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Read Me First! Welcome to the First Version of the xHarbour Language Reference Guide. This page informs you about what you can expect from the First Edition and what is not yet included. First, we'd like to thank you for patience and ongoing support while we were preparing this xHarbour Language Reference Guide. It is a huge documentation project and while not yet finished, we are confident that the large amount of quality information will prove valuable for every xHarbour programmer.
First stage (completed) We have started this First Edition with collecting most information about xHarbour's roots: the Clipper programming language. The topics covering Clipper are entirely re-written.
Second stage (completed) At the second stage, we focused on documenting all xHarbour's extensions to the Clipper language. There is a wealth of new features added to xHarbour that do not exist in Clipper (see "Notes for Clipper programmers" below). For example, a complete OOP model with new classes exists. New data types and operators are added. Additional preprocessor directives are available, plus numerous new functions and statements. As a matter of fact, the core documentation of xHarbour is about twice as much as the original Clipper Norton Guides. It is structured into the following sections: 1.
Operator Reference
2.
Statement Reference
3.
Command Reference
4.
Class Reference (textmode)
5.
Function Reference
6.
Preprocessor Reference
These sections should aid you in finding information on core features of xHarbour easily. In addition, a comprehensive Index and Full Text Search capabilities, plus a sophisticated Cross Reference Link System are all included. Please try these features for searching and finding the information you need in the xHarbour Documentation.
Third stage (ongoing) We are now entering the third documentation stage right after the release of this book. The result of this third stage will be provided as a free update to Version 1.0 of the xHarbour Language Reference Guide. We are commited to supply you with this free update promptly. However, there is still a bunch of work ahead and we hope you enjoy reading this Version 1.0 in the meantime. A list of items we will cover in the next version can be found on this page.
xHarbour Language Reference Guide
1
Read Me First!
Notes for Clipper programmers The xHarbour programming language has its roots in CA-Clipper, a powerful xBase dialect, and is modelled around Clipper. xHarbour extends Clipper and has received many new features, which can be detected very easily: Extended function arguments are highlighted in yellow. Refer to function AIns() for an example. All topics that do not exist in Clipper, but are added by xHarbour, are listed in the category xHarbour extensions.
Copyright Please read the Copyright note carefully before using this guide.
Your contributions This Version 1.0 of the xHarbour Language Reference Guide can only reflect the current stage of xHarbour and its documentation, as of Dec 2006. We are constantly adding new features to xHarbour and new information to the documentation. If you miss something particular, please let us know. This file should give you a clear idea of xHarbour.com commitment to the xHarbour programming language. We are grateful for any suggestion you may have to improve xHarbour. Thank you for your support.
BEGIN SEQUENCE ...............................................92 CLASS ..................................................................94 CLASSDATA .........................................................99 CLASSMETHOD .................................................102 DATA.................................................................103 DELEGATE .........................................................106 DESTRUCTOR ....................................................108 DO CASE............................................................110 DO WHILE .........................................................112 ENABLE TYPE CLASS..........................................114 ERROR HANDLER ..............................................116 EXIT PROCEDURE ..............................................118 EXPORTED: .......................................................120 EXTEND CLASS...WITH DATA ............................121 EXTEND CLASS...WITH METHOD.......................123 EXTERNAL .........................................................125 FIELD.................................................................126 FOR ...................................................................128 FOR EACH .........................................................130 FUNCTION ........................................................132 GLOBAL.............................................................135 HIDDEN:............................................................137 IF.......................................................................139 INIT PROCEDURE ..............................................141 INLINE METHOD ...............................................143 LOCAL ...............................................................145 MEMVAR ..........................................................147 MESSAGE ..........................................................148 METHOD (declaration) .....................................150 METHOD (implementation)..............................153 METHOD...OPERATOR ......................................156 METHOD...VIRTUAL ..........................................158 OPERATOR ........................................................159 OVERRIDE METHOD .........................................161 PARAMETERS....................................................163 pragma pack() ..................................................164 PRIVATE ............................................................165 PROCEDURE ......................................................167 PROTECTED: .....................................................169 PUBLIC ..............................................................170 REQUEST...........................................................172 RETURN ............................................................173 STATIC...............................................................174 SWITCH.............................................................176 TRY...CATCH ......................................................178 typedef struct ...................................................180 WITH OBJECT ....................................................183 COMMAND REFERENCE ......................................... 185 ? | ?? ................................................................186 @...BOX ............................................................188 @...CLEAR .........................................................190 @...GET.............................................................192 @...PROMPT.....................................................196 @...SAY .............................................................197 @...TO ..............................................................200 I
