SystemVerilog learning-06-class encapsulation

Class overview

Object oriented programming （object oriented programing,OOP）, The core idea is to combine data with relevant methods （ Also called subroutine , That is, some data processing task or function） Encapsulate into a class （class）. such , Users can build test platforms at a higher level of abstraction . Users can build test platforms and system level models at a more abstract level , Execute by calling a function — One action rather than simply changing the level of the signal . In a validation environment , Include stimulator、monitor、checker As well as other verification components, you can follow OOP The way to build .

For example, a packet , May be defined as a class , Class can contain instructions 、 Address 、 queue ID、 Members such as timestamps and data .packet This class can initialize the data in it , Set command , Read the status of this class and check the queue ID.

every last packet The data members of concrete objects instantiated by classes may be different , However packet Class as an abstract type to describe these data , Define the corresponding data members and the methods of operating these data members .

OOP The term

• class (class) : Contains member variables and member methods , Basic building blocks that contain variables and subroutines .Verilog Corresponding to this is the module (mod-ule).
• object (object): Instance of class after instantiation . stay Verilog in , You need to instantiate a module to use it .
• Handle (handle)︰ Pointer to object . stay Verilog in , You reference signals and methods outside the module through the instance name . One OOP A handle is like the address of an object , But it is stored in a pointer that can only point to a single data type .
• Prototype (prototype): The declarative part of the program , Include program name 、 Return a list of types and parameters . The body of the program contains the execution code .

Building functions

SV Don't like C++ Language also requires complex means of storage space development and destruction , Instead, it uses things like Java— Means of automatic development and recycling of sample space . therefore SV When defining a class , Just define the build function (constructor) , Without defining a destructor (destructor) .

When a class is defined , You need to define the build function , If not defined , The system will automatically help define an empty build function ( There are no formal parameters , The function body is also empty ). Objects are created , You need to declare first and then instantiate , At the same time .

class Packet;
	integer comnand ;
	function new ();
		command = IDLE ;
	endfunction
endclass
Packet p = new ;

Static members

Class members ( Variable / Method ) The default is dynamic (automatic) Life cycle , That is, the variables and methods of each object will open up a new space for it . If multiple objects want to share a member ( Variable / Method ), Then you can add keywords static.

Multiple objects can therefore share the same member variable or method . When accessing this member , There is no need to instantiate objects .

class Packet ;
	static integer fileID = $fopen( "data" , "r" ) ;
	...
endclass
Packet p;
c = $fgetc( p.fileID ) ; //OR Packet: : fileID

Member methods can also be declared static . Static methods cannot access non static members ( Variable / Method ), Otherwise, there will be compilation errors .

this usage

this It is used to specify the member of the object where the index is currently located ( Variable / Parameters / Method ).this Can only be used in non static members of a class 、 Used in constraint and override groups .this The use of can specify the scope of the pointed variable .

class Demo ;
	integer x;
	function new (integer x);
		this.x = x;
	endfunction
endclass

Assignment and copy

Declaring variables and creating objects are two processes , It can also be done in one step .

Packet p1;
p1 = new ;

If you will p1 Assign to another variable p2, Then there is still only one object , Only the handle to this object has p1 and p2.

This means p1 and p2 Represents two different objects . Creating p2 Object time , Will be taken from p1 Copy its member variables, such as integer、string And handles, etc , This copy method is called shallow copy (shallow copy) .

Packet p1;
Packet p2;
pl = new ;
p2 = new pl ;

Data hiding and encapsulation

Class members ( Variable / Method ) By default , That is, the of public attributes . This means that the member can be accessed both for the class itself and outside .

For commercial development , The provider of the class will restrict the external access of some class members , Then hide more details of class members . This approach also makes the external access interface of the class more compact , Reduce the maintenance workload of classes , It also makes the class easy to modify with l Older versions remain compatible . The way of data hiding makes the test and maintenance of classes easier .

If you use local, Only this class can access this member , and Neither subclasses nor externals can access .

If you use protected, It means that this class and its subclasses can access this member , and External inaccessible .

reference

1. Verification of West circuit department PPT