匿名结构和联合在 C11 中何时有用? [英] When are anonymous structs and unions useful in C11?

问题描述

C11 添加了匿名结构和联合"等内容.

C11 adds, among other things, 'Anonymous Structs and Unions'.

我四处寻找，但找不到关于匿名结构和联合何时有用的明确解释.我问是因为我不完全理解它们是什么.后来我知道它们是没有名称的结构或联合，但我一直(不得不?)将其视为错误，因此我只能设想使用命名结构.

I poked around but could not find a clear explanation of when anonymous structs and unions would be useful. I ask because I don't completely understand what they are. I get that they are structs or unions without the name afterwards, but I have always (had to?) treat that as an error so I can only conceive a use for named structs.

推荐答案

结构内部的匿名联合在实践中非常有用.考虑到您想要实现一个可区分的和类型(或 tagged union)，一个带有布尔值的聚合和一个浮点数或一个 char* (即一个字符串)，取决于布尔标志.使用 C11 你应该能够编码

Anonymous union inside structures are very useful in practice. Consider that you want to implement a discriminated sum type (or tagged union), an aggregate with a boolean and either a float or a char* (i.e. a string), depending upon the boolean flag. With C11 you should be able to code

typedef struct {
    bool is_float;
    union {
       float f;
       char* s;
    };
} mychoice_t;

double as_float(mychoice_t* ch) 
{ 
   if (ch->is_float) return ch->f;
   else return atof(ch->s);
}

使用 C99，您必须为联合命名，并且代码 ch->uf 和 ch->us 可读性更差且更冗长.

With C99, you'll have to name the union, and code ch->u.f and ch->u.s which is less readable and more verbose.

实现某些标记联合类型的另一种方法是使用强制转换.Ocaml 运行时 提供了大量示例.

Another way to implement some tagged union type is to use casts. The Ocaml runtime gives a lot of examples.

Common Lisp 的 SBCL 实现确实使用了一些 union 来实现 标记联合 类型.GNU make 也使用它们.

The SBCL implementation of Common Lisp does use some union to implement tagged union types. And GNU make also uses them.

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