英文:
Generate nested constant arrays in C at compile time
问题
我试图在C（C99）中在编译时生成一个大的常量查找表，查找表中的每个条目都有一个指向另一个常量数组的指针。我想一次性构建所有这些数组，但这是不可能的，因为花括号初始化器不能用于指针。是否有一些预处理宏或其他技巧，我可以使用来定义“匿名”数组或类似的东西，以便我可以将它们包含在嵌套数据结构中？
我已经创建了下面的示例，展示了我当前正在做的事情，以及我试图做的事情。
typedef enum {
RED = 0,
GREEN,
BLUE,
PURPLE,
YELLOW
} colours_t;
typedef struct {
const char *name;
const size_t number_of_preferences;
const colours_t *ordered_preferences;
} colour_preferences_t;
// 这种方式可以工作，但很繁琐
const colours_t bob_preferences[] = {RED, GREEN};
const colours_t alice_preferences[] = {BLUE, PURPLE, YELLOW};
const colours_t eve_preferences[] = {YELLOW, RED};
const colour_preferences_t array_of_preferences[3] = {
{"Bob", 2, bob_preferences},
{"Alice", 3, alice_preferences},
{"Eve", 2, eve_preferences}
};
// 这是我想要的方式（但它无效）
const colour_preferences_t array_of_preferences_invalid[3] = {
{"Bob", 2, {RED, GREEN}},
{"Alice", 3, {BLUE, PURPLE, YELLOW}},
{"Eve", 1, {YELLOW, RED}},
};
更新：多亏了下面的答案，我已经找到了解决方案。并且使用可变参数宏，甚至可以省去显式输入大小：
#define PREFERENCE_LIST(...) sizeof((colours_t[]) { __VA_ARGS__ })/sizeof(colours_t), (colours_t[]){ __VA_ARGS__ }
const colour_preferences_t array_of_preferences_short[3] = {
{"Bob", PREFERENCE_LIST(RED, GREEN)},
{"Alice", PREFERENCE_LIST(BLUE, PURPLE, YELLOW)},
{"Eve", PREFERENCE_LIST(YELLOW, RED)}
};
英文:
I'm trying to generate a large constant lookup table at compile time in C (C99), and each entry to the lookup table has a pointer to another const array. I'd like to be able to build all of these arrays all at once, but this isn't possible because a brace initialiser can't be used for a pointer. Is there some preprocessor macro or other trick I can use to define "anonymous" arrays or something similar so that I can include them in a nested data structure?
I've created an example below, showing what I am currently doing, and also what I am trying to do.
typedef enum {
RED = 0,
GREEN,
BLUE,
PURPLE,
YELLOW
} colours_t;
typedef struct {
const char *name;
const size_t number_of_preferences;
const colours_t *ordered_preferences;
} colour_preferences_t;
// this way works, but is tedious
const colours_t bob_preferences[] = {RED, GREEN};
const colours_t alice_preferences[] = {BLUE, PURPLE, YELLOW};
const colours_t eve_preferences[] = {YELLOW, RED};
const colour_preferences_t array_of_preferences[3] = {
{"Bob", 2, bob_preferences},
{"Alice", 3, alice_preferences},
{"Eve", 2, eve_preferences}
};
// this is how I'd like to do it (but it isn't valid)
const colour_preferences_t array_of_preferences_invalid[3] = {
{"Bob", 2, {RED, GREEN}},
{"Alice", 3, {BLUE, PURPLE, YELLOW}},
{"Eve", 1, {YELLOW, RED}},
};
Update: Thanks to the answers below, I have worked out a solution. And using a varadic macro, I can even drop typing out the size explicitly:
#define PREFERENCE_LIST(...) sizeof((colours_t[]) { __VA_ARGS__ })/sizeof(colours_t), (colours_t[]){ __VA_ARGS__ }
const colour_preferences_t array_of_preferences_short[3] = {
{"Bob", PREFERENCE_LIST(RED, GREEN)},
{"Alice", PREFERENCE_LIST(BLUE, PURPLE, YELLOW)},
{"Eve", PREFERENCE_LIST(YELLOW, RED)}
};
答案1
得分: 3
你需要创建对象，并且该对象的指针必须成为该字段的初始化器。您可以通过使用复合字面值来实现它。
const colour_preferences_t array_of_preferences_invalid[3] = {
{"Bob", 2, (const colours_t[]){RED, GREEN}},
{"Alice", 3, (const colours_t[]){BLUE, PURPLE, YELLOW}},
{"Eve", 1, (const colours_t[]){YELLOW, RED}},
};
英文:
you need to create the object, and pointer to this object has to be the initializer of this field. You can archive it by using the compound literals
const colour_preferences_t array_of_preferences_invalid[3] = {
{"Bob", 2, ( const colours_t[]){RED, GREEN}},
{"Alice", 3, (const colours_t[]){BLUE, PURPLE, YELLOW}},
{"Eve", 1, (const colours_t[]){YELLOW, RED}},
};
</details>
# 答案2
**得分**: 3
使用复合字面量可以像这样工作：
```c
const colour_preferences_t array_of_preferences_invalid[3] = {
{"Bob", 2, (colours_t[]){RED, GREEN}},
{"Alice", 3, (colours_t[]){BLUE, PURPLE, YELLOW}},
{"Eve", 1, (colours_t[]){YELLOW, RED}},
};
你可以在这里阅读有关复合字面量的详细信息。
英文:
It should work using a compound literal like this:
const colour_preferences_t array_of_preferences_invalid[3] = {
{"Bob", 2, (colours_t[]){RED, GREEN}},
{"Alice", 3, (colours_t[]){BLUE, PURPLE, YELLOW}},
{"Eve", 1, (colours_t[]){YELLOW, RED}},
};
You can read the details about compound literals here
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