though:

folly::Function<int() const> uf1c = folly::constCastFunction(std::move(uf1));
// uf1c() returns 1

Admittedly, seeing const function types as template parameters is unfamiliar. As far as I am aware it happens nowhere in the standard library. But it is the most consistent way to deal with the issue of const-correctness here. Const qualifiers are part of a function type, as a matter of fact. If you require a const-qualified function to be wrapped in a folly::Function, just declare it as that! More often than not you will find that you do not need the const qualifier. While writing the folly::Function implementation, a good set of unit tests had existed before the const function types got introduced. Not a single of those unit tests had to be changed: they all compiled and passed after the introduction of const function types. Obviously new ones were added to test the const-correctness. But in your day-to-day use of folly::Function you won’t have to worry about const very often.