In Bjarne Stroustrup’s paper P0557R1 “Concepts: The Future of Generic Programming” (January 2017), section 6 is titled “Concept Overloading,” which Stroustrup distinguishes from the traditional approach “using helper functions and tag dispatch.” More subtly, Stroustrup seems to distinguish between true “overloading” (asking the compiler to resolve among multiple viable overloads) and other means of giving-two-things-the-same-name:

Where we cannot overload, we need to use workarounds (e.g., traits, enable_if, or helper functions).

I’ll present two problems, each with two solutions (one in C++11 and one in C++20), which I think illustrate something interesting about “concept overloading” versus… other things.

1. Absolute value for signed and unsigned

Let’s write a function template that returns the absolute value of its integral input. If the type of the input is signed, we compute (x < 0) ? -x : x. If it’s unsigned, we simply return x.

Tag-dispatch solution

// A
template<class T>
T abs_impl(T x, std::false_type) {
    return (x < 0) ? -x : x;
}

template<class T>
T abs_impl(T x, std::true_type) {
    return x;
}

template<class T>
T abs_value(T x) {
    return abs_impl(x, std::is_unsigned<T>{});
}

int main() {
    assert(abs_value(-42) == 42);
    assert(abs_value(42u) == 42u);
}

C++20 Concepts solution

// B
template<std::signed_integral T>
T abs_value(T x) {
    return (x < 0) ? -x : x;
}

template<std::unsigned_integral T>
T abs_value(T x) {
    return x;
}

int main() {
    assert(abs_value(-42) == 42);
    assert(abs_value(42u) == 42u);
}

Notice that no type can satisfy both signed_integral and unsigned_integral at the same time, so this Concepts-based solution is equivalent to the C++14 enable_if approach denigrated by Stroustrup as a “workaround”:

// C
template<class T, std::enable_if_t<std::is_signed<T>::value, int> = 0>
T abs_value(T x) {
    return (x < 0) ? -x : x;
}

template<class T, std::enable_if_t<std::is_unsigned<T>::value, int> = 0>
T abs_value(T x) {
    return x;
}

int main() {
    assert(abs_value(-42) == 42);
    assert(abs_value(42u) == 42u);
}

In this kind of overload set, all the constraints are mutually exclusive, such that no more than one viable candidate is ever present. Overload resolution never has to choose the “best” overload from among a candidate set. As I understand it, this is not what we mean by “concept overloading.”

2. Advance for forward and random-access iterators

Let’s write a function template that returns its iterator argument advanced by two positions. For the general case of a forward iterator, we compute ++it twice. But if the input is additionally a random-access iterator, we can simply return it + 2.

Tag-dispatch solution

// D
template<class It>
It plus2_impl(It it, std::forward_iterator_tag) {
    ++it; ++it;
    return it;
}

template<class It>
It plus2_impl(It it, std::random_access_iterator_tag) {
    return it + 2;
}

template<class It>
It plus2(It it) {
    using Tag = typename std::iterator_traits<It>::iterator_category;
    return plus2_impl(it, Tag{});
}

The term “tag dispatch” is used to refer both to snippet D and to snippet A above. But notice the difference (not reflected in our terminology): In A, our abs_impl helpers were designed so that only one of them would be viable. In D, our plus2_impl helpers are designed so that the first overload is always viable, and then the second overload is sometimes also viable; when it is, it’s the better match (over.ics.rank/4.4.4).

C++20 Concepts solution (concept overloading)

// E
template<std::forward_iterator It>
It plus2(It it) {
    ++it; ++it;
    return it;
}

template<std::random_access_iterator It>
It plus2(It it) {
    return it + 2;
}

In this overload set, the constraints are not mutually exclusive. Like D, snippet E relies on overload resolution to choose the “best” overload from among a candidate set. Rather than rely on the best-match rules regarding inheritance, E relies on the fact that the second overload’s constraint subsumes the first overload’s constraint, and thus (over.match.best.general/2.6) the second overload is the best match whenever it is viable at all.

My understanding is that this is what Stroustrup refers to as “concept overloading.”

Final thoughts

UPDATE, 2021-06-08: I’ve changed my snippets F, G, and H to relax the signed_integral constraint into integral; originally, I’d relaxed the unsigned_integral constraint. Reddit commenter “Quincunx271” rightly pointed out that that way was strictly worse. Our new version of F has one generic overload that works correctly for all integral types, plus a faster overload for unsigned types. (This is the same overall pattern as D and E.) My original post’s F had a generic overload that worked correctly for unsigned types but incorrectly for signed types, and then an overload “rescuing” the correct behavior for signed types. That was unnecessarily silly of me.

Reddit commenter “sphere991” observes that in real life we could use if constexpr. I absolutely agree; I prefer if constexpr over any multi-function approach. But this post wasn’t about if constexpr; it was about “concept overloading.” :)

It seems to be the current wisdom that if you’re writing an overload set with C++20 Concepts, you needn’t go out of your way to make your constraints mutually exclusive. I claim that all of my examples are reasonable C++20; but I do have the impression that some commentators would go so far as to remove the constraint from one or the other of the overloads in snippet B, or relax it until it was no longer mutually exclusive. Thus:

// F
template<std::integral T>  // note: NOT signed_integral
T abs_value(T x) {
    return (x < 0) ? -x : x;
}

template<std::unsigned_integral T>
T abs_value(T x) {
    return x;
}

The tag-dispatch equivalent would be to replace false_type with an ellipsis:

// G
template<class T>
T abs_impl(T x, ...) {
    return (x < 0) ? -x : x;
}

template<class T>
T abs_impl(T x, std::true_type) {
    return x;
}

template<class T>
T abs_value(T x) {
    return abs_impl(x, std::is_unsigned<T>{});
}

or use a template of two parameters:

// H
template<class T, class U>
T abs_impl(T x, U) {
    return (x < 0) ? -x : x;
}

Personally, I would vastly prefer to read snippet A than to read either G or H.

I have used H in situations dispatching on multiple bools at a time — where we want to do one thing if b1 and b2 are both true, a different thing if b1 is true and b2 is false, and a third thing if b1 is false regardless of b2.)

For me personally, so far, my attitude carries over into C++20 Concepts: I find snippet B strictly more understandable at a glance than snippet F, because you can understand each candidate’s purpose in isolation, without even knowing that the other candidate exists. However, in cases where the overloads express a natural hierarchy (by which I mean “a hierarchy that the educated reader ought to know to be on the lookout for”), concept overloading could be the simplest approach. For example, I find snippet E reasonably understandable. I would not be tempted to rewrite it merely to introduce mutually exclusive constraints:

// J
template<std::forward_iterator It>
    requires (!std::random_access_iterator<It>)
It plus2(It it) {
    ++it; ++it;
    return it;
}

template<std::forward_iterator It>
    requires std::random_access_iterator<It>
It plus2(It it) {
    return it + 2;
}

Although honestly, if I saw that a coworker had already written snippet J, I would not be tempted to change it back to snippet E!

Concept overloading allows you to increase the number of viable candidates in your overload sets, without introducing artificial inheritance hierarchies of tag types á là std::forward_iterator_tag or priority_tag. But ultimately it is just another tool in the C++ toolbox; you can be allowed to use it, without feeling obliged to use it.