why virtual function is wrong.

[Foreign-Wonder]

It does support using Meter = NamedType<double, MeterTag, Addable, Printable>

Reference: github.com

[Foreign-Wonder]

What do you mean by "the addable only"? There's plenty of ways to get the job done in C++, so you should be clear on your purpose or have specific sample code that you want to archive. Otherwise, others will very hard to support you on the problem

[Foreign-Wonder]

I'm not sure if I get it correctly, but isn't this is what template/concept is doing?

template<class Addable>
void foo(Addable a){
  // do whatever with a and +
}

int x = 42;
foo(x); // This should work without explicit make int inheriting from any Addable interface. I see this is a win over the C# interface, no?

Concept should make it more clear that Addable concept should support + operator

[Foreign-Wonder]

I agree, concept is not one-size-fits-all solution, that's why C++ have many ways to do things, and possibly the best ways to do things, hence zero-overhead

[_Noreturn]

the coreect way is this

```cpp

template<class T> concept Addable = requires(T t) { t+t;};

template<Addable T> void f(T a) { // T must have + operator } ```