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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
class Foo {
public:
void za() { cout << "🍕"; }
int n = 42;
};
int main() {
Foo f;
f.za();
cout << f.n << '\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.
private member access
class Foo {
private:
void za() { cout << "🍕"; }
int n = 42;
};
int main() {
Foo f;
f.za();
cout << f.n << '\n';
}
c.cc:9: error: 'void Foo::za()' is private within this context
c.cc:3: note: declared private here
c.cc:10: 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:
void za() { cout << "🍕"; }
int n = 42;
};
int main() {
Foo f;
f.za();
cout << f.n << '\n';
}
c.cc:9: error: 'void Foo::za()' is protected within this context
c.cc:3: note: declared protected here
c.cc:10: 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.