CS253 Access

Member access

Class members (data & methods) can have public:, private:, or protected: access.
These are sections. Don’t put public on every member–that’s Java.
The default is private for class, public for struct.
- public: anyone can access them
- private: only methods of this class can access them
- protected: can be accessed by this class, and by immediately derived class

public member access

struct Foo {
    auto ff() { return "🍟"; }
    int n = 43;
};

int main() {
    Foo f;
    cout << f.ff() << f.n;
}

🍟43

class Foo {
  public:
    auto za() { return "🍕"; }
    int n = 42;
};

int main() {
    Foo f;
    cout << f.za() << f.n;
}

🍕42

A public data member is rarely the right thing to do, but it can be justified for a class that’s really just a struct in fancy clothing. At that point, you may as well just make it a struct.

private member access

class Foo {
  private:
    auto za() { return "🍕"; }
    int n = 42;
};

int main() {
    Foo f;
    cout << f.za() << f.n;
}

c.cc:9: error: 'auto Foo::za()' is private within this context
c.cc:3: note: declared private here
c.cc:9: error: 'int Foo::n' is private within this context
c.cc:4: note: declared private here

Only Foo can use private members.

protected member access

class Foo {
  protected:
    auto za() { return "🍕"; }
    int n = 42;
};

int main() {
    Foo f;
    cout << f.za() << f.n;
}

c.cc:9: error: 'auto Foo::za()' is protected within this context
c.cc:3: note: declared protected here
c.cc:9: error: 'int Foo::n' is protected within this context
c.cc:4: note: declared protected here

Only Foo and its immediately derived classes can use protected members.

Inheritance access

There is also public, private, or protected inheritance.
- public inheritance
  - public members in base class are my public members
  - protected members in base class are my private members
- private inheritance
  - public & protected members in base class are my private members
- protected inheritance
  - public & protected members in base class are my protected members
The default is private for class, public for struct.
- Ignore the default—always specify!

public inheritance

class B {
  public:    void pub() {}
  private:   void priv() {}
  protected: void prot() {}
};
class D : public B {
  public:
    D() {
        pub();
        priv();
        prot();
    }
};
int main() {
    D d;
    d.pub();
    d.priv();
    d.prot();
}

c.cc: In constructor 'D::D()':
c.cc:10: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:10: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:17: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:18: error: 'void B::prot()' is protected within this context
c.cc:4: note: declared protected here

private inheritance

class B {
  public:    void pub() {}
  private:   void priv() {}
  protected: void prot() {}
};
class D : private B {
  public:
    D() {
        pub();
        priv();
        prot();
    }
};
int main() {
    D d;
    d.pub();
    d.priv();
    d.prot();
}

c.cc: In constructor 'D::D()':
c.cc:10: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:10: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:16: error: 'void B::pub()' is inaccessible within this context
c.cc:2: note: declared here
c.cc:16: error: 'B' is not an accessible base of 'D'
c.cc:17: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:17: error: 'B' is not an accessible base of 'D'
c.cc:18: error: 'void B::prot()' is protected within this context
c.cc:4: note: declared protected here
c.cc:18: error: 'B' is not an accessible base of 'D'

protected inheritance

class B {
  public:    void pub() {}
  private:   void priv() {}
  protected: void prot() {}
};
class D : protected B {
  public:
    D() {
        pub();
        priv();
        prot();
    }
};
int main() {
    D d;
    d.pub();
    d.priv();
    d.prot();
}

c.cc: In constructor 'D::D()':
c.cc:10: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:10: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:16: error: 'void B::pub()' is inaccessible within this context
c.cc:2: note: declared here
c.cc:16: error: 'B' is not an accessible base of 'D'
c.cc:17: error: 'void B::priv()' is private within this context
c.cc:3: note: declared private here
c.cc:17: error: 'B' is not an accessible base of 'D'
c.cc:18: error: 'void B::prot()' is protected within this context
c.cc:4: note: declared protected here
c.cc:18: error: 'B' is not an accessible base of 'D'

Summary

public inheritance: Used for is-a relationships. The derived class gets the base class methods.
private inheritance: A sort of secret has-a relationship. The derived class gets the base class methods, but they’re private. It might be better to declare the base class as a private data member, instead.
protected inheritance: Used only by the confused, the desperate, and the wicked.

CS253: Software Development with C++

Fall 2020

Access