- After
inheriting
the
variables and functions from a
parent class, you can
do the following to the
child class:
- Add
one or more
new variables
to the
child class
If you need to remember more information, you must define new variables in the child class
- Add
one or more
new functions
to the
child class
If you need more functionality, you must define new functions in the child class
- Modify the behaviour of one or more existing functions in the child class !!!
- Add
one or more
new variables
to the
child class
- To
modify the behavior
of an (inherited) function in the
child class, you:
Define a function with the same name and same number and type of parameters in the child class - When you define
a function with the
same name
and
same number and type
of parameters
in the
child class,
the (inherited) function from the
parent class
is
overridden
- Example:
class parent { public: int i1; void f1() { i1 = i1 + 100; cout << "parent method increases i1" << endl; } }; class child : public parent { public: void f1() { i1 = i1 - 100; cout << "child method DEcreases i1" << endl; } };
int main(int argc, char **argv) { child y, *q; y.f1(); // invoke f1() in child q = &y; q->f1(); // invoke f1() in child }
- Example Program:
(Demo above code)
- Prog file: click here
- It is a common practice to override functions in a parent-class because the functionality of the function in the parent is unsuitable for the child class.
- Recall that we can
convert (cast)
a
child reference
and
parent reference
safely:
class parent { public: int i1; void f1() { i1 = i1 + 100; cout << "parent method increases i1" << endl; } }; class child : public parent { public: void f1() { i1 = i1 - 100; cout << "child method DEcreases i1" << endl; } };
parent *p_ptr; child c, *c_ptr; p_ptr = &c; p_ptr = c_ptr;
- $64,000 question:
- Which function
will the following construct invoke:
parent *p_ptr; child c; p_ptr = &c; p_ptr->f1(); <------ ????
- Is it:
- f1() in the child's class ?
- Or is it:
- f1() in the parent's class ?
- Which function
will the following construct invoke:
- Example Program:
(Demo above code)
- Prog file: click here
You will see that in this case , the invocation: parent *p_ptr; child c; p_ptr = &c; p_ptr->f1(); <------ invokes f1() in PARENT class
- Now consider
the
following program:
(notice the key word
virtual)
class parent { public: int i1; virtual void f1() { i1 = i1 + 100; cout << "parent method increases i1" << endl; } }; class child : public parent { public: virtual void f1() { i1 = i1 - 100; cout << "child method DEcreases i1" << endl; } };
parent *p_ptr; child c, *c_ptr; p_ptr = &c; p_ptr = c_ptr;
- $64,000 question:
- Which function
will the following construct invoke:
parent *p_ptr; child c; p_ptr = &c; p_ptr->f1(); <------ ????
- Is it:
- f1() in the child's class ?
- Or is it:
- f1() in the parent's class ?
- Which function
will the following construct invoke:
- Example Program:
(Demo above code)
- Prog file: click here
You will see that in this case , the invocation: parent *p_ptr; child c; p_ptr = &c; p_ptr->f1(); <------ invokes f1() in PARENT class
- If the variable
x
is a variable of type
Class_X , but
references a variable of the type
Class_Y ,
then the invocation
x->func()
will invoke:
- the function func()
in
Class_X
if the function
is real
- the function func() in Class_Y if the function is virtual
- the function func()
in
Class_X
if the function
is real
- In other words:
- A real function
will
"follow"
the
type of
the declared variable
(i.e., static)
- A virtual function will "follow" the type of the actual variable (i.e., dynamic)
- A real function
will
"follow"
the
type of
the declared variable
(i.e., static)
|
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- Polymorphism
means
multiple forms
Polymorphism is the ability to use one expression to convey multiple "meanings" (effects)/B>
- In terms on
object oriented programming:
Function polymorphism is the ability to one expression to invoke different functions (under different circumstances).
- Example:
class myOldClass { public: int i1; virtual void func1(int i) { i1 = i1 + i*i; } };
class myNewClass : public myOldClass { public: int i2; virtual void func1(int i) { i2 = i2 + 1000*i; } };Consider now the expression:
myOldClass A; myNewClass B; myOldClass *ptr; ptr = &A; ptr->func1(4); // Invokes func1() in myOldClass ptr = &B; ptr->func1(4); // Invokes func1() in myNewClass
Make sure you notice that:
- The same expression
ptr->func1(4);
is used to invoke two (multiple) functions !!
- In one case, it invokes
func1()
in
myOldClass
- In the other case, it invokes func1() in myNewClass
- The same expression
- Example Program:
(Demo above code)
- Prog file: click here
- Previously, we
assigned
a reference to a child/parant variable
explicitly:
myOldClass A; myNewClass B; myOldClass *ptr; ptr = &A; ptr->func1(); ptr = &B; ptr->func1();
- We can assign (pass)
a reference to a child/parant variable
as a parameter
Example:
void doSomething( myOldClass *ptr ) { ptr->func1(4); }
Consider now the expression:
myOldClass A; myNewClass B; doSomething( &A ); // Will invoke func1() in myOldClass doSomething( &B ); // Will invoke func1() in myNewClass
Here, the assignment ptr = &A and ptr = &B is done through passing of parameters
Make sure you notice that:
- The same function call
doSomething(.);
is used to invoke two (multiple) different functions !!
- In one case, it invokes
func1()
in
myOldClass
- In the other case, it invokes func1() in myNewClass
- The same function call
- Example Program:
(Demo above code)
- Prog file: click here
- We can also
pass a
a reference to a child/parant variable
by reference
to achieve function polymorphism
Example:
void doSomething( myOldClass & x ) { x.func1(4); }
The function call will become:
myOldClass A; myNewClass B; doSomething( A ); // Will invoke func1() in myOldClass doSomething( B ); // Will invoke func1() in myNewClass
Here, the reference &A and &B is done through passing of the parameter by reference
- Example Program:
(Demo above code)
- Prog file: click here
- We cannot
pass a
a reference to a child/parant variable
by VALUE
to achieve function polymorphism
Example:
void doSomething( myOldClass x ) { x.func1(4); }The compiler will use static binding in this case...
These function calls will both the function in myOldClass:
myOldClass A; myNewClass B; doSomething( A ); // Will invoke func1() in myOldClass doSomething( B ); // Will ALSO invoke func1() in myOldClass
- Example Program:
(Demo above code)
- Prog file: click here
- Recall the
desired solution
for the Jacobi iteration:
Vector Jacobi(Matrix P, Matrix Q, Vector b) { Vector v1, v2; int i; double d; for (i = 0; i < 3; i = i + 1) { v1.x[i] = 0; v2.x[i] = 1; } d = 0.1; while ( d > 0.0001 ) { v2 = P*v1 + Q*b; d = distance(v1, v2); v1 = v2; } return(v2); }
- When solving equation in
dense representation
we write:
DenseMatrix P1, Q1; Vector sol, b; sol = Jacobi( P1, Q1, b);
- When solving equation in
sparse representation
we write:
SparseMatrix P1, Q1; Vector sol, b; sol = Jacobi( P1, Q1, b);
Nothing else need to be changed !!!
- Now I can show you how it can be done using
function polymorphism
!!!