Table of Contents ACCEPT.............................................................202 APPEND BLANK................................................. 203 APPEND FROM ................................................. 205 AVERAGE ..........................................................208 CANCEL.............................................................210 CLEAR ALL......................................................... 211 CLEAR MEMORY ............................................... 212 CLEAR SCREEN..................................................213 CLEAR TYPEAHEAD ........................................... 214 CLOSE ...............................................................215 COMMIT...........................................................217 CONTINUE ........................................................218 COPY FILE ......................................................... 219 COPY STRUCTURE ............................................. 220 COPY STRUCTURE EXTENDED...........................221 COPY TO ...........................................................223 COUNT..............................................................227 CREATE .............................................................229 CREATE FROM ..................................................231 DO ....................................................................233 DEFAULT TO ..................................................... 234 DELETE..............................................................235 DELETE FILE ......................................................237 DELETE TAG......................................................238 EJECT ................................................................ 240 ERASE ...............................................................241 FIND..................................................................242 GO ....................................................................243 INDEX ...............................................................245 INPUT ...............................................................249 JOIN ..................................................................250 KEYBOARD........................................................252 LOCATE.............................................................254 MENU TO..........................................................256 PACK.................................................................258 QUIT .................................................................259 READ.................................................................260 RECALL..............................................................262 REINDEX ...........................................................264 RELEASE............................................................265 RENAME ...........................................................267 REPLACE ...........................................................269 RESTORE...........................................................271 RUN ..................................................................273 SAVE .................................................................274 SEEK..................................................................275 SELECT ..............................................................277 SET ALTERNATE ................................................279 SET AUTOPEN ................................................... 281 SET AUTORDER ................................................. 283 SET AUTOSHARE ............................................... 284 SET BACKGROUND TASKS.................................285 SET BACKGROUNDTICK ....................................286 SET BELL ...........................................................287 SET CENTURY....................................................288 SET COLOR........................................................289 II
SET CONFIRM ................................................... 290 SET CONSOLE ................................................... 291 SET CURSOR ..................................................... 292 SET DATE .......................................................... 293 SET DBFLOCKSCHEME ......................................295 SET DECIMALS .................................................. 297 SET DEFAULT .................................................... 298 SET DELETED .................................................... 299 SET DELIMITERS ............................................... 301 SET DESCENDING ............................................. 302 SET DEVICE ....................................................... 303 SET DIRCASE ..................................................... 305 SET DIRSEPARATOR .......................................... 306 SET EOL ............................................................ 307 SET EPOCH ....................................................... 308 SET ERRORLOG................................................. 309 SET ERRORLOOP............................................... 310 SET ESCAPE....................................................... 311 SET EVENTMASK .............................................. 312 SET EXACT ........................................................ 314 SET EXCLUSIVE ................................................. 316 SET FILECASE .................................................... 317 SET FILTER ........................................................ 318 SET FIXED ......................................................... 320 SET FUNCTION.................................................. 321 SET INDEX......................................................... 323 SET INTENSITY .................................................. 325 SET KEY.............................................................326 SET MARGIN..................................................... 327 SET MEMOBLOCK............................................. 328 SET MESSAGE ................................................... 330 SET OPTIMIZE................................................... 331 SET ORDER ....................................................... 332 SET PATH .......................................................... 334 SET PRINTER..................................................... 335 SET RELATION .................................................. 337 SET SCOPE ........................................................ 339 SET SCOPEBOTTOM.......................................... 341 SET SCOPETOP.................................................. 342 SET SCOREBOARD ............................................ 343 SET SOFTSEEK................................................... 344 SET STRICTREAD............................................... 346 SET TRACE ........................................................ 347 SET TYPEAHEAD ............................................... 348 SET UNIQUE ..................................................... 349 SET VIDEOMODE .............................................. 350 SET WRAP......................................................... 351 SKIP ..................................................................352 SORT................................................................. 354 STORE...............................................................356 SUM ................................................................. 357 TEXT ................................................................. 359 TOTAL...............................................................360 UNLOCK............................................................ 362 UPDATE............................................................ 363 USE ................................................................... 365
$ Substring operator (binary): search substring in string.
