Delphi in a Nutshell

Name

At Directive

Syntax

raise Exception at Address;

Description

Use the at directive to raise an exception with a specific address as the origin of the exception. The Address can be any integer expression.

Tips and Tricks

The at directive is not used in most applications, but it can be helpful when writing certain libraries or generic error-handling packages.

Example

The following example is a subroutine that raises an exception that uses the caller’s address as the exception address. Thus, you can call the example procedure anywhere in your application, and when the debugger stops the application, the position is not where the exception is truly raised, but where the RaiseExceptionInCaller procedure is called, which is much more useful and informative.

procedure RaiseExceptionInCaller;
  // Get the return address from the stack and save it in EAX,
  // which is the function's result. The value at [ESP] is the
  // address of RaiseExceptionInCaller; go back one more frame
  // to [ESP+4] to get the caller of RaiseExceptionInCaller.
  // If you need to add local variables to RaiseExceptionInCaller,
  // you might want to get an offset from EBP instead.
  function CallerAddress: Pointer;
  asm
    mov eax, [esp+4]
  end;
begin
  raise Exception.Create('Example') at CallerAddress;
end;

procedure Demo;
begin
  RaiseExceptionInCaller; // Debugger shows exception here.
end;

See Also

Raise Keyword