What is the difference between public, protected, and private inheritance?

Question

Accepted Answer

**public** — "is-a" relationship. **protected** — "implemented-as". **private** — "implemented-in-terms-of".

```cpp
class Base {
public:
    int public_member;
protected:
    int protected_member;
private:
    int private_member;
};

// Public inheritance
class PublicDerived : public Base {
public:
    void access() {
        public_member;      // OK
        protected_member;   // OK
        // private_member;  // Error
    }
};

int main() {
    PublicDerived obj;
    obj.public_member;      // OK
    // obj.protected_member;  // Error
}

// Protected inheritance
class ProtectedDerived : protected Base {
public:
    void access() {
        public_member;      // OK
        protected_member;   // OK
    }
};

// Private inheritance
class PrivateDerived : private Base {
public:
    void access() {
        public_member;      // OK
        protected_member;   // OK
    }
};
```

**Summary:**
- **public** — public stays public, protected stays protected.
- **protected** — public becomes protected, protected stays protected.
- **private** — public becomes private, protected becomes private.

C++: What is the difference between public, protected, and private inheritance?