Syntax $ $
Arguments
is a character value that is searched for in .
is a character value where is searched in.
is the expression of any type to search.
is the array to scan for a matching in.
Description The substring operator performs a case-sensitive search and returns .T. (true) when is found in , otherwise the result is .F. (false). The $ operator can also be used to search for a given value within any Array.
Info See also: Category: DLL:
<, <=, <> != #, = (comparison), ==, >, >=, IN, HAS, LIKE Character operators, Comparison operators, Operators xhbdll.dll
Example // The example demonstrates a case-sensitive search. PROCEDURE Main() LOCAL cString := "xHarbour" ? "X" $ cString ? "arb" $ cString ? 1 $ { 3, 2, 1 } RETURN
10
// result: .F. // result: .T. // result: .T.
Operator Reference
%
% Modulus operator (binary): calculates the remainder of a division.
Syntax %
Arguments
is the dividend of the division.
is the divisor of the division.
Description The modulus operator returns the remainder of dividing by .
Example // The example calculates the remainder of a division // between two numbers. PROCEDURE Main ? 5 % 0 ? 3 % 2 ? -2 % 3 ? 4 % 2 ? 5.4 % 2.3 RETURN
Operator Reference
// // // //
result: 0 result: 1 result: -2 result: 0
// result: 0.8
11
& (bitwise AND)
& (bitwise AND) Bitwise AND operator (binary): performs a logical AND operation.
Syntax & | --> cCharacter & | --> nNumeric
Arguments
A character expression of which all bits of each character are processed in a logical AND operation.
A numeric value all bits of which are processed. Numbers are always treated as integer values. If a number has a decimal fraction, it is truncated. |
The right operand of the bitwise AND operator can be specified as a character or a numeric value.
Description The bitwise AND operator performs a logical AND operation with the individual bits of both operands. The left operand is the value to process while the right operand provides the bit mask for the operation. The return value of the &-operator has the same data type as the left operand. Bits at identical positions in both operands are compared. The bit at the same position is set in the return value, when both operands have the bit set at the same position. If either operand has a bit not set, the corresponding bit in the return value is also not set. The bit mask to apply to the left operand can be specified as a numeric or as a character value. Depending on the left operand, bitwise AND operations are performed according to the following rules:
cString & cMask When both operands are character values, the bits of individual characters of both operands are compared. If the right operand has less characters, it is repeatedly applied until all characters of the left operand are processed. The return value has the same number of characters as the left operand.
cString & nMask When the left operand is a character value and the right operand is numeric, the bits set in the numeric value are compared with the bits of each character in the left operand.
nNumber & cMask When the left operand is numeric and the right operand is a character value, the bits set in the numeric value are compared consecutively with the bits of each character in the right operand.
12
Operator Reference
& (bitwise AND)
nNumber & nMask When both operands are numeric values, the bits of both values are compared. Note: Numeric operands are always treated as integer values. If a number has a decimal fraction, it is ignored.
Example // The example demonstrates bitwise AND operations using // operands of different data types. #define #define #define #define
STAT_OFF STAT_INIT STAT_IDLE STAT_ON
0 1 2 4
PROCEDURE Main LOCAL nStatus := STAT_INIT + STAT_IDLE ? 2 & 3
// result: 2
? "A" & 64
// result: "@"
? 64 & "A"
// result: 64
? "Z" & "A"
// result: "@"
? nStatus & STAT_IDLE
// result: 2
? nStatus & STAT_ON
// result: 0
? "One" & "Two" RETURN
Operator Reference
// result: "Dfe"
13
& (macro operator)
& (macro operator) Macro operator (unary): compiles a character string at runtime.
Syntax a) & b) "Text &. substitution" c) & d) :& [:= ] :&([]) e) &([])
Arguments
This is a character string indicating the name of a variable whose symbolic name is known at runtime. This applies to variables of type PRIVATE, PUBLIC or FIELD.
A character string holding a valid xHarbour expression to compile at runtime.
This is a variable holding a reference to an object value (Valtype() == "O") to which a message is sent.
