命名参数和可选参数是两个比较简单的特性，对于熟悉其他编程语言的开发者来说可选参数并不陌生，为参数提供默认值时就是可选参数：
注意：加密文件之后如果忘记了用来加密该文件的密码，再想恢复出原始文件就不可能了。当然，这与密码本身的复杂程度有关，要想保证文件的安全，最好采用较复杂的密码，例如混合运用字母、数字和特殊字符（如“$”符号等）。
然而必须说明的是，没有一种防范泄密的安全技术是绝对坚固的，因为密码有可能被未经授权的人获得。
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协变和逆变，是为解决问题而生的。而要理清解决什么样的问题，需要首先从理清几个简单的概念开始。首先我们进行一点操作：
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你的隐私选择 主题 亮 暗 高对比度
你的隐私选择 主题 亮 暗 高对比度
你的隐私选择 主题 亮 暗 高对比度
更详细的对比示例留待读者在实践中对此进行讨论，并行计算为托管代码在多核环境下的性能优化提供了统一的解决方案，而未来我们会做的更好。
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而这种原始转换（由子类转换为父类）方向相同的可变性，被称为协变（covariant）；其反向操作则被称为逆变（contravariant）。当同样的情形应用于泛型时，例如：
During the Privateness section, apparent the Look at packing containers and choose Implement. Now Swift entry will only present your pinned folders. (If you turn them back on, data files you Earlier removed from Fast access might show up again.)