Question

While browsing System.Zip (Delphi XE2) to see how it works, I found this function:

procedure VerifyWrite(Stream: TStream; var Buffer; Count: Integer);
begin
  if Stream.Write(Buffer, Count) <> Count then
    raise EZipException.CreateRes(@SZipErrorWrite) at ReturnAddress;
end;

It’s the at ReturnAddress part that sort of puzzles me.

I didn’t know that at was a valid keyword (the syntax highlighter doesn’t seem to recognise it either).

According to the IDE it’s declared as System.ReturnAddress, but I can only find it declared as a label somewhere in the (asm) code of procedure _HandleAnyException;. The system unit is full of references to it though.

So what I would like to know is this:

1. What is ReturnAddress?
2. What exactly does Raise Exception.Create ... at ReturnAddress do?

Bonuspoints if you can give a real-world example of where this would be a useful construct, or if you can advice against using it.

Answer 1

ReturnAddress is the address to which VerifyWrite would have returned when finished.

Raise Exception.Create... at ReturnAddress means that when the exception dialog is displayed, it would indicate the address of the exception as being at ReturnAddress. In other words, the exception message would read Exception <whatever> raised at <ReturnAddress>: <Exception Message>.

Here is an excerpt from the help file for Delphi 7. It’s nearly the same as the online version.

To raise an exception object, use an instance of the exception
class with a raise statement. For example,

raise EMathError.Create;

In general, the form of a raise statement is

raise object at address

where object and at address are both optional; see
Re-raising exceptions. When an address is specified,
it can be any expression that evaluates to a pointer
type, but is usually a pointer to a procedure or function.
For example:

raise Exception.Create('Missing parameter') at @MyFunction;

Use this option to raise the exception from an earlier point
in the stack than the one where the error actually occurred.

Note the last sentence in particular. It’s pretty specific about the use of at <address>.