| Description |
The $BoolEval compiler directive tells Delphi whether to continue a multi argument boolean expression evaluation when the result is known before evaluation completes.
{$BoolEval Off} (default) means no continue
{$BoolEval On} means continue checking
For example, by default, with an expression :
expr1 and expr2
expr2 is not evaluated if expr1 is false. With {$BoolEval On}, checking would have continued.
The example illustrates a normal use of the default.
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| Notes |
$BoolEval is equivalent to $B.
This directive can be used multiple times within your code.
The default value is $BoolEval Off
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| Related commands |
| $B |
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Whether to short cut and and or operations |
| And |
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Boolean and or bitwise and of two arguments |
| If |
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Starts a conditional expression to determine what to do next |
| Or |
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Boolean or or bitwise or of two arguments |
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| Example code : Checking string contents |
// Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.
unit Unit1;
interface
uses
SysUtils,
Forms, Dialogs;
type
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
end;
var
Form1: TForm1;
implementation
{$R *.dfm} // Include form definitions
procedure TForm1.FormCreate(Sender: TObject);
var
FullString, EmptyString : string;
begin
FullString := 'Fred';
EmptyString := '';
// Set full checking OFF
{$BoolEval Off}
// Check the 4th character of each string
if (Length(FullString) >= 4) and (FullString[4] = 'd')
then ShowMessage('FullString 4th character is d')
else ShowMessage('FullString 4th character is NOT d');
if (Length(EmptyString) >= 4) and (EmptyString[4] = 'd')
then ShowMessage('EmptyString 4th character is d')
else ShowMessage('EmptyString 4th character is NOT d');
// Set full checking ON
{$BoolEval On}
// Check the 4th character of each string
if (Length(FullString) >= 4) and (FullString[4] = 'd')
then ShowMessage('FullString 4th character is d')
else ShowMessage('FullString 4th character is NOT d');
// Now we must protect the code from errors
try
if (Length(EmptyString) >= 4) and (EmptyString[4] = 'd')
then ShowMessage('EmptyString 4th character is d')
else ShowMessage('EmptyString 4th character is NOT d');
except
on E : EAccessViolation do
ShowMessage(E.Message);
end;
end;
end.
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| Hide full unit code |
The following is typical of the output from the above code:
FullString 4th character is d
EmptyString 4th character is NOT d
FullString 4th character is d
Access violation at address 00442196 in module
'PROJECT1.EXE'. Read of address FFFFFFFF
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