Why Can C Objects of the Same Class Access Each Other's Private Data?

Understanding Data Accessibility in Same-Class Objects

In C , classes often contain private data members to encapsulate sensitive or internal information. However, it may seem counterintuitive that objects belonging to the same class can access each other's private data. This phenomenon raises the question, "Why is this allowed?"

Access Control in C

Unlike some other programming languages, C implements access control at the class level. This means that when a private data member is declared within a class, it is inherently inaccessible to all entities outside that class. However, there is an exception to this rule.

Exception: Same-Class Accessibility

Objects created from the same class share a common blueprint and, therefore, have access to each other's private data. This is because the access control rules are enforced on a per-class basis, not on a per-object basis.

Reasoning

This design decision is based on the purpose of private data members. Private data is intended to protect data from external access, but it does not need to be hidden from other objects within the same class. In fact, sharing private data between same-class objects often allows for efficient and reusable code.

Example

Consider the following code:

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 code compiles and runs successfully because object a can access the private mData data member of object b within the getData method. This feature facilitates data sharing and collaboration within a class.

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