SV class virtual method of polymorphism

概述

• Member methods of a class can have modifiersvirtual（虚方法）
• A virtual method is a basic polymorphic construct
• 一个虚方法可以覆盖基类的同名方法
• Declare virtual methods in parent and child classes,其方法名、参数名、The parameter directions should all be consistent
• 在调用虚方法时,它将调用句柄指向对象的方法,而不受句柄类型的影响

class BasePacket;
  int A = 1;
  int B = 2;
  
  function void printA;
    $display("BasePacket::A is %d", A);
  endfunction
  
  virtual function void printB;
    $display("BasePacket::B is %d", B);
  endfunction

endclass

class My_Packet extends BasePacket;
  int A = 3;
  int B = 4;

  function void printA;
    $display("My_Packet::A is %d", A);
  endfunction

  virtual function void printB;
    $display("My_Packet::B is %d", B);  
  endfunction

endclass

module tb;
  BasePacket P1 = new();
  My_Packet P2 = new();

  initial begin
    P1.printA;  // A is 1
    P1.printB;  // B is 2
    P1 = P2;    // The subclass handle is assigned to the superclass,The parent class handle points to an object of the child class
    P1.printA;  // A is 1
    P1.printB;  // B is 4
    P2.printA;  // A is 3
    P2.printB;  // B is 4
  end

endmodule

By default, the parent class handle will look up and call the parent class method,When the subclass handle is assigned to the superclass,After the parent class handle points to the object of the child class,The parent class handle lookup method will be extended to the child class,If there is a method with the same name in the subclass,Then the method of the same name of the subclass will be executed.故P1.printB()和P2.printB()打印结果都是4.
virtual关键字的作用：Although the handle types are not the same,But calling the function will take precedence over the subclass's implementation,That is, if the subclass has a method with the same name, the method in the subclass is called.

---

Variables can also be declared asvirtual：不可以

• People call it a virtual method,No dummy variables
• The scope of parent class access methods can be extended to subclasses,But the variable scope accessed is only the parent class variable scope
• The above code subclass method can not be usedvirtual声明,But the parent class must be declared
• 上述代码,no parent classvirtual,子类用virtual,则不行,In this way, the parent class object is looking for a method,I don't know where to look for subclasses,Will only look in the parent class scope

---

Some suggested points：

• Classes are not recommended when encapsulatinglocal、protected
• When a class inherits,In order to facilitate access to more variables later,When a subclass inherits a parent class, try not to have variables with the same name