[maattdd]

Inheritance (the subtyping part of it) is considered the OOP way to write sum type.

sum Expr { Int; Add(Int,Int) }

VS

class Expr { }

class Add extends Expr { Expr left; Expr right; }

[OvbiousError]

You should probably look up what a sum type is, it has nothing to do with summations. Your example doesn't contain a sum type. A C++ example:

    std:variant<int,std::string> sum_type_instance = 5;

[maattdd]

My sum type example is exactly this (but I didn't use C++ std::variant<> syntax to not confuse the reader).

The most common example of a sum type is the "Expression problem" - please read some literature before commenting on a topic.

(Btw, it's called sum type for a reason: summation. The cardinality of the sum type is the sum of the cardinality of its variants)

[Maxatar]

You seem to have a very narrow C++ view of what sum types are. Sum types are related to the expression problem, and the given example is the canonical instance of the expression problem.

https://en.wikipedia.org/wiki/Expression_problem

[gpderetta]

The expression problem enters the room.

[searealist]

That's not C++.

[maattdd]

Any reader who comment here hopefully has enough knowledge to understand the implied C++.

struct Add {} std::variant<Add, int> Expr;

OR

class Expr {} class Add : Expr {}