[jcelerier]

> You can’t strictly enforce the invariant I just stated via OOP.

but you can strictly enforce it in pretty much every relevant OOP language - Java, C#, C++, C, Rust, D, ... by defining a class / struct / record with these two members, which is the only thing that matters. No one programs in abstract design principles, only in actual programming languages.

[oivey]

Ok, show me in C++. You have these three structs, which you cannot alter the signature of (maybe from an external library, maybe you don't want to introduce a type hierarchy in someone else's code, etc).

  struct Name {
    std::string firstName
    std::string lastName
  }
  struct AnotherName {
    std::string lastName
    std::string firstName
  }
  struct YetAnotherName {
    std::string middleName
    std::string lastName
    std::string firstName
  }

Write a single function that returns firstName and lastName concatenated. Bonus: write it for any struct containing firstName and lastName. The only way I can think to do it is via templates, which aren't traditional OOP and have their own downsides. Concepts in C++20 look like they make this much easier, but, again, not expressed via traditional object orientation and still infects your code with templates.

This isn't theoretical. I often don't want and often cannot use inheritance-based polymorphism. If I'm using a language where that is the only option, I'm struck writing tons of redundant, error prone, pointless, and brittle glue code. The amount of glue explodes combinatorially. That glue code can contain errors that the type checker won't find.

The inverse of this problem is also interesting. Someone wrote a function to concatenate the strings in Name. I can't put AnotherName into it unless the original author had the forethought to make their function templated. I guess the future of C++ is that all code ever lives in headers.

[jcelerier]

> The goal of OOP or any programming system is to help you enforce invariants to improve correctness of a program.

enforcing invariants means reducing the number of types that satisfy the invariant. I wouldn't call doing what you ask for "enforcing" any invariant.

> Bonus: write it for any struct containing firstName and lastName.

well,

    auto concatenate(auto t) { return t.firstName + " " + t.lastName; }

satisfies your condition - here's one that'll also handle middle names: https://gcc.godbolt.org/z/YfTYs6MMn ; again I don't think anyone wants this when they say "enforcing invariants", this does exactly the opposite of what is actually wanted.

> I guess the future of C++ is that all code ever lives in headers.

or in modules, which makes it very similar than other languages with generics instantiation

[oivey]

Ahh you’re, right, I forgot about fully auto’d functions. I think want more structure than just auto everything, although the compilers should find mistakes with that. Concepts will be nice.

[jcelerier]

The concept version here would look like:

    template<typename T>
    concept WesternishName = requires (T t) { 
      { t.firstName } -> convertible_to<string>; 
      { t.lastName } -> convertible_to<string>;
    };

    auto concatenate(WesternishName auto t) { return t.firstName + " " + t.lastName; }