A exception ekon16

Exception Handling
10 print “hello EKON”;
20 Goto 10;

“Exceptions measure the stability of code
before it has been written”

1

Agenda EKON
• What are Exceptions ?
• Prevent Exceptions (MemoryLeakReport)
• Log Exceptions (Global Exception Handler)
• Check Exceptions (Test Driven)
• Compiler Settings

2

Some Kind of wonderful ?
• One of the main purposes of exception
handling is to allow you to remove error-
checking code altogether and to separate error
handling code from the main logic of your
application.
• [DCC Fatal Error] Exception Exception:
Compiler for personality "Delphi.Personality"
and platform "Win32" missing or unavailable.

UEB: 8_pas_verwechselt.txt

3

Types of Exceptions
Throwable
• Checked (Handled) Exceptions
• Runtime Exceptions (unchecked)
• Errors (system)
• Hidden, Deprecated or Silent
EStackOverflow = class(EExternal) end deprecated;

UEB: 15_pas_designbycontract2.txt

4

Kind of Exceptions
Some Structures
• try finally
• try except
• try except finally
• try except raise
• Interfaces or Packages (design & runtime)
• Static, Public, Private (inheritance or delegate)

UEB: 10_pas_oodesign_solution.txt

5

Cause of Exceptions
Bad Coordination
• Inconsistence with Threads
• Access on objects with Null Pointer
• Bad Open/Close of Input/Output Streams or
I/O Connections (resources)
• Check return values, states or preconditions
• Check break /exit in loops
• Access of constructor on non initialized vars

6

Who the hell is general failure
and why does he read on my
disk ?!
Die Allzweckausnahme!
Don’t eat exceptions silent

7

Top 5 Exception Concept
1. Memory Leak Report
• if maxform1.STATMemoryReport = true then
• ReportMemoryLeaksOnShutdown:= true; Ex.
298_bitblt_animation3
2. Global Exception Handler Logger
3. Use Standard Exceptions
4. Beware of Silent Exceptions (Ignore but)
5. Safe Programming with Error Codes

Ex: 060_pas_datefind_exceptions2.txt

8

Memory Leaks I
• The Delphi compiler hides the fact that the string variable is a
heap pointer to the structure but setting the memory in advance
is advisable:
path: string;
setLength(path, 1025);
• Check against Buffer overflow
var
Source, Dest: PChar;
CopyLen: Integer;
begin
Source:= aSource;
Dest:= @FData[FBufferEnd];
if BufferWriteSize < Count then
raise EFIFOStream.Create('Buffer over-run.');

9

Memory Leaks II
• Avoid pointers as you can
• Ex. of the win32API:

pVinfo = ^TVinfo;
function TForm1.getvolInfo(const aDrive: pchar; info: pVinfo):
boolean;
// refactoring from pointer to reference
function TReviews.getvolInfo(const aDrive: pchar; var info: TVinfo):
boolean;

“Each pointer or reference should be checked to see if it is null. An
error or an exception should occur if a parameter is invalid.”

244_script_loader_loop_memleak.txt

10

Global Exceptions
Although it's easy enough to catch errors (or exceptions) using
"try / catch" blocks, some applications might benefit from having
a global exception handler. For example, you may want your own
global exception handler to handle "common" errors such as
"divide by zero," "out of space," etc.

Thanks to TApplication's ‘OnException’ event - which occurs
when an unhandled exception occurs in your application, it only
takes three (or so) easy steps get our own exception handler
going:

11

Global Handling

1. Procedure AppOnException(sender: TObject;
E: Exception);
if STATExceptionLog then
Application.OnException:= @AppOnException;

12

Global Log
2. procedure TMaxForm1.AppOnException(sender: TObject;
E: Exception);
begin
MAppOnException(sender, E);
end;
procedure MAppOnException(sender: TObject; E: Exception);
var
FErrorLog: Text;
FileNamePath, userName, Addr: string;
userNameLen: dWord;
mem: TMemoryStatus;

13

Log Exceptions
3. Writeln(FErrorLog+ Format('%s %s [%s] %s %s
[%s]'+[DateTimeToStr(Now),'V:'+MBVERSION,
UserName, ComputerName, E.Message,'at: ,Addr]));

try
Append(FErrorlog);
except
on EInOutError do Rewrite(FErrorLog);
end;

14

Use Standards
(CL.FindClass('Exception'),'EExternal');
(CL.FindClass('TOBJECT'),'EExternalException');
(CL.FindClass('TOBJECT'),'EIntError');
(CL.FindClass('TOBJECT'),'EDivByZero');
(CL.FindClass('TOBJECT'),'ERangeError');
(CL.FindClass('TOBJECT'),'EIntOverflow');
(CL.FindClass('EExternal'),'EMathError');
(CL.FindClass('TOBJECT'),'EInvalidOp');
(CL.FindClass('EMathError'),'EZeroDivide');
"Die andere Seite, sehr dunkel sie ist" - "Yoda, halt's Maul und iß Deinen
Toast!"

