DATA

 

DATA

 

Alternate syntax for VAR: instance variable for the objects.

Syntax

      DATA <DataName1> [,<DataNameN>] [ AS <type> ] [ INIT <uValue> ]
      [[EXPORTED | VISIBLE] | [PROTECTED] | [HIDDEN]] [READONLY | RO]

Arguments

<DataName1> Name of the DATA

<type> Optional data type specification from the following: Character, Numeric, Date, Logical, Codeblock, Nil.

<uValue> Optional initial value when creating a new object.

EXPORTED Specifies that this DATA is accessible to functions and methods outside of the class. VISIBLE is a synonym for EXPORTED.

PROTECTED Specifies that this DATA is only accessible to functions and methods within this class and its subclasses.

HIDDEN Specifies that this DATA is only accessible to the class where it was defined, and is not inherited by the subclasses.

READONLY Restricts assignment to the variable. If specified with the EXPORTED clause, assignment is only permitted from the current class and its subclasses. If specified with the PROTECTED clause, assignment is only permitted from the current class. RO is a synonym for READONLY.

Description

DATA elements can also be thought of as the “properties” of an object. They can be of any data type, including codeblock. Once an object has been created, the DATA elements are referenced with the colon (:) as in MyObject:Heading := “Last name”. Usually a class also defines methods to manipulate the DATA.

You can use the “AS <type>” clause to enforce that the DATA is maintained as a certain type. Otherwise it will take on the type of whatever value is first assigned to it.

Use the “INIT <uValue>” clause to initialize that DATA to <uValue> whenever a new object is created.

VAR can be a synonym for DATA, or it can use a slightly different syntax for compatibility with other dialects.

Examples

      CREATE CLASS TBColumn

         DATA Block      // Code block to retrieve data for the column
         DATA Cargo      // User-definable variable
         DATA ColorBlock // Code block that determines color of data items
         DATA ColSep     // Column separator character
         DATA DefColor   // Array of numeric indexes into the color table
         DATA Footing    // Column footing
         DATA FootSep    // Footing separator character
         DATA Heading    // Column heading
         DATA HeadSep    // Heading separator character
         DATA Width      // Column display width
         DATA ColPos     // Temporary column position on screen

         METHOD New()    // Constructor

      ENDCLASS

Compliance

Harbour

Platforms

All

Seealso

Object Oriented Programming, CLASS, METHOD, CLASSDATA, VAR

CLASSDATA

 

CLASSDATA

Define a CLASSDATA variable for a class (NOT for an Object!)

Syntax

      CLASSDATA <DataName1> [,<DataNameN>] [ AS <type> ] [ INIT <uValue> ]

Arguments

<DataName1> Name of the DATA

<type> Optional data type specification from the following: Character, Numeric, Date, Logical, Codeblock, Nil

<uValue> Optional initial value at program startup

Description

CLASSDATA variables can also be thought of as the “properties” of an entire class. Each CLASSDATA exists only once, no matter how many objects are created. A common usage is for a counter that is incremented whenever an object is created and decremented when one is destroyed, thus monitoring the number of objects in existence for this class.

You can use the “AS <type>” clause to enforce that the CLASSDATA is maintained as a certain type. Otherwise it will take on the type of whatever value is first assigned to it. Use the “INIT <uValue>” clause to initialize that DATA to <uValue> whenever the class is first used.

Examples

      CREATE CLASS TWindow
         DATA   hWnd, nOldProc
         CLASSDATA lRegistered AS LOGICAL
      ENDCLASS

Compliance

Harbour

Platforms

All

Seealso

Object Oriented Programming, CLASS, METHOD, DATA

HBClass()

HBClass()

HBClass() is used in the creation of all classes

Syntax

      oClass := HBClass():New("TMyClass")

        -or-

      HBClass() is usually accessed by defining a class with the commands
      defined in hbclass.h:

        CLASS HBGetList    // Calls HBClass() to create the HBGetList class
           ...
        ENDCLASS

Returns

An instance of the HBClass Class. This special object’s :New() method can then create the classes you define.

