[TeMPOraL]

Yeah, but that's the thing: in case of dynamic dispatch in OOP, the function is tied to the argument you're passing. In a typical OOP language, your object carries an extra pointer that's hidden from your view - a pointer to a table of pointers to functions, specific for that object's class. Dynamic dispatch goes through that pointer on the object.

Doing what you describe with a struct would require at least keeping explicit function pointers on the structure; the OOP mechanism described above is a generalization of that.

(There are other, neater approach too, allowing late binding dispatched on the types of more than one argument - see e.g. methods in Common Lisp.)