# Difference between revisions of "Object Oriented Programming with Free Pascal and Lazarus/zh TW"

## 介紹

Pascal 有很多很好的教材，但這篇教學主要是讓初學者能更往前進到 物件導向的程式設計，物件導向算是 Pascal 的延伸，從 Turbo-Pascal 提出，一直到 Delphpi，Free Pascal／Lazarus。

• 從選單選取項目，
• 開啟或儲存檔案，
• 連線到網路，
• 進行數字運算，等等。

## 物件 - 現實世界的例子

### 血液樣本

• 試管，即內科醫師用來進行檢驗的地方。
• 內部流程 (或稱方法標準操作流程 (SOP)) 來指引護士或技師來採樣
• 要用哪一種試管，
• 如何進行採樣
• 如何儲存樣本並送回實驗室。
• 試管上的標籤訊息代表著
• 採樣編號
• 患者的姓名與生日
• 採樣日期
• 是否需要檢驗。
• 在實驗室裡檢驗請求單會伴隨著樣本作業，指出
• 採樣編號
• 請求檢驗的醫師編號
• 請求檢驗的醫師所要求的檢驗內容
• 患者完整的資訊
• 所希望看到的診斷確認。

• 在實驗室中 - 內部方法決定了
• 樣本要如何來分析
• 要用什麼樣的機械
• 機械要如何去校正與操作
• 結果要用什麼樣的格式寫下來，然後
• 回報給醫生。

### Another Example - Automobile

If you don't like blood, the same sort of reasoning could be applied to a car taken to a garage for repair. It might consist of:

• the physical vehicle
• documents held by the owner: registration or license (including license plates), insurance, receipts for purchase, parts, repairs etc
• the fuel consumption history
• the drivers allowed to use the vehicle, with their license particulars
• service records held by the garage
• methods or procedures to be followed for routine checking and maintenance
• methods to be followed for non-routine repairs etc
• billing information for the customer

## Programming Example

Enough of this pre-occupation with Real-World examples! Let us proceed to the main purpose: programming in Object Pascal.

Let us consider the creation of a simple Form with a few controls for an application in Free Pascal/Lazarus.

On invoking the Lazarus IDE, the programmer is presented with a blank template Form design, on which he is encouraged to place various controls or objects.

Note that the pre-made blank Form is already an Object, with its own properties such as position (Top and Left), size (Height and Width), colour, default font for adding text etc.

If a Button control is placed on the Form (type TButton), it will have its own series of properties, which can be examined in the Object Inspector window.

Several of the properties have names similar to those for the Form; this is because many properties are Inherited from some common Ancestor class, which lays out how properties are to be defined and handled by the descendant classes.

As well as properties, the Object Inspector offers a tab called Events, which gives access to Event Handlers which are methods instructing the application how to deal with things such as a mouse click on a button (OnClick) or some change in the position, size or other properties (OnChange).

The physical image of the Button on the Form, together with all its properties and Event Handler methods, should be regarded as a single entity or Object in Pascal.

## Object-Oriented Extensions to standard Pascal

The Pascal record structure is extended by defining an

### Object

An Object is a special kind of record. The record contains all the ﬁelds that are declared in the object's deﬁnition (just like a conventional record), but now procedures and functions can be declared as if they were part of the record and are held as pointers to the methods associated with the object's type.

For example, an object could contain an array of real values, together with a Method for calculating the average.

```Type
Average = Object
NumVal: Integer;
Values: Array [1..200] of Real;
Function Mean: Real; { calculates the average value of the array }
End;
```

Objects can ”inherit” ﬁelds and methods from ”parent” objects. This means that these ﬁelds and methods can be used as if they were included in the objects declared as a ”child” object.

Furthermore, a concept of visibility is introduced: ﬁelds, procedures and functions can be declared as public, protected or private. By default, ﬁelds and methods are public, and can be exported outside the current unit. Protected fields or methods are available only to objects descended from the current ancestor object. Fields or methods that are declared private are only accessible in the current unit: their scope is limited to the implementation of the current unit.

### Class

Objects are not used very often by themselves in Free Pascal and Lazarus; instead, Classes are used very widely. A Class is defined in almost the same way as an Object, but is a pointer to an Object rather than the Object itself. Technically, this means that the Class is allocated on the Heap of a program, whereas the Object is allocated on the Stack.

