Constant class properties in SV

To make class properties read-only, a const specifier is used. Since class objects are dynamic in nature, SystemVerilog provides two types of constants

Global constants
Instance constants

In both types, class properties have to specify with the const keyword.

The class methods can not use a const specifier.

Global constants

During variable declaration, an initial value is assigned, such class properties known as global constants. The value of a variable can not be changed after the declaration of the variable.

Global constant Example

class transaction;
  bit [31:0] data;
  const int id = 1;
  
  function void display();
    $display("data = %0d and id = %0d", data, id);
  endfunction
endclass

module class_example;
  transaction tr;
  
  initial begin
    tr = new();
    tr.data = 100;
    //tr.id = 2;  // Invalid const usage
    tr.display();
  end
endmodule

Output:

data = 100 and id = 1

Instance constants

During variable declaration, an initial value is not assigned, such class properties are known as instance constants. An instance constant allows a user to assign value in run time only once in the class constructor,

Instance constant Example

class transaction;
  bit [31:0] data;
  const int id;
  
  function new();
    data = 100;
    id = 1; // run time assignment only once
  endfunction
  
  function void display();
    $display("data = %0d and id = %0d", data, id);
  endfunction
endclass

module class_example;
  transaction tr;
  
  initial begin
    tr = new();
    tr.data = 200;
    //tr.id = 2; // Invalid const usage
    tr.display();
  end
endmodule

Output:

data = 200 and id = 1

System Verilog Tutorials