因此，例如，您可以对一些具有明显结果(在一定范围内)的计算放置一个断言，以快速查找是否有错误。但是你不会想要这样做，因为超越界的结果可能是有效的，并不重要，所以不应该崩溃你的应用程序(假设你只是使用它来显示进度条的进度)。

另一方面，在获取元素时检查数组上的下标是否有效是前提条件。当要求无效下标时，数组对象没有合理的下一个操作，因为它必须返回非可选值。

文档的完整文本(尝试选项 – 点击Xcode中的assert和precondition)：

前提

Check a necessary condition for making forward progress.

Use this function to detect conditions that must prevent the@H_301_19@ program from proceeding even in shipping code.

• In playgrounds and -Onone builds (the default for Xcode’s Debug@H_301_19@ configuration): if condition evaluates to false,stop program@H_301_19@ execution in a debuggable state after printing message.

• In -O builds (the default for Xcode’s Release configuration):@H_301_19@ if condition evaluates to false,stop program execution.

• In -Ounchecked builds,condition is not evaluated,but the@H_301_19@ optimizer may assume that it would evaluate to true. Failure@H_301_19@ to satisfy that assumption in -Ounchecked builds is a serIoUs@H_301_19@ programming error.

断言

Traditional C-style assert with an optional message.

Use this function for internal sanity checks that are active@H_301_19@ during testing but do not impact performance of shipping code.@H_301_19@ To check for invalid usage in Release builds; see precondition.

• In playgrounds and -Onone builds (the default for Xcode’s Debug@H_301_19@ configuration): if condition evaluates to false,stop program@H_301_19@ execution in a debuggable state after printing message.

• In -O builds (the default for Xcode’s Release configuration),@H_301_19@ condition is not evaluated,and there are no effects.

• In -Ounchecked builds,but the@H_301_19@ optimizer may assume that it would evaluate to true. Failure to satisfy that assumption in -Ounchecked builds is a serIoUs programming error.