Description

HBClass is a class that … The class methods are as follows:

New() Create a new instance of the class

Examples

      FUNCTION TestObject()
         LOCAL oObject

         oObject := HBClass():New( "TMyClass" )
         oObject:End()

         RETURN NIL

Compliance

Object Oriented syntax in Harbour is compatible with CA-Cl*pper.

However CA-Cl*pper only allowed creation of objects from a few standard classes, and did not let the programmer create new classes.

In Harbour, you can create your own classes–complete with Methods, Instance Variables, Class Variables and Inheritance. Entire applications can be designed and coded in Object Oriented style.

Platforms

All

Files

Library is core

Seealso

__objHasData(), Object Oriented Programming, CLASS

VAR

VAR

Alternate syntax for VAR: instance variable for the objects.

Syntax

      VAR <DataName1> [, <DataNameN>] [ AS <type> ] [ INIT <uValue> ]
      [[EXPORTED | VISIBLE] | [PROTECTED] | [HIDDEN]] [READONLY | RO]

Arguments

<DataName1> Name of the VAR

<type> Optional data type specification from the following: Character, Numeric, Date, Logical, Codeblock, Nil.

<uValue> Optional initial value when creating a new object.

EXPORTED Specifies that this VAR is accessible to functions and methods outside of the class. VISIBLE is a synonym for EXPORTED.

PROTECTED Specifies that this VAR is only accessible to functions and methods within this class and its subclasses.

HIDDEN Specifies that this VAR is only accessible to the class where it was defined, and is not inherited by the subclasses.

READONLY Restricts assignment to the variable. If specified with the EXPORTED clause, assignment is only permitted from the current class and its subclasses. If specified with the PROTECTED clause, assignment is only permitted from the current class. RO is a synonym for READONLY.

Description

VAR elements can also be thought of as the “properties” of an object. They can be of any data type, including codeblock. Once an object has been created, the VAR elements are referenced with the colon (:) as in MyObject:Heading := “Last name”. Usually a class also defines methods to manipulate the VAR.

You can use the “AS <type>” clause to enforce that the VAR is maintained as a certain type. Otherwise it will take on the type of whatever value is first assigned to it.

Use the “INIT <uValue>” clause to initialize that VAR to <uValue> whenever a new object is created.

VAR can be a synonym for VAR, or it can use a slightly different syntax for compatibility with other dialects.

Examples

      CREATE CLASS TBColumn

         VAR Block      // Code block to retrieve data for the column
         VAR Cargo      // User-definable variable
         VAR ColorBlock // Code block that determines color of data items
         VAR ColSep     // Column separator character
         VAR DefColor   // Array of numeric indexes into the color table
         VAR Footing    // Column footing
         VAR FootSep    // Footing separator character
         VAR Heading    // Column heading
         VAR HeadSep    // Heading separator character
         VAR Width      // Column display width
         VAR ColPos     // Temporary column position on screen

         METHOD New()   // Constructor

      ENDCLASS

Compliance

Harbour

Platforms

All

Seealso

Object Oriented Programming, CLASS, METHOD, CLASS VAR, VAR

ON ERROR

ON ERROR

Designate a method as an error handler for the class

Syntax

      ON ERROR <MethodName>( [<params,...>] )

Arguments

<MethodName> Name of the method to define

<params, …> Optional parameter list

Description

ON ERROR is a synonym for ERROR HANDLER. It names the method that should handle errors for the class being defined.

Examples

      CREATE CLASS TWindow
         ON ERROR  MyErrHandler()
      ENDCLASS

Compliance

Harbour

Platforms

All

Seealso

Object Oriented Programming, ERROR HANDLER, CLASS, METHOD, DATA

METHOD

METHOD

Declare a METHOD for a class in the class header

Syntax

      METHOD <MethodName>( [<params,...>] ) [CONSTRUCTOR]

      METHOD <MethodName>( [<params,...>] ) INLINE <Code,...>

      METHOD <MethodName>( [<params,...>] ) BLOCK  <CodeBlock>

      METHOD <MethodName>( [<params,...>] ) EXTERN <NAME>([<args,...>])

