第十三號艦隊
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Redefinition type in the same scope for C23

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C23有一項特性

Structure, union and enumeration types may be defined more than once in the same scope with the same contents and the same tag; if such types are defined with the same contents and the same tag in different scopes, the types are compatible.

以下的程式碼在C23是合法的

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struct A {
    int a;
};

struct A {
    int a;
};

而在C23之前會被歸類成 redefinition of 'struct A'
有了這個，在C語言寫類Generic 會比較方便
例如

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#include <stdio.h>
#define Result_t(T, E) struct Result_##T##_##E { bool is_ok; union { T value; E error; }; }

#define Ok(T, E) (struct Result_##T##_##E){ .is_ok = true, .value = (T) _OK_IMPL
#define _OK_IMPL(...) __VA_ARGS__ }

#define Err(T, E) (struct Result_##T##_##E){ .is_ok = false, .error = (E) _ERR_IMPL
#define _ERR_IMPL(...) __VA_ARGS__ }

typedef const char *ErrorMessage_t;

Result_t(int, ErrorMessage_t) my_func(int i)
{
    if (i == 42) return Ok(int, ErrorMessage_t)(100);
    else return Err(int, ErrorMessage_t)("Cannot do the thing");
}

int main()
{
    Result_t(int, ErrorMessage_t) x = my_func(42);

    if (x.is_ok) {
        printf("%d\n", x.value);
    } else {
        printf("%s\n", x.error);
    }
}

目前只有GCC 14.1支持

不過事情總不可能永遠事事順利，總是有會碰到支援舊的Compiler這種事
這邊看到不錯的解決方法

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#if __STDC_VERSION__ >= 202311L
    #define Neat_SString(cap) \
    struct SString_##cap \
    { \
        unsigned int len; \
        unsigned char chars[ cap + 1 ]; /* + 1 for the nul */ \
    }

    #define NEAT_DECL_SSTRING(cap)
#else
    #define Neat_SString(cap) \
    struct SString_##cap

    #define NEAT_DECL_SSTRING(cap) \
    struct SString_##cap \
    { \
        unsigned int len; \
        unsigned char chars[ cap + 1 ]; /* + 1 for the nul */ \
    }
#endif

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