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

Question

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

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.)

Answer 1

Because that's how it works in C++. In C++ access control works on per-class basis, not on per-object basis.

Access control in C++ is implemented as a static, compile-time feature. I think it is rather obvious that it is not really possible to implement any meaningful per-object access control at compile time. Only per-class control can be implemented that way.

Some hints of per-object control are present in protected access specification, which is why it even has its own dedicated chapter in the standard (11.5). But still any per-object features described there are rather rudimentary. Again, access control in C++ is meant to work on per-class basis.