Here is a simple example of a typical Class declaration: <delphi>{-------------------------------------------} { Example of Class declaration from the LCL } {-------------------------------------------}

``` TPen = class(TFPCustomPen)
private
FColor: TColor;
FPenHandleCached: boolean;
FReference: TWSPenReference;
procedure FreeReference;
function GetHandle: HPEN;
function GetReference: TWSPenReference;
procedure ReferenceNeeded;
procedure SetHandle(const Value: HPEN);
protected
procedure DoAllocateResources; override;
procedure DoDeAllocateResources; override;
procedure DoCopyProps(From: TFPCanvasHelper); override;
procedure SetColor
(const NewColor: TColor; const NewFPColor: TFPColor); virtual;
procedure SetFPColor(const AValue: TFPColor); override;
procedure SetColor(Value: TColor);
procedure SetMode(Value: TPenMode); override;
procedure SetStyle(Value: TPenStyle); override;
procedure SetWidth(value: Integer); override;
public
constructor Create; override;
destructor Destroy; override;
procedure Assign(Source: TPersistent); override;
property Handle: HPEN read GetHandle write SetHandle; deprecated;
published
property Color: TColor read FColor write SetColor default clBlack;
property Mode default pmCopy;
property Style default psSolid;
property Width default 1;
end;</delphi>
```

Note that this class is defined as an instance of another parent or ancestor class (TFPCustomPen) from which it inherits all its properties and methods. It has some fields of its own, grouped under

• private - this means that items defined here are only available or visible to other classes or procedures/function defined within the same program unit (this example is from Graphics, so any of the other classes such as TBitMap, TPicture etc in the same unit can use them). They are essentially local variables (eg FColor, FPenHandleCached) or locally used methods (GetHandle, SetHandle) but can be used or referred to in items declared in the protected or public sections.
• protected - this means that items defined here are only available or visible to classes that are descended from this ancestor class, and inherit its properties or methods
• public - this means that items defined here are available to any programming unit that includes the current unit in its Uses clause
• published - is the same as a public section, but the compiler also generates type information that is needed for automatic streaming of these classes. Often the list of published items appear in the Object Inspector of Lazarus; if there is no published list, all the public fields usually appear in the Object Inspector.

### Methods

A method is just like a standard procedure or function, but can have some additional directives.

Some of the methods defined above are labelled with the directive virtual; others are labelled with the override directive.

• virtual means that the type or actual instance of a method is not known at compile-time, but is selected at run-time depending on what sub-program actually calls the method. It could be considered a place-holder in the definition of the class.
• override means that at run-time the locally given definition can take the place of a definition inherited from an ancestor class, particularly if it was virtual. If you particularly want to use the method defined in the ancestor class, it is sometimes necessary to call it specifically with the inherited clause.

Methods with no virtual or override directive are static methods (the usual kind in Pascal). Those with a virtual or override directive are dynamic.

Special instances of methods are:

• create - a constructor for a class, which takes care of allocating memory, collecting together all the information needed and configuring/initializing the various properties.
• destroy - a destructor for a class, which removes all the parts of the class from the system in an orderly and logical way, and returns all its resources for the system to re-use.

### Properties

Properties are just like ordinary fields in a conventional Pascal record, but they can have read and/or write specifiers.

• read specifier is a field, or a function that returns a result of the correct type for the property. In the example above, the property Color has a read specifier FColor, which is a local variable containing the value to be used. If a property has a read but no write specifier, it is read-only.
• write specifier is a field, or a procedure that will store the value of the property in a specific location. In the example above, Color has a write specifier SetColor that is a procedure (defined in the protected section) for writing the color value to some specified location. If a property has a write but no read specifier, it is write-only.
• default - note that it is possible to set a default value for a property. For example, Color here is given the default value clBlack, or black, at the time of creation. It could subsequently be given a different value, by a programming assignment statement, or in the Object Inspector.
• index specifier is an integer that the read or write methods, shared between properties, can use to identify which property is desired. Note that if index is used the read or write specifiers must be a function or a procedure respectively and cannot be a normal field/variable.

