Comment by danhau

12 hours ago

> But then anyone could just instantiate an invalid Name without calling the parse_name function and pass it around wherever

This is nothing new in C. This problem has always existed by virtue of all struct members being public. Generally, programmers know to search the header file / documentation for constructor functions, instead of doing raw struct instantiation. Don‘t underestimate how good documentation can drive correct programming choices.

C++ is worse in this regard, as constructors don‘t really allow this pattern, since they can‘t return a None / false. The alternative is to throw an exception, which requires a runtime similar to malloc.

2 comments

danhau

maccard 10 hours ago

In C++ you can do: struct Foo { private: int val = 0; Foo(int newVal) : val(newVal) {} public: static optional<Foo> CreateFoo(int newVal) { if (newVal != SENTINEL_VALUE) { return Foo(newVal); } return {}; } };

    int main(int argc, char* argv[]) {
      if (auto f = CreateFoo(argc)) {
        cout << "Foo made with value " << f.val;
      } else {
        cout << "Foo not made";
      }
    }

apaprocki 10 hours ago

In C++ you would have a protected constructor and related friend utility class to do the parsing, returning any error code, and constructing the thing, populating an optional, shared_ptr, whatever… don’t make constructors fallible.