Re: [sv-ec] Query related to virtual methods in a class

Arturo,

Your replay is a little confusing to me...

Arturo Salz wrote:

> Don,
>
>  
>
> ...
>
>  
>
First you note that "an extended class is not required to override a 
virtual method

> As for your question. An extended class is not required to override 
> (or re-implement) a virtual method introduced by any of its ancestors.
>
Then you state that if the parent class is virtual with a virtual 
method, the exended class must provide an implementation for that 
virtual method.for the class to be come concrete.

> If the parent class is virtual (i.e., abstract) and the virtual method 
> is undefined then the extended class must provide an implementation 
> for the virtual method in order for the class to become concrete - and 
> be able to create objects of that class. If, on the other hand, the 
> parent class is not virtual and does provide an implementation of the 
> virtual method then the extended class does not need to override the 
> virtual method; it will simply inherit the virtual method implemented 
> by its closest ancestor. Perhaps, if you provide an example we can 
> identify the exact problem you are seeing.
>
>  
>
>             Arturo
>
So,  as I understand this, implementing virtual methods in extended 
classes are optional.  Of course, if they are implemented, they must 
follow the prototype declared in the base class.

Using (and modifying code from Gord's email) to elaborate


module top;
  virtual class base;
    virtual task t;
    endtask // t
   
    virtual task t2;
      $display("in base::t2");
    endtask // t2
   
    virtual task t3;
      $display("in base::t3");
    endtask // t3
  endclass
 
  virtual class mid1 extends base;
    task t;
      $display("in mid1::t");
    endtask // t
  endclass
 
  class mid2 extends base;
    task t2;
      $display("in mid2::t2");
    endtask // t2
  endclass
 
  class mid3 extends base;
  endclass // mid3

  class bot extends mid1;
  endclass // bot
 
 
//  mid1 m1 = new;
  mid2 m2 = new;
  mid3 m3 = new;
  bot  b  = new;
 
  initial begin
//    m1.t;  // prints:    in mid1:t
//    m1.t2;  // prints:     in base:t2
   
    $display("entering mid2");
    m2.t;
    m2.t2;
    m2.t3;
    $display("leaving mid2");
    $display;

    $display("entering mid3");
    m3.t;
    m3.t2;
    m3.t3;
    $display("leavinging mid3");
    $display;

    $display("entering bot");
    b.t;
    b.t2;
    b.t3;
    $display("leavinging bot");
  end
endmodule


The results from simulating the code above:
# entering mid2
# in mid2::t2
# in base::t3
# leaving mid2
#
# entering mid3
# in base::t2
# in base::t3
# leavinging mid3
#
# entering bot
# in mid1::t
# in base::t2
# in base::t3
# leavinging bot


My conclusions are, none of the extended classes mid1, mid2, mid3 are 
required to included and define the virtual task declared in virtual 
base classes.

The reason I am asking all this is due to a sentence in section 7.19:
"In general, if an abstract class has any virtual methods, all of the 
methods must be overridden (and provided with a method body) for the 
subclass to be instantiated."

This implies to me that the virtual methods must be defined in the 
extended classes.  Ummm, that is not what I am seeing from simulation.

(Note that I did not make any of the task "pure".)


 

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Don Mills
LCDM Engineering     (Logic, Coding, & Design Methodology) 
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