Try-except: try [statements] except [[[on E:ExceptionType do statement]] [else statement] | [statements] end;
Try-finally: try [statements] finally [statements] end;
Exception-safe return of a new object
When a function returns an object (as opposed to using one that's passed in by the caller), be careful an exception doesn't cause the object to leak.
function MakeStrings: TStrings; begin // Create a new object before entering the try-block. Result := TStringList.Create; try // Execute code that uses the new object and prepares it for the caller. Result.Add('One'); MightThrow; except // If execution reaches this point, then an exception has occurred. We cannot // know how to handle all possible exceptions, so we merely clean up the resources // allocated by this function and then re-raise the exception so the caller can // choose what to do with it. Result.Free; raise; end; // If execution reaches this point, then no exception has occurred, so the // function will return Result normally. end;
Naive programmers might attempt to catch all exception types and return
nil from such a function, but that's just a special case of the general discouraged practice of catching all exception types without handling them.
Simple try..finally example to avoid memory leaks
finally to avoid leaking resources (such as memory) in case an exception occurs during execution.
The procedure below saves a string in a file and prevents the
TStringList from leaking.
procedure SaveStringToFile(const aFilename: TFilename; const aString: string); var SL: TStringList; begin SL := TStringList.Create; // call outside the try try SL.Text := aString; SL.SaveToFile(aFilename); finally SL.Free // will be called no matter what happens above end; end;
Regardless of whether an exception occurs while saving the file,
SL will be freed. Any exception will go to the caller.
Try-except nested inside try-finally
except block may be nested inside a
AcquireResource; try UseResource1; try UseResource2; except on E: EResourceUsageError do begin HandleResourceErrors; end; end; UseResource3; finally ReleaseResource; end;
EResourceUsageError occurs in
UseResource2, then execution will jump to the exception handler and call
HandleResourceError. The exception will be considered handled, so execution will continue to
UseResource3, and then
If an exception of any other type occurs in
UseResource2, then the exception handler show here will not apply, so execution will jump over the
UseResource3 call and go directly to the
finally block, where
ReleaseResource will be called. After that, execution will jump to the next applicable exception handler as the exception bubbles up the call stack.
If an exception occurs in any other call in the above example, then
HandleResourceErrors will not be called. This is because none of the other calls occur inside the
try block corresponding to that exception handler.
Try-finally nested inside try-except
finally block may be nested inside a
try AcquireResources; try UseResource; finally ReleaseResource; end; except on E: EResourceUsageError do begin HandleResourceErrors; end; end;
If an exception occurs inside
UseResource, then execution will jump to
ReleaseResource. If the exception is an
EResourceUsageError, then execution will jump to the exception handler and call
HandleResourceErrors. Exceptions of any other type will skip the exception handler above and bubble up to the next
except block up the call stack.
ReleaseResource will cause execution to go to the exception handler, skipping the
finally block, either because the corresponding
try block has not been entered yet or because the
finally block has already been entered.
Try-finally with 2 or more objects
Object1 := nil; Object2 := nil; try Object1 := TMyObject.Create; Object2 := TMyObject.Create; finally Object1.Free; Object2.Free; end;
If you do not initialize the objects with
nil outside the
try-finally block, if one of them fails to be created an AV will occur on the finally block, because the object won't be nil (as it wasn't initialized) and will cause an exception.
Free method checks if the object is nil, so initializing both objects with
nil avoids errors when freeing them if they weren't created.