A character string indicating the name of an instance variable of the object. If no assignment operator is used in the expression, the value of the instance variable is retrieved by the macro operator. Otherwise, the value is assigned to the instance variable.
A character string indicating the name of a method of the object to execute. Calling a method with the macro operator requires the parentheses () be added to the expression. Optionally, a list of parameters can be specified within the parentheses. The list of is passed to the method.
A character string indicating the name of a function or procedure to execute. Calling a function with the macro operator requires the parentheses () be added to the expression. Optionally, a list of parameters can be specified within the parentheses. The list of is passed to the function or procedure.
Description One extremely powerful feature of xHarbour is the Macro operator (&). It allows for compiling and executing program code at runtime of an application. The program code is supplied in form of a character string and can be as simple as a variable name or may include very complex expressions. 14
Operator Reference
& (macro operator)
Despite its power, there are some limitations a programmer must be aware of when using the Macro operator. As a first rule, no white space characters may follow the macro operator. This is required to distinguish the & sign from the bitwise AND operator. The macro expression must follow the & operator immediately. All entities that do not have a symbolic name at runtime of an application cannot be accessed by the Macro operator. This applies to memory variables of type GLOBAL, LOCAL and STATIC, to STATIC declared functions and procedures, as well as commands and statements. Also, the Macro operator cannot resolve multiple line expressions, it can only compile expressions programmed within one line of code. As indicated in the syntax description, there are some typical scenarios where the Macro operator is used. Examples for these scenarios follow: a)
Acessing and creating variables The values of PRIVATE, PUBLIC and FIELD variables can be retrieved by using a character string indicating the symbolic name of a variable. LOCAL LOCAL LOCAL LOCAL
cPrivate := "myPrivate" cPublic := "myPublic" cField := "LastName" x, y, z
PUBLIC myPublic := "A public variable" PRIVATE myPrivate := 10 USE Customer x := &cPrivate y := &cPublic z := &cField ? x, y, z // result: 10 A public variable Miller
The macro operator is used in this example as the right-hand-side expression of the assignments. As a result, the values of the variables whose names are stored in LOCAL variables are retrieved and assigned to the LOCAL variables x, y and z. When the macro operator is used on the left-hand-side of an assignment, a value is assigned to the corresponding variable. If the variable does not exist prior to the assignment, it is created as a PRIVATE variable: LOCAL cPrivate := "myPrivate" LOCAL cPublic := "myPublic" LOCAL cField := "LastName" PUBLIC
myPublic
:= "A public variable"
USE Customer &cPrivate := 10
// creates a PRIVATE variable // named myPrivate
&cPublic := "New text" FIELD->&cField := "McBride" ? &cPrivate, &cPublic, &cField // result: 10 New text McBride
Operator Reference
15
& (macro operator)
Note that PUBLIC variables cannot be created using this technique while field variables must be aliased to assign a value. b)
Text substitution Text substitution is a special situation where the macro operator appears within a character string. LOCAL cText PRIVATE cMacro := "xHarbour" cText := "This is a &cMacro. test." ? cText // result: This is a xHarbour test." cMacro := CDoW(Date()) cText := "Today is &cMacro.!" ? cText // result: Today is Tuesday!"
