1. static;

    a. 直接声明一个static变量，每次调用时，都会保持上次的值；

        注意static的值，刚开始的时候必须要进行初始化；

    b. static function；

        static function声明后，其返回值就是static的，同时这个function也是会申请一段空间，不会释放，所以该function的调用，可以时实例化后再调用，也可以是静态调用； 

Packet::get_pkt_ctr(); // Static call using :: operator；

pkt[5].get_pkt_ctr(); // Normal call using instanc

        static function中，不能调用function外部的automatic的变量或者方法，因为生命周期不同；

        static function内部的变量，可以声明成automatic的；

2. pure virtual methods

A virtual method in an abstract class may be declared as a prototype without providing an implementation. This is called a pure virtual method and shall be indicated with the keyword pure together with not providing a method body

virtual class BasePacket;

        pure virtual function integer send(bit[31:0] data); // No implementation

endclass

class EtherPacket extends BasePacket;

        virtual function integer send(bit[31:0] data);

                // body of the function

                ...

        endfunction

endclass

3. interface class；参考环境即可；

interface class PutImp#(type PUT_T = logic);

        pure virtual function void put(PUT_T a);

endclass

interface class GetImp#(type GET_T = logic);

        pure virtual function GET_T get();

endclass

class MyQueue #(type T = logic, int DEPTH = 1);

        T PipeQueue[$:DEPTH-1];

        virtual function void deleteQ();

                PipeQueue.delete();

        endfunction

endclass

class Fifo #(type T = logic, int DEPTH = 1) extends MyQueue#(T, DEPTH) implements PutImp#(T), GetImp#(T);

         virtual function void put(T a);

                PipeQueue.push_back(a);

        endfunction

        virtual function T get();

                get = PipeQueue.pop_front();

        endfunction

endclass