Difference between revisions of "CLASS"

Revision as of 16:05, 18 December 2009

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Purpose

Create a user-defined class

Syntax

CLASS <class name>

[OF | AS | EXTENDS <base class> [, ...]]

[PRIVATE | PUBLIC]

ENDCLASS

See Also

ACLASS(), ADDPROPERTY(), AMEMBERS(), CLASS - Methods, CLASS - Parameters, CLASS - Properties, CLASS - Scoping, COMPOBJ(), CREATEOBJECT(), DEFINE CLASS, DISPLAY CLASSES, DODEFAULT(), FOREACH, LIST CLASSES, LOADOBJECT(), METHOD, NEWOBJECT(), OBJECT(), PRINT_HTML(), PRINT_JSON(), PRINT_R(), PRINT_XML(), REMOVEPROPERTY(), REQUIRE_ONCE(), SAVEOBJECT(), SQL SELECT, WITH

Description

Fundamental to object-oriented programming is the concept of Objects and classes. An object is a self-contained unit of data and functions that manipulate that data. A class is a specification of an object. A class contains memory variable specifications known as properties and functions that perform actions on the object known as methods. An object is an instance of a class.

The NEW operator is used to define a new object based on a class. The class name must be postfixed with parentheses when the new operator is used. The syntax is therefore as follows:

<object> = NEW <class>()

e.g. myobject = new myclass()

The CLASS...ENDCLASS construct is used to create a user-defined class. The beginning of a class is specified with CLASS <class name>, where <class name> can be any valid name up to 32 characters. The ENDCLASS command is used to complete the class construct. The CLASS...ENDCLASS construct is built using the commands describe in this section. Any Recital command can be used in building methods in the class, however these commands cannot be used outside the method definition inside a CLASS...ENDCLASS construct.

OF | AS | EXTENDS <base-name> [, ...]

A key feature of the object-oriented code is reusability through a mechanism called inheritance, that is, one class can inherit the members and their implementation from another class. Building new classes out of existing classes allows for the reusing of proven classes and the incorporation of system classes into user defined classes. Inheritance enables developers to build a hierarchy of descending objects. The inheriting class is called a derived class, and the class from which the derived class inherits is called a base class. The OF | AS | EXTENDS clause is used to inherit the <base name>. You can inherit multiple classes by specifying a class name comma-separated list.

PRIVATE | PUBLIC

A user-defined class by default is defined as PRIVATE, which means that it will be visible only at the level where the class is instantiated. If the PUBLIC clause is specified, then when the class is instantiated, it will be visible at all levels until it is RELEASED from memory.

Example

class NullData dynamic
    property mCHARACTER
    property mNUMERIC
endclass
 
oNULLDATA = new NullData() 
oNULLDATA.mCHARACTER = ""
oNULLDATA.mNUMERIC = 0
//Note: {} cannot be used to delimit a date here 
oNULLDATA.mDATE = ctod("")
oNULLDATA.mLOGICAL = .f.
 
// Example of Constructor & Destructor
class OpenTable
    property cALIAS
    property nRECNUM
 
    method Constructor
        parameters cTABLENAME, cTAGNAME
        local cTMPALIAS
        cTMPALIAS = basename(cTABLENAME)
        cTMPALIAS = iif(at('.',cTMPALIAS) = 0, cTMPALIAS, left(cTMPALIAS,at('.',cTMPALIAS) - 1))
        if select(cTMPALIAS) = 0 
            use &(cTABLENAME + iif(empty(cTAGNAME), '', " order " + cTAGNAME)) in workarea()
        else
            select select(cTABLENAME)
            set order tag &cTAGNAME
        endif
        this.cALIAS = alias()
        this.nRECNUM = recno()
    return  && Constructor
 
    method Destructor
       close &(this.cALIAS)
    return  && Destructor
endclass
 
set exclusive off
oCOMPANY = new OpenTable("/usr/recital/unixdeveloper/demo/state.rdb", "state")
? oCOMPANY.cALIAS 
? oCOMPANY.nRECNUM
 
// Example of dynamically adding properties using the ADDPROPERTY method
class Box 
endclass 
 
oDIALOG = new Box()
oDIALOG.AddProperty("myprop", "hello world") 
dialog box oDIALOG.myprop
release oDIALOG

Products

Recital Server, Recital