By default all the methods and members of the class are accessible outside the class from object pointer. What if we don't want some members and some methods to be not accessable from outside. For this purpose Systemverilog provides local reserve word, when added to a class member to method, becomes local to the object and can not be accessed from outside the object. Even the child class which inherits the parent class can not access the local members to the parent class.

In case we want the local members to accessable to a child class, then Systemverilog provides reserve word protected, when added infront of a member, instead of local. A protected class property or method has all of the characteristics of a local member, except that it can be inherited; it is visible to subclasses.

  1 program class_data_hiding;
  2   class A;
  3     integer data;
  4     local   integer addr;
  5     protected integer cmd;
  6     static integer credits;
  7     function new();
  8       begin
  9         data = 100;
 10         addr = 200;
 11         cmd  = 1;
 12         credits = 10;
 13       end
 14     endfunction
 15     task printA();
 16       begin
 17         $write ("value of data %0d in A\n", data);
 18         $write ("value of addr %0d in A\n", addr);
 19         $write ("value of cmd  %0d in A\n", cmd);
 20       end
 21     endtask
 22   endclass
 23   
 24   class B extends A;
 25     task printB();
 26       begin
 27         $write ("value of data %0d in B\n", data);
 28         // Below line will give compile error
 29         //$write ("value of addr %0d in B\n", addr);
 30         $write ("value of cmd  %0d in B\n", cmd);
 31       end
 32     endtask
 33   endclass
 34   
 35   class C;
 36     A a;
 37     B b;
 38     function new();
 39       begin
 40         a = new();
 41         b = new();
 42         b.data = 2;
 43       end
 44     endfunction
 45     task printC();
 46       begin
 47         $write ("value of data %0d in C\n", a.data);
 48         $write ("value of data %0d in C\n", b.data);
 49         // Below line will give compile error
 50         //$write ("value of addr %0d in C\n", a.addr);
 51         //$write ("value of cmd  %0d in C\n", a.cmd);
 52         //$write ("value of addr %0d in C\n", b.addr);
 53         //$write ("value of cmd  %0d in C\n", b.cmd);
 54       end
 55     endtask
 56   endclass
 57   
 58   initial begin
 59     C c = new();
 60     c.a.printA();
 61     c.b.printB();
 62     c.printC();
 63     $write("value of credits is %0d\n",c.a.credits); 
 64     $write("value of credits is %0d\n",c.b.credits); 
 65     c.a.credits ++;
 66     $write("value of credits is %0d\n",c.a.credits); 
 67     $write("value of credits is %0d\n",c.b.credits); 
 68     c.b.credits ++;
 69     $write("value of credits is %0d\n",c.a.credits); 
 70     $write("value of credits is %0d\n",c.b.credits); 
 71   end
 72 
 73 endprogram

 value of data 100 in A
 value of addr 200 in A
 value of cmd  1 in A
 value of data 2 in B
 value of cmd  1 in B
 value of data 100 in C
 value of data 2 in C
 value of credits is 10
 value of credits is 10
 value of credits is 11
 value of credits is 11
 value of credits is 12
 value of credits is 12

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