Why do objects of the same class have access to each other's private data?

Question

class TrivialClass {
public: 
  TrivialClass(const std::string& data) :
    mData(data) {};

  const std::string& getData(const TrivialClass& rhs) const {
    return rhs.mData;
  };

private:
  std::string mData;
};

int main() {
  TrivialClass a("fish");
  TrivialClass b("heads");

  std::cout << "b via a = " << a.getData(b) << std::endl;
  return 0;
}

This codes works. It is perfectly possible for object a to access private data from object b and return it. Why should this be so? I would think that private data is private. (I started out by trying to understand copy constructors in the pimpl idiom, but then I discovered that I didn't even understand this simple situation.)

user_71131 · Answer

In addition to all the answers above, consider custom copy constructors, assignment operators and all the other functions you would write for a class which operate on other instances. You would need accessor functions for all those data members.

Why do objects of the same class have access to each other's private data?

7 Answers