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Try...finally statements

Sometimes you want to ensure that specific parts of an operation are completed, whether or not the operation is interrupted by an exception. For example, when a routine acquires control of a resource, it is often important that the resource be released, regardless of whether the routine terminates normally. In these situations, you can use a try...finally statement.

The following example shows how code that opens and processes a file can ensure that the file is ultimately closed, even if an error occurs during execution.

Reset(F);

try

... // process file F

finally

CloseFile(F);

end;

The syntax of a try...finally statement is

try statementList1 finally statementList2 end

where each statementList is a sequence of statements delimited by semicolons. The try...finally statement executes the statements in statementList1 (the try clause). If statementList1 finishes without raising exceptions, statementList2 (the finally clause) is executed. If an exception is raised during execution of statementList1, control is transferred to statementList2; once statementList2 finishes executing, the exception is re-raised. If a call to the Exit, Break, or Continue procedure causes control to leave statementList1, statementList2 is automatically executed. Thus the finally clause is always executed, regardless of how the try clause terminates.

If an exception is raised but not handled in the finally clause, that exception is propagated out of the try...finally statement, and any exception already raised in the try clause is lost. The finally clause should therefore handle all locally raised exceptions, so as not to disturb propagation of other exceptions.

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Try...finally语句

有时想要确保操作中指定的部分被完成，不管该操作是否被异常中断。例如，当一个例程获得了对某个资源的控制时，释放该资源通常是重要的，而不管例程是否正常终止。对于这些情况，可以使用try...finally语句。

下面的例子说明了打开并处理文件的代码确保文件最终被关闭，即使在执行中可能有错误发生。

Reset(F);

try

... //处理文件F

finally

CloseFile(F);

end;

try...finally语句的语法是

try statementList1 finally statementList2 end

这里的每个语句列表statementList都是分号隔开的语句序列。try...finally语句首先试图执行statementList1中的语句（try子句）。如果statementList1完成并且没有引发任何异常，那么statementList2（finally子句）被执行。如果在statementList1执行中引发了某个异常，那么控制即可被传递到statementList2；一旦statementList2完成执行，异常就被再引发。如果有对Exit、Break或Continue等标准过程的调用导致控制离开statementList1，那么statementList2被自动执行。由此可见，无论try子句以哪一种方式终止（正常结束或由于引发异常而中断），finally子句都总是被执行。

如果异常被引发但在finally子句中未处理，那么该异常将向try...finally语句之外传播，而且在try子句中已引发的任何异常都将丢失。因此，finally子句应处理局部引发的所有异常，从而不干扰对其他异常的传播。

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Try...finally statements

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Try...finally语句

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