Const Correctness Issue of std::function

Create: December 30, 2019 | 307 words | 1 min read

The const type qualifier is one of the jewels of the C++ language design. Surrounding by this feature, we devise the "const correctness" practice to prevent const objects from getting mutated. The const correctness rule is straight-forward to follow for implementation of the most classes, but it is harder to heed for classes with type erasure. Unfortunately, the standard library type std::function is implemented by type erasure; and due to short-sightedness, it becomes one of the ill-behaved citizens that doesn't follow the const-correctness rule.

The Problem

std::function has one const member operator(), yet it can mutate the underlying function. For example,

const std::function<int()> f {[x=0]() mutable { return ++x; }};
f(); // returns 1
f(); // returns 2

The Document N43481 first formalized this concern. It states that

This violates not only basic tenets of const-correctness, but also the data race avoidance guarantees in [res.on.data.races]/p3, which states that "A C++ standard library function shall not directly or indirectly modify objects accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's non-const arguments, including this".

The Fix

Implementations of a function-like class should have separate specializations for const and non-const.

template<class Sig> class function; // not defined

template<class R, class... Args>
  class function<R(Args...)>;

template<class R, class... Args>
  class function<R(Args...) const>;

operator() of the const specialization should be annotated as const, but the constructor of the const specialization would not accept mutable function objects.

function<int() const> f1 {[x=0]() { return x; }};
f1() // ok;

function<int() const> f2 {[x=0]() mutable { return ++x; }}; // Does not compile

On the other hand, operator() of the non-const specialization would not have const type signature, so you cannot invoke the const version of such functions at all:

function<int()> f1 {[x=0]() mutable { return ++x; }};
f1(); // ok

const function<int()> f2 {[x=0]() mutable { return ++x; }};
f2(); // Does not compile

The Future

I don't expect std::function itself to have any change that breaks backward-compatibility. As of the time of this writing (December 2019), my bet is on the proposed std::unique_function 2, which is a drop-in replacement of std::function that fixes the const-correctness bug among other features. Once we have an alternative in standard, std::function can be deprecated just like std::auto_ptr. In the meantime, we can always implement unique_function on our own, and I have a small library to implement that on Github.