      METHOD <MethodName>( [<params,...>] ) SETGET

      METHOD <MethodName>( [<params,...>] ) VIRTUAL

      METHOD <MethodName>( [<param>] )      OPERATOR <op>

      METHOD <MethodName>( [<params,...>] ) CLASS <ClassName>

Arguments

<MethodName> Name of the method to define

<params, …> Optional parameter list

Description

Methods are “class functions” which do the work of the class. All methods must be defined in the class header between the CLASS and ENDCLASS commands. If the body of a method is not fully defined here, the full body is written below the ENDCLASS command using this syntax:

METHOD <MethodName>( [<params, …>] ) CLASS <ClassName>

Methods can reference the current object with the keyword “Self:” or its shorthand version “::”.

CLAUSES:

CONSTRUCTOR Defines a special method Class Constructor method, used to create objects. This is usually the New() method. Constructors always return the new object.

INLINE Fast and easy to code, INLINE lets you define the code for the method immediately within the definition of the Class. Any methods not declared INLINE or BLOCK must be fully defined after the ENDCLASS command. The <Code, …> following INLINE receives a parameter of Self. If you need to receive more parameters, use the BLOCK clause instead.

BLOCK Use this clause when you want to declare fast ‘inline’ methods that need parameters. The first parameter to <CodeBlock> must be Self, as in:

METHOD <MethodName> BLOCK {| Self, <arg1>, <arg2>, …, <argN> | … }

EXTERN If an external function does what the method needs, use this clause to make an optimized call to that function directly.

SETGET For calculated Data. The name of the method can be manipulated like a Data element to Set or Get a value.

VIRTUAL Methods that do nothing. Useful for Base classes where the child class will define the method’s behavior, or when you are first creating and testing a Class.

OPERATOR Operator Overloading for classes. See example Tests/TestOp.prg for details.

CLASS <ClassName> Use this syntax only for defining a full method after the ENDCLASS command.

Examples

      CREATE CLASS TWindow
         VAR    hWnd, nOldProc
         METHOD New( ) CONSTRUCTOR
         METHOD Capture() INLINE  SetCapture( ::hWnd )
         METHOD End() BLOCK  {| Self, lEnd | iif( lEnd := ::lValid(),;
                                 ::PostMsg( WM_CLOSE ), ), lEnd }
         METHOD EraseBkGnd( hDC )
         METHOD cTitle( cNewTitle ) SETGET
         METHOD Close() VIRTUAL
      ENDCLASS

      METHOD New( ) CLASS TWindow
         local nVar, cStr
         ... <code> ...
         ... <code> ...
      RETURN Self

Tests

      TestOp.prg

Compliance

Harbour

Platforms

All

Seealso

HBClass(), Object Oriented Programming, DATA, CLASS

MESSAGE

MESSAGE

Route a method call to another Method

Syntax

      MESSAGE <MessageName>   METHOD <MethodName>( [<params,...>] )

      MESSAGE <MessageName>() METHOD <MethodName>( [<params,...>] )

Arguments

<MessageName> The pseudo-method name to define

<MethodName> The method to create and call when <MessageName> is invoked.

<params, …> Optional parameter list for the method

Description

The MESSAGE command is a seldom-used feature that lets you re-route a call to a method with a different name. This can be necessary if a method name conflicts with a public function that needs to be called from within the class methods.

For example, your app may have a public function called BeginPaint() that is used in painting windows. It would also be natural to have a Window class method called :BeginPaint() that the application can call. But within the class method you would not be able to call the public function because internally methods are based on static functions (which hide public functions of the same name).

The MESSAGE command lets you create the true method with a different name (::xBeginPaint()), yet still allow the ::BeginPaint() syntax to call ::xBeginPaint(). This is then free to call the public function BeginPaint().

Examples

      CREATE CLASS TWindow
         VAR    hWnd, nOldProc
         METHOD New( ) CONSTRUCTOR
         MESSAGE BeginPaint METHOD xBeginPaint()
      ENDCLASS

Compliance

Harbour

Platforms

All

Seealso

METHOD, DATA, CLASS, Object Oriented Programming