How to use concept to restrict typename to numeric? C++20 features

Question

I am learning some modern C++20 features. One of them is concept. I have read https://en.cppreference.com/w/cpp/language/constraints and https://www.modernescpp.com/index.php/defintion-of-concepts to get some examples to follow.

For now, I want to design a concept such that I can only accept numeric data type. In the traditional method, one can use

template<typename NumericType,
        typename = typename std::enable_if<std::is_arithmetic<NumericType>::value,NumericType>::type>

as suggested in Class template for numeric types

or one may also use static_assert() inside the definition of template class/function

static_assert(std::is_arithmetic<NumericType>::value, "NumericType must be numeric");

I am wondering what the syntax for concept should be? For now, I am doing

template<typename NumericType>
concept Numeric = std::is_arithmetic<NumericType>::value ;

and find that

template<Numeric T>
void f(T){}

can do the trick I expected (basically just duck typing). I am wondering am I correct or is there any difference?

Tufthunter

not sure what the question is, but what you did is same what std concepts do, see for example: en.cppreference.com/w/cpp/concepts/floating_point (except they use _v instead of ::value to save typing) – Counterstroke

std::floating_point doesn't quite do the trick. If you have integers mixed with floating point types, you may have issues. For example, check coliru.stacked-crooked.com/a/659565761faf0dac – Ona

Answer 1

I am wondering am I correct or is there any difference?

You're correct.

Using SFINAE / std::enable_if to constrain templates has the same essential effect of concept-constrained templates.

SFINAE has additional (negative) side-effects (see WG21 P0225R0 Why I want Concepts, and why I want them sooner rather than later).

SFINAE is less a technology and more hack of C++. From this point of view the most important thing concepts can do is to be human readable!

Also take a look at: Do C++20 Concepts replace other forms of constraints? for a borderline case.

I am wondering what the syntax for concept should be?

Your syntax is right. Perhaps

template<class T> concept Numeric = std::is_arithmetic_v<T>;

is a bit shorter.

Chapfallen

Answer 2

This worked for me:

template <typename T>
concept NumericType= std::integral<T> || std::floating_point<T>;

Montage

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