15

Top Ten II
6. Name Exceptions by Source (ex. a:=
LoadFile(path) or a:= LoadString(path)
7. Free resources after an exception (Shotgun
Surgery (try... except.. finally) pattern
missed, see wish at the end))
8. Don’t use Exceptions for Decisions (Array
bounds checker) -> Ex.
9. Never mix Exceptions in try… with too many
delegating methods) twoexcept
10. Remote (check remote exceptions, cport)

16

Testability
• Exception handling on designtime
{$IFDEF DEBUG}
Application.OnException:= AppOnException;
{$ENDIF}
• Assert function
accObj:= TAccount.createAccount(FCustNo,
std_account);
assert(aTrans.checkOBJ(accObj), 'bad OBJ cond.');
• Logger
LogEvent('OnDataChange', Sender as TComponent);
LogEvent('BeforeOpen', DataSet);

17

Check Exceptions
• Check Complexity
function IsInteger(TestThis: String): Boolean;
begin
try
StrToInt(TestThis);
except
on E: ConvertError do
result:= False;
else
result:= True;
end;
end;

18

Module Tests as Checksum

19

Test on a SEQ Diagram

20

Compiler & runtime checks
Controls what run-time checking code is generated. If such a check fails, a
run-time error is generated. ex. Stack overflow

• Range checking
Checks the results of enumeration and subset type
operations like array or string lists within bounds
• I/O checking
Checks the result of I/O operations
• Integer overflow checking
Checks the result of integer operations (no buffer
overrun)
• Missing: Object method call checking
Check the validity of the method pointer prior to
calling it (more later on).

21

Range Checking
{$R+}
SetLength(Arr,2);
Arr[1]:= 123;
Arr[2]:= 234;
Arr[3]:= 345;
{$R-}
Delphi (sysutils.pas) throws the ERangeError exception
ex. snippet


22

I/O Errors
The $I compiler directive covers two purposes! Firstly to include a file of
code into a unit. Secondly, to control if exceptions are thrown when an API
I/O error occurs.
{$I+} default generates the EInOutError exception when an IO error occurs.
{$I-} does not generate an exception. Instead, it is the responsibility of the
program to check the IO operation by using the IOResult routine.
{$i-}
reset(f,4);
blockread(f,dims,1);
{$i+}
if ioresult<>0 then begin

UEB: 9_pas_umlrunner.txt

23

Overflow Checks
When overflow checking is turned on (the $Q compiler
directive), the compiler inserts code to check that CPU
flag and raise an exception if it is set. ex.
The CPU doesn't actually know whether it's added signed
or unsigned values. It always sets the same overflow flag,
no matter of types A and B. The difference is in what
code the compiler generates to check that flag.
In Delphi ASM-Code a call@IntOver is placed.

UEB: 12_pas_umlrunner_solution.txt

24

Silent Exception, but
EStackOverflow = class(EExternal) ex. webRobot
end deprecated;

{$Q+} // Raise an exception to log
try
b1:= 255;
inc(b1);
showmessage(inttostr(b1));
//show silent exception with or without
except
on E: Exception do begin
//ShowHelpException2(E);
LogOnException(NIL, E);
end;
end;

UEB: 33_pas_cipher_file_1.txt

25

Exception Process Steps
The act of serialize the process:
Use Assert {$C+} as a debugging check to test
that conditions implicit assumed to be true are
never violated (pre- and postconditions). ex.
Create your own exception from Exception class
Building the code with try except finally
Running all unit tests with exceptions!
Deploying to a target machine with global log
Performing a “smoke test” (compile/memleak)

26

Let‘s practice
• function IsDate(source: TEdit): Boolean;
• begin
• try
• StrToDate(TEdit(source).Text);
• except
• on EConvertError do
• result:= false;
• else
• result:= true;
• end;
• end;
Is this runtime error or checked exception handling ?

27

Structure Wish
The structure to handle exceptions could be improved. Even in XE3, it's
either try/except or try/finally, but some cases need a more fine-tuned
structure:

try
IdTCPClient1.Connect;
ShowMessage('ok');
IdTCPClient1.Disconnect;
except
ShowMessage('failed connecting');
finally //things to run whatever the outcome
ShowMessage('this message displayed every time');
end;

28

Exceptional Links:
• Exceptional Tools:
http://www.madexcept.com/
• http://www.modelmakertools.com/
• Report Pascal Analyzer:
http://www.softwareschule.ch/download/pascal_analyzer.pdf
• Compiler Options:
• http://www.softwareschule.ch/download/Compileroptions_EKON13.pdf
• Example of code with maXbox
http://www.chami.com/tips/delphi/011497D.html
• Model View in Together:
www.softwareschule.ch/download/delphi2007_modelview.pdf

maXbox

29

Q&A
max@kleiner.com
www.softwareschule.ch

30

A exception ekon16

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A exception ekon16