Nested Exceptions

Code executed in an exception handler can itself raise and handle exceptions. As long as these exceptions are also handled within the exception handler, they do not affect the original exception. However, once an exception raised in an exception handler propagates beyond that handler, the original exception is lost. This is illustrated by the Tan function below.

type

ETrigError = class(EMathError); function Tan(X: Extended): Extended; begin try

Result := Sin(X) / Cos(X); except on EMathError do raise ETrigError.Create('Invalid argument to Tan'); end; end;

If an EMathError exception occurs during execution of Tan, the exception handler raises an ETrigError. Since Tan does not provide a handler for ETrigError, the exception propagates beyond the original exception handler, causing the EMathError exception to be destroyed. To the caller, it appears as if the Tan function has raised an ETrigError exception.

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