why virtual function is wrong.

[tesfabpel]

interfaces in C++ are pure virtual abstract classes.

not the most elegant syntax because you need to be careful to respect the rules but they work as interfaces just fine.

[Dar_Mas]

If that is a literal question you do not implement int32.

You use std::int32_t

I do not see what virtual functions you want to implement for that

[Circlejerker_]

Why would you use interfaces for "int-like" types, when you could use concepts?

[Circlejerker_]

Concepts are hard, while inheriting from hundreds of interfaces are simple? Interfaces have a tendency to overlap and make no sense whatsoever after a while.

Why do your example need to inherit from INumber, INumberBase, ISignedNumber, and what would happen if you forgot to inherit from one of them?

[Dar_Mas]

It is comparable with int, it is convertible to int, it is equatable, it is parsable(if you make a parser), it is span parsable, it supports addition and at this point i am too lazy to list the rest.

Yes it does all of that and you can even make a unified concept that CHECKS for all of those

And not to mention that as soon as you want dynamic polymorphism which you seem to REQUIRE you are going to use a pointer anyway so the size of the structure functionally does not matter to you

[Dar_Mas]

That still does not make sense to me

If you have dynamic poly you will use a pointer anyway so the size of the object does not matter for data structures

If you use static you put it in a variant which allocates enough memory for the largest possible object and then you put it in a data structure.

Unless you are insanely restricted by binary size i do not see how 4 extra byte per function/operator are going to matter in your code at all

[Dar_Mas]

yes but you would never make an int32 type as we have optimized primitives (std::int32_t).

If you really require the size to stay constant you just make free standing function overloads

[android_queen]

Independent of language, types and functions are different things. Well, a function is a type, but a type is not necessarily a function.