Re: [sv-ec] member initialization at declaration vs. constructor - order issue

>From: "Daniel Mlynek" <daniel.mlynek@aldec.com>

>I've hit  another problem connected with order of execution of declaration
>initialization vs.. constructor (imho allowing declaration initialization
>for class members was good idea at all)
>LRM doesn't define how it should be done. My guess is that:
>1st initialization from declaration should be done in order from base class
>to child class 
>2nd constructor chain should be executed - as described in LRM from base to
>child

The problem with this ordering is that an initializer on a declaration in
the child class could reference a property in the base class that is
initialized by the base class constructor.  The child class should not
be dependent on how the construction of the base class was done.  Therefore
the base class should be completely initialized before the initialization
of the child class is started.


>But for classes inheriting some other class LRM says:
>"Every class has a default (built-in) new method. The default constructor
>first calls its base class constructor (super.new() as described in 8.14)
>and then proceeds to initialize each member of the current object to its
>default (or uninitialized value)."
>Most interesting is "proceeds to initialize each member of the current
>object to its default (or uninitialized value)." I thought that
>uninitialized value is same as default, anyway why constructor need to do
>some initialization if the value should be default (which means that value
>is hold in variable if no initialization or assignment was done  ).
>But one can understand this sentence differently - default value is a value
>from initialization at declaration - because it has to be assigned

I believe this is how you must interpret the sentence, though I agree it
is not entirely clear.

The order should be

1. Call the base class constructor.
2. Initialize each member to the default value specified in its declaration,
   if any, else to the default initialization value.
3. Execute the body of the constructor.

Though the LRM does not say so, it would also make sense to initialize the
properties in the order they were declared, since the initializer of a
property might refer to a property declared earlier.

This leaves only one small hole where you could refer to the value of a
property before it is initialized.  That is the one you use in your
example: the actual arguments to the super.new() call, if you specify
it explicitly.  In your example, you refer to a property of the local
class, which is not initialized until after the super.new() call.  You
could also refer to a property of the base class, which might be
initialized by the super.new() call itself.  I think we just have to
regard that as bad code.

To make that code illegal, you would have to specify special scoping
rules to the actual arguments of the super.new() call.  You would have
to regard the scope the same way as if you were inside a static method
of the class: you have access to the locals of the new() method and to
static properties, but not to nonstatic properties.  Another way to
do it would be to treat the value of 'this' as if it were null until
after the super.new() call returned.

>I totally don't understand what was the goal of adding this sentence to LRM.

I suspect it was intended to answer your question about the order.  You
just have to interpret "default" in that sentence as meaning the value on
the declaration.  That would explain why the sentence treats it as something
different from the uninitialized value.
 

>What should be the results in your opinion? 

123 undefined 123 123

Making sure that the base class is initialized before the derived
class is more important than someone trying to pass an uninitialized
property as an argument to the base constructor.  Even with your
suggested ordering, I could still reference an uninitialized value
in the base class anyway.

With your suggested ordering, there would be a problem with

        class C;
                int i;
                function new(input int v);
                        i = v;
                        $display("C %d", i);
                        $display("C %d", v);
                endfunction
        endclass
 
        class CC extends C;
                int j=i;
                function new;
                        super.new(123);
                        $display("CC %d",i);
                        $display("CC %d",j);
                endfunction
        endclass

With your suggested ordering, the result of this would be

  123 123 123 undefined

With my suggested ordering, the result of this would be

  123 123 123 123

>Is the LRM description clear in this matter in your opinion?

It could use some improvement.


>Do you finding class member initialization at declaration useful feature -
>or are you regretting that someone have vote it in into the standard ?

It would certainly be simpler to have left it out and require users to
insert assignments into the constructor instead (after any super.new()
call).  But as long as it is understood to be equivalent to that, it does
not cause any real problems.


Steven Sharp
sharp@cadence.com


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