Original
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.
See also
Raising and handling exceptions
译文
嵌套异常
执行在异常处理程序中的代码可以自身引发和处理异常。当这些异常也在异常处理程序中被处理时，不影响最初的异常。不过，一旦在异常处理程序中引发的异常传播到处理程序之外，最初的异常就丢失了。下面通过Tan函数举例说明。
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;
如果在函数Tan的执行中出现了EMathError异常，那么异常处理程序会引发ETrigError异常。因为Tan函数未提供对ETrigError异常的处理程序，所以异常将传播到最初的异常处理程序之外，最终导致EMathError异常被销毁。对于调用者来说，就象Tan函数引发了一个ETrigError异常。