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What is a class handle ?

A class variable such as pkt below is only a name by which that object is known. It can hold the handle to an object of class Packet, but until assigned with something it is always null. At this point, the class object does not exist yet.

Class Handle Example

 
// Create a new class with a single member called
// count that stores integer values
class Packet;
  int count;
endclass
 
module tb;
    // Create a "handle" for the class Packet that can point to an
    // object of the class type Packet
    // Note: This "handle" now points to NULL
  Packet pkt;
 
    initial begin
      if (pkt == null)
        $display ("Packet handle 'pkt' is null");
 
      // Display the class member using the "handle"
      // Expect a run-time error because pkt is not an object
      // yet, and is still pointing to NULL. So pkt is not
      // aware that it should hold a member
      $display ("count = %0d", pkt.count);
    end
endmodule
 
Simulation Log
ncsim> run
Packet handle 'pkt' is null
count = ncsim: *E,TRNULLID: NULL pointer dereference.
          File: ./testbench.sv, line = 18, pos = 33
         Scope: tb
          Time: 0 FS + 0

./testbench.sv:18       $display ("count = %0d", pkt.count);
ncsim> exit

Click to try this example in a simulator!   

What is a class object ?

An instance of that class is created only when it's new() function is invoked. To reference that particular object again, we need to assign it's handle to a variable of type Packet.

Class Object Example

 
// Create a new class with a single member called
// count that stores integer values
class Packet;
  int count;
endclass
 
module tb;
    // Create a "handle" for the class Packet that can point to an
    // object of the class type Packet
    // Note: This "handle" now points to NULL
  Packet pkt;
 
    initial begin
      if (pkt == null)
        $display ("Packet handle 'pkt' is null");
 
      // Call the new() function of this class
      pkt = new();
 
      if (pkt == null)
        $display ("What's wrong, pkt is still null ?");
      else
        $display ("Packet handle 'pkt' is now pointing to an object, and not NULL");
 
      // Display the class member using the "handle"
      $display ("count = %0d", pkt.count);
    end
endmodule
 
Simulation Log
ncsim> run
Packet handle 'pkt' is null
Packet handle 'pkt' is now pointing to an object, and not NULL
count = 0
ncsim: *W,RNQUIE: Simulation is complete.

Click to try this example in a simulator!   

What happens when both handles point to same object ?

If we assign pkt to a new variable called pkt2, the new variable will also point to the contents in pkt.

 
// Create a new class with a single member called
// count that stores integer values
class Packet;
  int count;
endclass
 
module tb;
    // Create two "handles" for the class Packet
    // Note: These "handles" now point to NULL
  Packet pkt, pkt2;
 
    initial begin
 
 
      // Call the new() function of this class and 
      // assign the member some value
      pkt = new();
      pkt.count = 16'habcd;
 
      // Display the class member using the "pkt" handle
      $display ("[pkt] count = 0x%0h", pkt.count);
 
      // Make pkt2 handle to point to pkt and print member variable
      pkt2 = pkt;
      $display ("[pkt2] count = 0x%0h", pkt2.count);
    end
endmodule
 
Simulation Log
ncsim> run
[pkt] count = 0xabcd
[pkt2] count = 0xabcd
ncsim: *W,RNQUIE: Simulation is complete.

Now we have two handles, pkt and pkt2 pointing to the same instance of the class type Packet. This is because we did not create a new instance for pkt2 and instead only assigned a handle to the instance pointed to by pkt.

Click to try this example in a simulator!   

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