Down Casting

SystemVerilog
1

I have a problem with using $cast

If pc is a parent class and cc is a child class, This creates an error


pc pc;
cc cc;

initial begin
  pc = new();
  $cast(cc, pc);
end

but this doesn’t create errors


pc pc;
cc cc;
cc cc2;

initial begin
  cc = new();
  pc = cc;
  $cast(cc2, pc);
end

I think downcast doesn’t create errors when parent object was upcast before.
Is it right?

2

In reply to HanP:

Remember that there is a distinction between classes variables and handles to class objects.

A class variable can only store handles to class objects that are the same type or extensions of that type.

$cast is called a dynamic cast because it checks at runtime if the source handle type is the same or an extension of the destination class variable type.

In your first $cast, the source handle type is not an extension of the destination class type, it is the opposite. Therefore an error.

In your second $cast, the source handle type is an extension of the destination class type. Therefore no error.

3

In reply to dave_59:

Thank you dave.

I already know that the source handle should be an extension of the destination class type.
But, In my second $cast, pc is upcast of cc but pc is the parent class handle of cc2 class.
So, i think pc is still not the extension of the cc class.
Isn’t it right?

Then, Is it impossible that make cc downcast of pc what was not upcast before?

4

In reply to HanP:

class base;
endclass
class ext1 extends base;
endclass
class ext2 extends base;
endclass

base base_h;
ext1 ext1_h;
ext2 ext2_h;
initial begin
  ext1_h = new;
  base_h = ext1; // upcast
  $cast (ext2_h, base_h); // this is an error even though there was an upcast before.
end