Shallow Copy

Following is the example of "Shallow copy".

class A;

  int var_i;

 

  function new();

    $display("Class A is created.");

  endfunction

 

  function void display(string inst_name = "");

    $display ("%0s:: Value of var_i=%0d", inst_name, var_i);

  endfunction

endclass // class A

class B;

  int var_j;

  A a_inst;

 

  function new();

    $display("Class B is created.");

    a_inst = new();

  endfunction

 

  function void display(string inst_name = "");

    $display ("%0s:: Value of var_j=%0d", inst_name, var_j);

  endfunction

endclass // class B

module top();

  initial begin

    B b_inst1, b_inst2;

    b_inst1 = new();

    b_inst1.var_j = 10;

    b_inst1.a_inst.var_i = 20;

    b_inst2 = new b_inst1;

    b_inst1.display("b_inst1");

    b_inst1.a_inst.display("b_inst1.a_inst");

    b_inst2.display("b_inst2");

    b_inst2.a_inst.display("b_inst2.a_inst");

    b_inst1.var_j = 30;

    b_inst1.a_inst.var_i = 40;

    $display ("Value of b_inst1.var_j changed to 30.");

    $display ("Value of b_inst1.a_inst.var_i changed to 40.");

    b_inst1.display("b_inst1");

    b_inst1.a_inst.display("b_inst1.a_inst");

    b_inst2.display("b_inst2");

    b_inst2.a_inst.display("b_inst2.a_inst");

  end

endmodule

Following is the result of the above simulation:
b_inst1:: Value of var_j=10
b_inst1.a_inst:: Value of var_i=20
b_inst2:: Value of var_j=10
b_inst2.a_inst:: Value of var_i=20

Value of b_inst1.var_j changed to 30.
Value of b_inst1.a_inst.var_i changed to 40.

b_inst1:: Value of var_j=30
b_inst1.a_inst:: Value of var_i=40
b_inst2:: Value of var_j=10
b_inst2.a_inst:: Value of var_i=40

In the above code, statement "b_inst2 = new b_inst1", creates a shallow copy for instance "b_inst2". That means, creating a new object "b_inst2" whose class properties are copied from the object "b_inst1". All the class variables and instance handles are copied from "b_inst1" to "b_inst2". However, the instance handles within "b_inst1" and "b_inst2" points to the same object as shown in the below image, though new() is written for that instance handles.

I hope this post helped to understand shallow copy. Please provide feedback/suggestions in the comment section.