标准库不要求检查索引值，所用索引的下标越界是没有定义的，这样往往会导致严重的运行时错误。
The library is not required to check the value of the index. Using an index that is out of range is undefined and usually results in a serious run-time error.
标准库不要求检查索引值，所用索引的下标越界是没有定义的，这样往往会导致严重的运行时错误。
The library is not required to check the value of the index. Using an index that is out of range is undefined and usually results in a serious run-time error.
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