August 2021
Intermediate to advanced
450 pages
5h 35m
Chinese
Content preview from C++语言导学（原书第2版）
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13.5.2 optional
一个optional<A>可看作一种特殊的variant（类似于一个variant<A，nothing>），或看作一个指针A*要么指向某个对象要么为nullptr的思想的一种推广。
对于可能返回一个对象也可能什么都不返回的函数，optional是很有用的：
有了这个版本，我们就可以这样编写如下代码：
这种风格对一些不喜欢异常（参见3.5.3节）的人很有吸引力。注意对*的奇怪使用——optional被当作指向对象的指针而非对象本身。
空optional对象{}等价于nullptr。例如：
如果我们试图访问一个未保存值的optional，结果是未定义的——不会有异常抛出。因此，optional不保证类型安全。
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