Constraint for object dynamic array

SystemVerilog
,
1

The size constraint for dynamic array of primitive data type works fine.

when changing from primitive to class object, it does not work, because this will incur null pointer error.

class my_cons_1;
  rand my_class my_q1[];    // my_class is a custom class with rand variables here
  rand int my_size;
  constraint my_con{
   my_q1.size() == my_size;          // doesn't work here because no object constructed.
   //other constraint over my_q1 elements

  }
endclass

my question is:
Q1:why doesn’t constraint solver call class’s default constructor when solving the constraint in this case?
Q2:are there any other ways to achieve this? I know I could randomize my_size outside first and passed in as an normal int, and construct my object array inside pre_randomize function, but I’d like to do the randomization all at once if possible.

Thanks,
Jeff

2

In reply to Jeff_Li_90:

Let me address your second question first. The recommended practice is constructing a dynamic array with a maximum size first and constructing each element first. Then let randomization prune the array to the desired size. This has the benefit of preserving random stability since the number of constructed objects affects the random seeding. If calling randomize() causes the construction of a random number of objects, then a modification of constraints could lead to a disturbance in randstate for randomization statements that occur after it.

That should also cover most of your first question, but other reasons include the difficulty in passing arguments to the constructors during randomization.

3

In reply to dave_59:

Got it, Thanks Dave.

coded added for record.


class A;
  rand int x;
  constraint my_con{
    x inside {1,2,3,4,5};
  }
endclass


module tb_top;

  class B#(parameter int MAX = 32);
    rand A my_q1[];    
    rand int my_size;
   function new();

   endfunction
    
    function void pre_randomize();   // pre-construct dynamic array of obj to MAX size
      my_q1 = new[MAX];
     foreach(my_q1[i])
       my_q1[i] = new();
    endfunction

  constraint my_con{
   my_q1.size() == my_size;         // use size to prune dynamic array to desired size.
   foreach(my_q1[i])
      my_q1[i].x == 2;
   
  }
endclass
  
  initial begin
    B b;
    b = new();
    b.randomize() with{my_size == 3;};
    b.randomize() with{my_size == 4;};
    foreach(b.my_q1[i])
      $display("i: %0d, x %0d",i,b.my_q1[i].x);
  end
  
  
endmodule