RE: [sv-ec] Virtual function call from base class constructor

Gord,

You are right - do I owe you a book? :-)  I forgot about the time that
the call to t() takes place.

Chris

-----Original Message-----
From: Gordon Vreugdenhil [mailto:gordonv@model.com] 
Sent: Wednesday, June 25, 2008 10:01 AM
To: Chris Spear
Cc: Daniel Mlynek; sv-ec@eda.org
Subject: Re: [sv-ec] Virtual function call from base class constructor

Chris,

I disagree.  During construction, you cannot safely call an
overridden method since the derived class state would not have
been initialized yet.  As with C++ (ISO C++ 1998, Sec 12.7),
SV should clarify that during construction you effectively
change your vtbl as construction progresses so that you only
get "the most derived" override vis-a-vis the type of "new"
that you are currently executing.

So, in your example, the call to "t" in A::new should *always*
call A::t since that is "the most derived" at that point of
the call to A::new whereas the call to "t" in A::f should call either
A::t or B::t depending on the actual object type.

Gord.


Chris Spear wrote:
> Daniel,
> 
>  
> 
> The calls to t() are actually this::t(), so it makes sense that you 
> always get the extended function. Even when you call the constructor
for 
> class A, the handle is of type B, so this::t() expands into B::t().  
> Here is a more complete example:
> 
>  
> 
> class A;
> 
>    virtual function t;
> 
>      $display("base class A");
> 
>    endfunction
> 
>    
> 
>    function new;
> 
>       $display("In %m");
> 
>       t();        // This should call A::t or B::t because t() is
virtual
> 
>       f();
> 
>    endfunction
> 
>  
> 
>     function f();
> 
>       $display("In %m");
> 
>         t();        // This should call A::t or B::t because t() is
virtual
> 
>     endfunction
> 
> endclass
> 
>  
> 
> class B extends A;
> 
>    virtual function t;
> 
>       $display("Extended class B");
> 
>    endfunction
> 
>  
> 
>    function f();
> 
>       $display("In %m");
> 
>       t();        // this should call B::t bacause t is virtual
> 
>    endfunction
> 
> endclass
> 
>  
> 
> module top;
> 
>    
> 
>     initial begin
> 
>        A a;
> 
>        B b;
> 
>        $display("\nConstructing A");
> 
>        a = new;
> 
>        $display("\nCalling a.f()");
> 
>        a.f();
> 
>        $display("\nCalling a.t()");
> 
>        a.t();
> 
>  
> 
>        $display("\nConstructing B");
> 
>        b = new;
> 
>        $display("\nCalling b.f()");
> 
>        b.f();
> 
>        $display("\nCalling b.t()");
> 
>        b.t();
> 
>     end
> 
> endmodule
> 
>  
> 
> /*********************************************************  
> Chris Spear              Verification Specialist            
> Synopsys, Inc.           Phone 508-263-8114      ..  __@    
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>  
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> 
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Gordon Vreugdenhil                                503-685-0808
Model Technology (Mentor Graphics)                gordonv@model.com


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