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

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

It is surprising that objects of the same class can access each other's private data. Given that private data is supposed to be private, why does this occur?

Understanding Class-Level Access Control

In C , access control is implemented on a per-class basis, not per-object basis. This means that the access privileges for a particular piece of data are determined by the class it belongs to, not by the object that holds it.

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;
};

Here, the getData method of TrivialClass can access the mData member of another TrivialClass object. This is because access control is determined by the class itself. The private access specifier makes mData private only within the TrivialClass class and its derived classes.

Implications of Class-Level Access Control

Class-level access control has several implications:

• Objects of the same class can share access to private data, even if they are not derived from each other.
• It is not possible to restrict access to private data on a per-object basis.
• Encapsulation and data hiding are not completely enforced at the object level.

Conclusion

While class-level access control may seem counterintuitive, it allows C programmers to define and interact with classes and objects in a way that leverages the static nature of the language. It is important to understand the implications of this model when designing and implementing C code.

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