Re: [sv-ec] Abstract classes and virtual methods

I had the following comments on Dave's latest proposal for mantis 1308.

1. Overriding a virtual method

   Page 1, 1st paragraph, 3rd sentence:

   I still don't like this sentence. Both Gordon and I have previously
   suggested different ways to phrase this section. I have tried to refine
   my previous suggestion a bit more and now suggest the following.

   From:
      "A virtual method shall override a method in all of its base (parent)
      classes, whereas a non-virtual method shall only override a method
      in that class and its descendants."
   To:
      "The implementation of a virtual method shall override any implementations
      of that same virtual method that were made within any of its parent
      class whenever an object of that subclass is accessed through a handle
      to the defining subclass or a handle to any of its base classes. The
      implementation of a non-virtual method may only override that method
      within the subclass in which it is defined and shall not affect its
      parent classes."

2. Typos - Example on page 1

   From:
      $display("A is ",A);
   To:
      $display("A is %d",A);

   From:
      endfuncion : printA
   To:
      endfunction : printA

   From:
      $display("B is ",B);
   To:
      $display("B is %d",B);

   From:
      endfuntcion : printB
   To:
      endfunction : printB

   From:
      endclass : BasePacket
   To:
      endclass : My_Packet

3. Example on page 1

   It would be best to rework this example to make it a bit more clear
   what happens when a virtual method is overridden. The way that it is
   currently shown, there is no change in behavior between the two
   implementations. It is good to show how the values of A and B are displayed
   but there should also be some different behavior exhibited by the methods
   in the BasePacket and My_Packet classes. The example in section 7.13 shows
   one way to have an overridden method exhibit a difference in behavior (i.e.
   one negates the value returned and the other doesn't). Below I show a way
   to modify the calls to $display() to make the example more clear.

   From:
      class BasePacket;
         int A = 1;
         int B = 2;
         function void printA;
            $display("A is %d",A);
         endfunction : printA
         virtual function void printB;
            $display("B is %d",B);
         endfunction : printB
      endclass : BasePacket
      class My_Packet extends BasePacket;
         int A = 3;
         int B = 4;
         function void printA;
            $display("A is %d",A);
         endfunction : printA
         virtual function void printB;
            $display("B is %d",B);
         endfunction : printB
      endclass : My_Packet
   To:
      class BasePacket;
         int A = 1;
         int B = 2;
         function void printA;
            $display("A is %d",A);
         endfunction : printA
         virtual function void printB;
            $display("B is %d",B);
         endfunction : printB
      endclass : BasePacket
      class My_Packet extends BasePacket;
         int A = 3;
         int B = 4;
         function void printA;
            $display("MyPacket::A is %d",A);    <---- changed here
         endfunction : printA
         virtual function void printB;
            $display("My_Packet::B is %d",B);   <---- changed here
         endfunction : printB
      endclass : My_Packet

4. Results from example on page 1

   This example uses an overridden property. This makes the example
   interesting (possibly more interesting than what Dave intended).

   Sub-clause 7.13 describes overridden members.
   3rd paragraph:

         LinkedPacket lp = new;
         Packet p = lp;

      In this case, references to p access the methods and class properties of
      the Packet class. So, for example, if class properties and methods in
      LinkedPacket are overridden, these overridden members referred to
      through p get the original members in the packet class. From p, new and
      all overridden members in LinkedPacket are now hidden.

   If all of that is true then what is the correct value for the line
   marked below?

      For this case a virtual method was redefined in a subclass and a property
      was also overridden by that same subclass definition. If we now access
      that overridden property through a handle to the base class using the
      virtual method, which value of that overridden property should be used?

   We need to be explicit about what should be returned in this case and not
   just rely on an example to define the desired semantics. What is shown
   below appears to be in contradiction with 7.13.

   BasePacket p1 = new;
   My_Packet  p2 = new;
   initial begin
      p1.printA;  // displays 'A is 1'
      p1.printB;  // displays 'B is 2'

      p1 = p2;
      p1.printA;  // displays 'A is 1'
      p1.printB;  // displays 'B is 4'   <----- is this correct?

      p2.printA;  // displays 'A is 3'
      p2.printB;  // displays 'B is 4'

5. Typos page 2

   From:
      In the that
   To:
      In that

   From:
      having a method body..
   To:
      having a method body.

6. Sub-clause 7.20

   The version of the LRM that I am looking at has this as 7.19. I don't
   understand why you have changed it to 7.20

7. Typos page 3

   From:
      as show below
   To:
      as shown below

8. Sub-clause 7.13

   This may be beyond the scope of this mantis item, but since we are
   looking at this section of the document, I would like to add a couple
   of lines to the example in this section. The existing example remains
   the same, just add two extra statements to the end.

   From:
      LinkedPacket lp = new;
      Packet p = lp;
      j = p.i;           // j=1, not 2
      j = p.get();       // j=1, not -1 or -2
   To:
      LinkedPacket lp = new;
      Packet p = lp;
      j = p.i;           // j=1, not 2
      j = p.get();       // j=1, not -1 or -2
      j = lp.i;          // j=2                  <---- added this
      j = lp.get();      // j=-2                 <---- added this


Neil


Rich, Dave wrote On 03/02/06 18:57,:
> New proposal has been uploaded to mantis 1308 to reflect changes to
> “pure virtual” syntax and to match C++ as stated below.
> 
>  
> 

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Neil Korpusik                                     Tel: 408-720-4852
Senior Staff Engineer                             Fax: 408-720-4850
Frontend Technologies - ASICs & Processors (FTAP)
Sun Microsystems
email: neil.korpusik@sun.com
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