Access Violation

[Keeganscurlyperm]

From: Reasonably new user of Delphi

Under what circumstances would I get an 'access violation' message during the running of my Delphi program?

 

[LucieLastic]

Attempting to access something that has not been created. Or trying to free/access something that has already been freed.

lou

 

[Ronin441]

For example:

var mystrings: TStringList;
begin
  mystrings.Add('bad');  // this will cause an AV

  mystrings := TStringList.Create;
  mystrings.Add('good');  // this is OK

  ShowMessage(mystrings[5]);  // this will cause a list index out of bounds

  mystrings.Free;

  mystrings.Add('also bad);  //  this will also cause an AV
end;

-- Doug Burbidge mailto:doug@ultrazone.com

http://home.iprimus.com.au/dougburbidge/

 

[BobbaFet]

Also, trying to close a form that still has enabled Timers
will cause an EAccessViolation ... BobbaFet

Everyone has a right to my opinion.
E-mail me at caswegkamp@hotmail.com

http://cwcon.50megs.com/

 

[sggaunt]

I can see that if you read this from the help:--
"
EAccessViolation is raised when an application

• Dereferences a nil pointer.
• Writes to a code page.
• Attempts to access a memory address for which there is no virtual memory allocated to the application.
"
you might be confused.
There are probobly lots of other ways to get an AV as well as the previous answers, Look at error trapping with Try/Except Try/Finally constructs though in my experiance these won't always work with AV's and the debugger may miss the actual cause/line of the error.
Weez gave the best generalisation I think.
What is happening is that the Delphi compiler is trying to stop you sticking things where you shouldnt in the memory. Unlike a lot of 'C' compilers which will let you write all over it.
You will learn all the ways this can happen (and how to avoid it) only with experiance.

Steve