When a character string contains a macro variable, the macro is substituted with the contents of the macro variable. Note that the end of the macro variable within the text string is indicated with a period. This period does not appear in the result string but serves as "end-of-macro" marker. c)
Runtime compilation The most common situation for using the Macro operator is the compilation of expressions at runtime. Expressions must be syntactically correct and may only refer to entities whose symbolic names are known at runtime. If, for example, a function is called only within a Macro expression and nowhere else in the program code, the function symbol must be declared with the REQUEST statement so that it is linked to the executable file. PROCEDURE Main REQUEST CDoW, Date LOCAL cExpr := "CDoW(Date())" ? &cExpr // result: Tuesday RETURN
The creation of code blocks as complex data types is also a very common usage scenario for the Macro operator. PROCEDURE Main LOCAL i, cExpr, cFPos REQUEST FieldGet, FieldPut USE Customer aBlocks := Array( FCount() ) FOR i:=1 TO FCount() cFPos := LTrim( Str(i) ) cExpr := "{|x| IIf(x==NIL,FieldGet(" + cFPos + ")," + ; "FieldPut(" + cFPos + ",x)) }" aBlocks[i] := &cExpr NEXT AEval( aBlocks, {|b| QOut(Eval(b)) } ) USE RETURN
16
Operator Reference
& (macro operator)
Within the FOR..NEXT loop, the syntax for code blocks accessing different field variables is created as character strings. Each string differs by the value of the loop counter variable which identifies individual field variables by theri ordinal position. The character strings are compiled to code blocks and stored in an array. d)
Sending messages to objects The possibility of sending messages to objects is another field of Macro operator usage that allows for highly dynmic application development. Messages are encoded as character strings: #include "hbclass.ch" PROCEDURE Main LOCAL obj := TestClass():new( "Text to display" ) LOCAL cMsg := "display" obj:&cMsg()
// displays: Text to display
cMsg := "cValue" obj:&cMsg := "New text" obj:display() RETURN
// displays: New text
CLASS TestClass EXPORTED: DATA cValue METHOD new( cString ) CONSTRUCTOR METHOD display ENDCLASS METHOD new( cString ) CLASS TestClass ::cValue := cString RETURN self METHOD display() CLASS TestClass ? ::cValue RETURN self
When the send operator (:) is followed by the Macro operator, the contents of the variable following the Macro operator is interpreted as message to send to the object. Note that calling an object's method requires the parentheses be added to the expression. e)
Calling functions There are two possibilities for calling functions using the macro operator: one is to encode a complete function call as a macro expression, the other is to use only the function name. The difference between both possibilities lies in the type of variables that can be passed to a Macro operator called function. PROCEDURE Main LOCAL cMacro LOCAL nValue := 2 PRIVATE nNumber := 3 cMacro := "TestFunc( nNumber )"
// result: 30
? &cMacro cMacro := "TestFunc"
Operator Reference
17
& (macro operator) ? &cMacro( nValue ) RETURN
// result: 20
FUNCTION TestFunc( n ) RETURN 10 * n
When the macro expression includes parentheses for the function call, only PRIVATE, PUBLIC or FIELD variables can be passed as parameters, since the variable names appear in the macro string. LOCAL variables, in contrast, can be passed when the parentheses are "hard coded", i.e. the macro string contains only the name of the function to call.
One or more expressions to execute, separated with commas.
This is the symbolic name of a function, method or procedure to execute.
An optional comma separated list of parameters to pass to the called function, method or procedure
This is a variable holding a reference to an object value (Valtype() == "O") that should execute a method.
Description Parentheses are used to execute functions, procedures or methods, and to group expressions and define their order of execution. When using the parentheses as a grouping operator, all items appearing inside the parentheses must be valid expressions. The grouping operator can be nested to any depth. The nesting level influences execution order of expressions, i.e. expressions on a deeper nesting level are evaluated first while expressions with the same nesting level are evaluated from left to right. When the parantheses are used as execution operator, expressions between the parentheses are passed as arguments to the called function, method or procedure. Multiple expressions must be separated with commas. When no expression appears within the parentheses, no argument is passed.
Info See also: Category: LIB: DLL:
& (macro operator) Special operators, Operators xhb.lib xhbdll.dll
Example // The example demonstrates different scenarios for using // the () operator. PROCEDURE Main LOCAL x
Description This operator multiplies the value of with the value of and returns the result.
Info See also: Category: LIB: DLL:
%, **, +, -, /, = (compound assignment), SET DECIMALS, SET FIXED Mathematical operators, Operators xhb.lib xhbdll.dll
Example // The example shows the behavior of the "*" operand in different // situations. PROCEDURE Main ? 3 * 1 ? 3 * -1 ? -4 * -2 RETURN
Operator Reference
// result: 3 // result: -1 // result: 8
21
**
** Exponentiation (binary): raises a number to the power of an exponent.
Syntax ** ^
Arguments
is numeric value to raise to the power defined by .
is the power to raise .
Description This operator is a binary operator that raises to the power of .
Info See also: Category: LIB: DLL:
%, *, +, -, /, = (compound assignment), SET DECIMALS, SET FIXED Mathematical operators, Operators xhb.lib xhbdll.dll
Example // The example shows some results of the "**" operator. PROCEDURE Main ? 5 ** 0 ? 5 ** 1 ? 5 ** 2 ? 5 ** -2 ? -5 ** 2 ? 5 ^ 0 ? 5 ^ 1 ? 5 ^ 2 ? 5 ^ -2 ? -5 ^ 2 RETURN
22
// // // // //
result: result: result: result: result:
1.00 5.00 25.00 0.04 25.00
// // // // //
result: result: result: result: result:
1.00 5.00 25.00 0.04 25.00
Operator Reference
+
+ Plus operator: add values, concatenate values and unary positive.
