August 2021
Intermediate to advanced
450 pages
6h 57m
Chinese
Content preview from C++程序设计语言（第4部分：标准库）（原书第4版）
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39.3 类facet
一个locale就是一个facet集合。一个facet表示一个特定的文化侧面，例如数值输出时如何表示（num_put），如何从输入读取日期（time_get）以及在文件中如何保存字符（codecvt）。39.4节列出了标准库facet。
用户可以定义新的facet，例如确定季节名如何打印的facet（见39.3.2节）。
facet在程序中表示为std：：locale：：facet派生类的对象。类似其他locale特性，facet也定义在<locale>中：
类facet设计为无公有函数的基类。其构造函数是protected的，以防止创建“普通facet”对象，其析构函数是virtual的，以确保派生类对象正确销毁。
facet一般通过保存在locale中的指针来管理。传递给facet构造函数一个参数0表示locale应在facet的最后一个引用释放后删除它。反之，非零参数确保locale永远也不会删除facet。在极少数情况下，facet的生命期由程序员直接控制而不是通过locale间接控制，此时非零参数就很有意义了。
每种facet接口都必须有独立的id：
用户使用id为每个提供新facet接口的类定义一个id类型的static成员（例如，见39.4.1节）。locale机制用id标识facet（见39.2节和39.3.1节）。在显而易见的locale实现中，id用作facet指针向量的索引，从而实现高效的map<id，facet*>。 ...
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