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

Oops - minor fix below (cut and paste from another example
I had lying around...)



Gordon Vreugdenhil wrote:

> Don,
> 
> Until a pure virtual method is overridden, all class types in
> the inheritance chain must be abstract (i.e. be "virtual class").
> So derived classes may or may not override a pure virtual
> method but if they don't, they must remain abstract.
> 
> module top;
>    virtual class base;
>       pure virtual task t;
>       virtual task t2;
>           $display("in base::t2");
>       endtask
>    endclass
> 
>    virtual class mid extends base;
>    endclass
> 
>    class bot extends mid;
>       task t;
>          super.t;

"super.t" is illegal here since "t" is pure virtual in mid (and base).

Gord.

>           $display("in bot::t");
>       endtask
>    endclass
> endmodule
> 
> It would be an error if "mid" was not an abstract class
> since the method "t" is still pure virtual in mid.  Once
> "bot" overrides "t", all methods have implementations and
> "bot" is no longer required to be abstact.
> 
> 
> If base::t was not pure, then "mid" would not have to
> be abstract.  The override in bot would still be valid.
> 
> Basically the rule is that not specifying anything in
> an intermediate class with respect to a method does not
> impact the method inheritance from the base class
> (pure virtual remains pure virtual, etc).
> 
> Gord.
> 
> 
> Don Mills wrote:
> 
>> I have second question on the same train of thought that Kakoli 
>> presented.  If a virtual base class contains a virtual method, then 
>> when the base class is extended, is the extended child class required 
>> to include the virtual method?  Or is the virtual method only a 
>> prototype if the child class wished to use that method?  I assumed 
>> that base class virtual methods were required to exist and be defined 
>> in extended classes, but that is not the case with all simulators.  Is 
>> my understanding wrong here?
>>
>>  
>> dm
>>
>>
>>     -----Original Message-----
>>     From: Kakoli Bhattacharya
>>     Sent: Oct 19, 2006 6:07 AM
>>     To: sv-ec@server.eda.org
>>     Subject: [sv-ec] Query related to virtual methods in a class
>>
>> Hello,
>>
>> Consider the following code:
>>
>> virtual class Parent;
>> integer it;
>> int i;
>> virtual task virtual_task(input reg rg);  //This is the declaration
>> endtask
>> endclass
>>
>> class Child1 extends Parent;
>> shortint si;
>> longint li;
>> virtual task virtual_task1(input reg rg1);
>> endtask
>> endclass
>>
>> class Child2 extends Child1;
>> real r1;
>> shortreal sr1;
>> time t1;
>> realtime rt1;
>> task virtual_task(input reg rg1);  // This is the redefinition
>> it = rg1;
>> i = rg1;
>> si = rg1;
>> li = rg1;
>> endtask
>> enclass
>>
>> My problem concerns the virtual task 'virtual_task'.
>> When it has been declared in the parent class 'Parent' then it takes 
>> 'rg' as its argument.
>> During its redefinition in the child class 'Child2' it takes 'rg1' as 
>> its argument.
>> Is it necessary that the names of the variables also have to be 
>> identical?
>>
>> In LRM (1800-2005) Section 7.19 it is stated that "Virtual methods 
>> provide prototypes for subroutines,
>> i.e., all of the information generally found on the first line of a 
>> method declaration: the encapsulation criteria, the type and number of 
>> arguments, and the return type if it is needed.
>> Later, when subclasses override virtual methods, they must follow the 
>> prototype exactly."
>>
>> Prototype however do not mean identical variable name.But later on in 
>> the same section it is also stated that "Thus, all versions of the 
>> virtual method look identical in all subclasses"
>>
>> So what does the word 'identical' exactly mean overhere? Does it imply 
>> identical argument name also?
>>
>> Thanks,
>> Kakoli
>>
>>
>>
>> ==========================================================
>> Don Mills
>> LCDM Engineering     (Logic, Coding, & Design Methodology) 
>> mills@lcdm-eng.com                        www.lcdm-eng.com
>> ==========================================================
>>
> 

-- 
--------------------------------------------------------------------
Gordon Vreugdenhil                                503-685-0808
Model Technology (Mentor Graphics)                gordonv@model.com