In reply to dave_59:
In reply to haithamx15:
I think you are mixing two different concepts. Method overloading would be defining getArea() more than once within the same class with different prototypes. But with virtual method, the prototype is fixed in the Base class. You call a virtual method using a base Base class variable, but you don’t know if the variable hold a handle to the base or extended class object.
Overloading is in the same class understandable!
Dave look at the following code I’ve just coded to understand the concept of Virtual Classes better!
class Base;
string address;
function new(string address);
this.address = address;
endfunction
task show();
$display("BASE CLASS: Address = %0s",address);
endtask
endclass
class Rectangle extends Base;
string data;
function new(string address, string data);
super.new(address);
this.data = data;
endfunction
virtual task show();
$display("RECTANGLE CLASS: Addr=%0s, data=%0s",this.address,this.data);
endtask
endclass
class Circle extends Base;
string command;
string data;
function new(string address, string data, string command);
super.new(address);
this.data = data;
this.command = command;
endfunction
virtual task show();
$display("CIRCLE CLASS: Addr=%0s, data=%0s, CMD=%0s",this.address,this.data,this.command);
endtask
endclass
module tb;
Base b;
Circle c;
Rectangle r ;
Circle c2;
Base ba; //handle to base!
initial begin
b = new("Adr: BASE");
r = new ("Adr: Rect","RectData");
c = new ("Adr: Circle","CircData","BURST");
$display("\n \n \n \n ");
b.show(); //normal call
r.show(); //normal call
c.show(); //normal call
$display("\n \n \n \n ");
ba = c;
ba.show();
$cast(c2,ba);
c2.show();
end
endmodule
The output is like this:
BASE CLASS: Address = Adr: BASE
RECTANGLE CLASS: Addr=Adr: Rect, data=RectData
CIRCLE CLASS: Addr=Adr: Circle, data=CircData, CMD=BURST
BASE CLASS: Address = Adr: Circle
CIRCLE CLASS: Addr=Adr: Circle, data=CircData, CMD=BURST
and when modifying the base class and make the show() method virtual this is the output.
BASE CLASS: Address = Adr: BASE
RECTANGLE CLASS: Addr=Adr: Rect, data=RectData
CIRCLE CLASS: Addr=Adr: Circle, data=CircData, CMD=BURST
CIRCLE CLASS: Addr=Adr: Circle, data=CircData, CMD=BURST
CIRCLE CLASS: Addr=Adr: Circle, data=CircData, CMD=BURST
So cast made the it look like as if show() method was virtual, but by making it virtual in the first place it would behave as it should be. Now what we can understand from this?
-
Virtual Method calls the correct method of the object that the class variable/handle pointing to! (is it determined at compile time or at run time?)
-
dynamic casting was able to provide us with the same result to use the methods of the correct object/class but that required the programmer to know if that was a valid casting. else it would have returned a null pointer. (All done at runtime)
Dave, please correct me If I’m wrong. And I have another question. I still don’t grasp why would we need cast for? is there a practical example that shows the significance of it? I thoroughly watched your video of OOP and I understood it preciesly but wasn’t able to think of a case where we need it.
By the way, thank you dave for those tutorial videos, they are really professionaland helpful.