Syntax + + + +
Arguments
is a numeric value to which the value of is added.
is a date value to which days are added.
is a character string to concatenate with .
Description Depending on the data types of the operands, the Plus operator performs different operations.
Unary positive sign A numeric expression preceeded by the "+" operator performs no operation on the operand.
Numeric addition When both operands are numeric values, the right operand is added to the left operand and the result is a numeric value.
Date addition When eigher operand is a date value and the other is a numeric, the value of is added as days to . The returned value is a date.
String concatenation If both operands are character strings, the value of is joined to the end of . The result is a character string containing both operands.
Hash operation If both operands are hashes, a set-oriented operation is performed so that the resulting hash value contains unique keys of both operands (refer to the hash operator ).
is the name of a memory or field variable of Numeric or Date data type. When is a field variable, it must be specified with an alias name or must be declared as field variable using the FIELD statement.
Description The increment operator increases the value of its operand by one. When used in an expression, the position of the operator is important for the result of the expression. When the operator appears to the left of the operand (prefix notation), the operand's value is first incremented and then used in the expression. When the operator appears to the right of the operand (postfix notation), the operand's value is first used in the expression and then incremented.
Example // The example demonstrates the use of the ++ operator and // outlines the importance of prefix and postfix notation. PROCEDURE Main LOCAL nValue1 := 0 LOCAL nValue2 ? nValue1 ++ ? nValue1
// result: 0 // result: 1
? ++ nValue1 ? nValue1
// result: 2 // result: 2
nValue2 := ++ nValue1 ? nValue1 ? nvalue2
// result: 3 // result: 3
nValue2 := nValue1 ++ ? nValue1 ? nvalue2 RETURN
Operator Reference
// result: 4 // result: 3
25
-
Minus operator: add values, concatenate values and unary negative.
Syntax - - - - -
Arguments
is a numeric value from which the value of is subtracted.
is a date value from which the date value is subtracted.
is a date value from which days are subtracted.
is a character string to add to the end of after all trailing blanks from are removed.
Description Depending on the data types of the operands, the Minus operator performs the following operations:
Unary negative sign when the Minus operator precedes a numeric operand, it performs the equivalent of multiplying the operand by -1. This changes the operand's sign from plus to minus and vice versa.
Numeric subtraction When both operands are numeric values, the right operand is subtracted from the left operand and the result is a numeric value.
Date subtraction When the left operand is a date value and the right operand is a numeric, the value of is subtracted as number of days from . The returned value is a date. When both operands are of Date data type, the operator calculates the difference in days between left and right operand and returns a numeric value. When the left operand is a Numeric value and the right operand is a Date, a runtime error is raised sine this is not allowed.
26
Operator Reference
-
String concatenation If both operands are character data types, the value of is joined to , returning a character string. Trailing blanks in are removed and appended to the end of the result string.
Hash operation If both operands are hashes, a set-oriented operation is performed so that the resulting hash value contains unique keys of the left operand which are not contained in the right operand (refer to the hash operator ).
is the name of a memory or field variable of Numeric or Date data type. When is a field variable, it must be specified with an alias name or must be declared as field variable using the FIELD statement.
Description The decrement operator decreases the value of its operand by one. When used in an expression, the position of the operator is important for the result of the expression. When the operator appears to the left of the operand (prefix notation), the operand's value is first decremented and then used in the expression. When the operator appears to the right of the operand (postfix notation), the operand's value is first used in the expression and then decremented.
Example // The example demonstrates the use of the -- operator and // outlines the importance of prefix and postfix notation. PROCEDURE Main LOCAL nValue1 := 0 LOCAL nValue2 ? nValue1 -? nValue1
// result: 0 // result: -1
? -- nValue1 ? nValue1
// result: -2 // result: -2
nValue2 := -- nValue1 ? nValue1 ? nvalue2
// result: -3 // result: -3
nValue2 := nValue1 -? nValue1 ? nvalue2 RETURN
28
// result: -4 // result: -3
Operator Reference
->
-> Alias operator (binary): identifies a work area.