#### Examples

<delphi>

``` TFooClass = class
private
FIntProp: Integer;
public
property IntProp: Integer read FIntProp write FIntProp;
end;
```

</delphi>

<delphi>

``` TFooClass = class
private
function GetListProp(AIndex: Integer): String;
property SetListProp(AIndex: Integer; AValue: String);
public
// this type of property may not be in the published section of a class
property ListProp[AIndex: Integer]: String read GetListProp write SetListProp;
end;
```

</delphi>

<delphi>

``` TFooClass = class
private
function GetValue(const AIndex: Integer): Integer;
procedure SetValue(const AIndex: Integer; AValue: Integer);
public
// note that the read and write methods are shared
property Value1: Integer index 1 read GetValue write SetValue;
property Value2: Integer index 2 read GetValue write SetValue;
property Value3: Integer index 3 read GetValue write SetValue;
property Value4: Integer index 4 read GetValue write SetValue;
// index may be a const or a number or you can even use an enumerated type
// for example:
// property Value: Integer index ord(seSomeEnum) read SomeFunction write SomeProcedure;
end;
```

</delphi>

## Free Pascal Language Extensions

FPC includes several language extensions to its "standard" Pascal syntax to support object oriented programming.

These extensions are described in the indicated chapters of the FPC Language Reference Guide: http://www.freepascal.org/docs.var. Links to tutorial pages for each concept are included above as well. The Language Reference Guide includes syntax diagrams and further details not contained in this introductory tutorial. Of the four language features listed above, Objects and Classes form the basis of object oriented programming (OOP) in FPC and Lazarus. For those new to OOP, the Objects section includes more introductory concepts and the Classes section minimizes repetition by emphasizing the similarities and differences to the Objects syntax. In general, the Classes implementation seems to be more widely in use including Delphi Lazarus developers. Often the word "objects" is used to refer to what is actually a "class" in the Classes dialect of OOP in FPC. These documents will be worded to minimize any terminology confusion, but outside of this document, the term "object" oftentimes refers to objects created from a Class. In fact, the FPC run time library (RTL) includes a class library with a base class called TObject.

Users familiar with the older Turbo Pascal OOP implementation may initially want to skip the section on Classes since the Objects implementation is based on the older Turbo Pascal dialect. The section on Classes should be familiar to Delphi users since it is based on Delphi syntax. Be aware that some of the writeup in the Classes section may refer to concepts from the Objects section. For Macintosh developers familiar with the various Apple, THINK and MPW Object Pascal dialects, neither the FPC Objects or Classes dialects provide a direct migration path. As of March 2009, there are discussions on the Mac Pascal Mailing list about potentially providing some compiler support (new syntax) for accessing Apple's Objective C / Cocoa framework.

### General Concepts of Object Oriented Pascal

OOP provides different ways to manage and encapsulate data and to manage program flow compared with other available programming language features and constructs. OOP often lends itself to modeling certain applications such as Graphic User Interfaces (GUI's) and physical systems in a more natural feeling manner. However OOP is not appropriate for all applications. Program control is not as explicit as the more basic Pascal procedural constructs. To obtain the most benefit from OOP, understanding of large class libraries is often required which can entail a steep learning curve. Maintaining large OOP application code has its advantages and disadvantages compared to maintaining strictly procedural code. There are many sources for learning OO analysis, design and programming techniques which are beyond the scope of this guide.

There are numerous programming languages which incorporate OOP features as extensions or the basis of their language. As such, there are many different terms for describing OO concepts. Even within FPC, some of the terminology overlaps. In general, OOP usually consists of the concept of a programming object (or information unit) which explicitly combines and encapsulates a related set of data and procedures which act on the data. This data is usually persistent during program execution but with mechanisms to reduce some of the problems inherent in declaring global variables. In addition, OOP languages enable objects to be incrementally modified and/or extended based on previously defined objects. This feature is usually referred to by the terms inheritance and polymorphism. Many OOP languages use the terms method or message referring to procedures which belong to an object. Much of the power of OOP is realized by late (run time) dynamic binding of methods rather than compile binding. This dynamic binding of methods is similar to using procedural variables and procedural parameters but with greater syntactic cohesion, encapsulation with the data it is related to and also inheritance of behavior of previously defined methods. The following wiki pages provide a starting point for discovering more about analysis, design and programming in an object orient manner.

## Further information

This has only scratched the surface of the topic. For more details, readers are strongly recommended to read the Free Pascal manuals, especially Chapters 5 (Objects) and 6 (Classes)