- There is a close relationship
between the
child class
and its
parent class:
- All variables
in the
parent class
are also defined in the
child class
- All functions (methods) in the parent class are also defined in the child class
- All variables
in the
parent class
are also defined in the
child class
- In other words:
Anything (assignment, function call, etc) that you can do with a variable of the parent class, you can do it with a variable of the child class !!!
- Because of the
close relationship
between the child class and its
parent class, we can
convert (cast)
a
variable of the child class
to a
variable of the parent class
- Example:
class parent { public: int i1; void f1() { cout << "parent method" << endl; } }; class child : public parent { public: int i2; void f2() { cout << "child method" << endl; } };
int main(int argc, char **argv) { parent *p; child y, *q; y.i1 = 1; q = &y; // This is normal... p = &y; // This is NOT normal ! // But is it allowed because y has // everything that p needs to do its job p->i1 = 1234; // You can use i1 in the "child" variable through q p->f1(); // You can use f1() in the "child" variable through q }
- Example Program:
(Demo above code)
- Prog file: click here
- After you
convert (cast)
a
variable of the child class
to a
variable of the parent class -
e.g., using:
parent *p; child y, *q; p = &y; // p is a (parent *) variable // But p points to a (child *)you can no longer access variables and functions in the child class
- Example:
class parent { public: int i1; void f1() { cout << "parent method" << endl; } }; class child : public parent { public: int i2; void f2() { cout << "child method" << endl; } };
int main(int argc, char **argv) { parent *p; child y, *q; y.i1 = 1; p = &y; // y has a mamber i2... p->i2 = 1234; // **** Illegal: i2 is not in parent class !!! }
- Example Program:
(Demo above code)
- Prog file: click here
- After you
convert (cast)
a
variable of the child class
to a
variable of the parent class -
e.g., using:
parent *p; child y, *q; p = &y; // p is a (parent *) variable // But p points to a (child *)
you can convert (cast) the parent class reference to a child class reference using:
parent *p; child y, *q; p = &y; // p is a (parent *) variable // But p points to a (child *) q = (child *) p; // Forced conversion...
- This is a useful conversion,
because you can
get back
the variables and functions that
are defined in the
child class
- Example:
class parent { public: int i1; void f1() { cout << "parent method" << endl; } }; class child : public parent { public: int i2; void f2() { cout << "child method" << endl; } };
int main(int argc, char **argv) { parent *p; child y, *q; y.i1 = 1; p = &y; // y has a mamber i2... // But cannot be accessed through p... q = (child *) p; // Forced conversion... q->i2 = 1234; // **** Now legal !!! }
- Example Program:
(Demo above code)
- Prog file: click here
- Because a
variable of the parent class
do NOT
have every variable and
every function
in the
child class,
the
converion (casting)
of a
reference of the child class
to a
reference of the parent class
can result in
ERROR
- Example:
class parent { public: int i1; void f1() { cout << "parent method" << endl; } }; class child : public parent { public: int i2; void f2() { cout << "child method" << endl; } };
int main(int argc, char **argv) { parent x, *p; child *q; x.i1 = 1; p = &x; // p points to a parent variable (object) q = (child *) p; // Forced conversion... q->i1 = 1234; // OK q->i2 = 1234; // **** Can cause problems !!! // **** Access a non-existing variable !! }
- Example Program:
(Demo above code)
- Prog file: click here
- Because the
conversion
of a
parent reference
to a
child reference
may cause
program execution errors,
the conversion
requires an
explicit cast operation
parent x, *p; child *q; q = &x; q = p; // Not allowed q = (child *) &x; q = (child *) p; // Need explicit cast operation