This is the symbolic alias name of a work area as specified with the ALIAS option of the USE command.
The field name whose value is retrieved from the work area specified with .
Instead of using a symbolic name, a work area can be identified by its numeric ordinal position . This value can be obtained by calling the Select() function. The numeric work area must be enclosed in parentheses () when using it with the alias operator.
One or more comma separated expressions to execute in the work area identified with or . FIELD
The FIELD keyword is a reserved alias name that identifies a field variable in the current work area. MEMVAR
The MEMVAR keyword is a reserved alias name that identifies a memory variable of type PRIVATE or PUBLIC. An abbreviation of this alias is M->.
Description The alias operator "points" to an unselected work area identified with its symbolic name or its ordinal number . This allows for retrieving values of field variables from unselected work areas or executing expressions in the specified work area. The alias operator implicitly selects the work area specified with the left operand and then executes the instructions of the right operand. When the operation is complete, the original work area becomes current again. FIELD and MEMVAR are reserved alias names that allow for resolving name conflicts between memory and field variables. If memory and field variables with the same symbolic name exist, unaliased variables are resolved as field variables. This behaviour is influenced by the compiler switch Operator Reference
29
->
/v. It is, however, strongly recommended to avoid identical variable names for memory and field vraiables, and to use the alias operator to identify a variable unambiguously.
Info See also: Category: LIB: DLL:
DbSelectArea(), FIELD, FieldName(), FieldPos(), FieldGet(), MEMVAR, Select(), USE Special operators, Operators xhb.lib xhbdll.dll
Example // The example demonstrates usage of the alias operator. // The last aliased expression generates a runtime error. PROCEDURE Main LOCAL nArea USE Customer ALIAS Cust ? FIELD->Lastname
// current work area // result: Miller
nArea := Select() SELECT 100 ? Used()
// work area number // select free work area // result: .F.
? Cust->Lastname ? Cust->(Recno())
// result: Miller // result: 1
? (nArea)->Lastname ? (nArea)->(LastRec())
// result: Miller // result: 1576
? FIELD->Lastname USE RETURN
30
// runtime error
Operator Reference
.AND.
.AND. Logical AND operator (binary).
Syntax .AND.
Arguments
and are logical expressions to AND.
Description The logical .AND. operator yields the result of a logical AND operation with left and right operand. The result is only .T. (true) when the value of both operands is also .T. (true), in all other cases the result is .F. (false). When multiple .AND. operators appear in an expression, the result is already determined when one operand has the value .F. (false). In this case, remaining .AND. operators are not evaluated for optimization reasons. This so called shortcut optimization can be switched off using the compiler switch /z.
Example // This example shows results of .AND. operations // using different operands: PROCEDURE Main ? .T. .AND. ? .T. .AND. ? .F. .AND. ? .F. .AND. RETURN
Operator Reference
.T. .F. .T. .F.
// // // //
result: result: result: result:
.T. .F. .F. .F.
31
.NOT.
.NOT. Logical NOT operator (unary).
Syntax ! .NOT.
Arguments
is a logical expression to logically negate.
Description The .NOT. operator (alternatively "!" as an abbreviation) is a unary logical operator that negates the value of its operand. This yields the logical inverse of .
Example // The example shows the logical inverse of the operands: PROCEDURE Main ? .NOT. (.T.) ? .NOT. (.F.) ? ! 1 > 2 RETURN
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// result: .F. // result: .T. // result: .T.
Operator Reference
.OR.
.OR. Logical OR operator (binary).
Syntax .OR.
Arguments
and are logical expressions.
Description The logical .OR. operator yields the result of a logical OR operation with left and right operand. The result is only .T. (true) when the value of one operand is also .T. (true), in all other cases the result is .F. (false). When multiple .OR. operators appear in an expression, the result is already determined when one operand has the value .T. (true). In this case, remaining .OR. operators are not evaluated for optimization reasons. This so called shortcut optimization can be switched off using the compiler switch /z.
Example // This example shows results of .OR. operations // using different operands: PROCEDURE Main ? .T. .OR. .T. ? .T. .OR. .F. ? .F. .OR. .T. ? .F. .OR. .F. RETURN
