Some SystemVerilog constructor insights

Can you predict the printed messages of the two statements within the initial block? 🧐

//----------------------------------//
class C;
  integer c1 = 1;
  integer c2 = 1;
  integer c3 = 1;
  function new(integer a);
    c2 = 2;
    c3 = a;
  endfunction
endclass

//---------------------------------//
class D extends C;
  integer d1 = 4;
  integer d2 = c2;
  integer d3 = 6;
  function new;
    super.new(d3);
  endfunction
endclass

//---------------------------------//
module test;
  
  initial begin
    C c0 = new(5);
    D d0 = new;
    $display("c0 variables c1:%0d, c2:%0d, c3:%0d", c0.c1,  c0.c2,  c0.c3);
    $display("d0 variables c1:%0d, c2:%0d, c3:%0d, d1:%0d, d2:%0d, d3:%0d", 
              d0.c1, d0.c2, d0.c3, d0.d1, d0.d2, d0.d3);
  end

endmodule        

Before unveiling the answer, let's explore some key points mentioned in SystemVerilog LRM regarding Constructors:

• SystemVerilog does not require the complex memory allocation and deallocation of C++.
• Construction of an object is straightforward; and garbage collection, as in Java, is implicit and automatic.
• Example

class Packet

   integer command;

   function new();
     command = IDLE;
   endfunction
endclass

Packet p = new; // new is called a class constructor        

• The new operation is defined as a function with no return type, and like any other function, it shall be nonblocking.
• If a class does not provide an explicit user-defined new method, an implicit new method shall be provided automatically.
• The order of execution of a derived class :

1. The new method of a derived class shall first call its base class constructor super.new().
2. After the base class constructor call (if any) has been completed, each property defined in the class shall be initialized to its explicit default value or its uninitialized value if no default is provided.
3. After the properties are initialized, the remaining code in a user-defined constructor shall be evaluated. The default constructor has no additional effect after the property initialization. The value of a property prior to its initialization shall be undefined.

Applying those points to our example

For c0 object:
==============
c1 | 1 | 1 |  
----->>-----
c2 | 1 | 2 |
----->>-----
c3 | 1 | 5 |

So the first printed message will be : c0 variables c1:1, c2:2, c3:5

For d0 object (Derived class):
=============================
c1 | 1 | 1 |  
----->>-----
c2 | 1 | 2 |
----->>-----
c3 | 1 | x | 
----->>-----
d1 | x | 4 |  
----->>-----
d2 | x | 2 |
----->>-----
d3 | x | 6 |

Note: c3 has an undefined value since the constructor call from D passes in the value of d3, which is
undefined when the super.new(d3) call is made

So the second printed message will be : d0 variables c1:1, c2:2, c3:x, d1:4, d2:2, d3:6        

Would you be willing to attempt the same approach with the following example to see if you've grasped it? 🤔 Let me know how it goes!

class C
  int c1 = 1;
  int c2 = 2;
  int c3 = 3;
  function new(int a);
    c2 = 2;
    c3 = a;
  endfunction
endclass

//---------------------------------//
class D extends C;
  int d1 = 4;
  int d2 = c2;
  int d3 = 6;
  function new;
    super.new(d3);
  endfunction
endclass

//---------------------------------//
module test;
  
  initial begin
    C c0 = new(5);
    D d0 = new;
    $display("c0 variables c1:%0d, c2:%0d, c3:%0d", c0.c1,  c0.c2,  c0.c3);
    $display("d0 variables c1:%0d, c2:%0d, c3:%0d, d1:%0d, d2:%0d, d3:%0d", 
              d0.c1, d0.c2, d0.c3, d0.d1, d0.d2, d0.d3);
  end  
endmodule;