在通用选择中处理 size_t

Question

是否可以定义一个支持所有标准（无符号）整数类型的泛型宏，以及size_t？

foo((size_t)42)只要size_t引用标准的无符号整数类型之一，就可以使用下面的示例。但是标准是否保证在任何情况下都是这种情况，或者也size_t可以指代扩展整数类型？

void foo_bool(_Bool x);
void foo_uchar(unsigned char x);
void foo_ushort(unsigned short x);
void foo_uint(unsigned x);
void foo_ulong(unsigned long x);
void foo_ullong(unsigned long long x);

#define foo(x) _Generic((x), \
    _Bool: foo_bool, \
    unsigned char: foo_uchar, \
    unsigned short: foo_ushort, \
    unsigned: foo_uint, \
    unsigned long: foo_ulong, \
    unsigned long long: foo_ullong)(x)

Answer 1

是否可以定义支持所有标准（无符号）整数类型的类型通用宏，并且还可以size_t？

是的 - 嵌套。

size_t 通常匹配标准整数类型，如unsigned, unsigned long, unsigned long long，但这不是C 所要求的。技巧在于如何使用_Generic何时size_t与标准类型相同以及何时不是标准类型。类似的问题也适用于uintmax_t.

void foo_size_t(size_t x) { (void)x; puts("size_t"); }
void foo_bool(_Bool x) { (void)x; puts("bool"); }
void foo_uchar(unsigned char x){ (void)x; puts("unsigned char"); }
void foo_ushort(unsigned short x){  (void)x; puts("unsigned short"); }
void foo_uint(unsigned x) { (void)x; puts("unsigned"); }
void foo_ulong(unsigned long x){ (void)x; puts("unsigned long"); }
void foo_ullong(unsigned long long x){ (void)x; puts("unsigned long long"); }
void foo_none(){ puts("TBD"); }

#define foo(x) _Generic((x), \
  _Bool: foo_bool, \
  unsigned char: foo_uchar, \
  unsigned short: foo_ushort, \
  unsigned: foo_uint, \
  unsigned long: foo_ulong, \
  unsigned long long: foo_ullong, \
  default: foo_none)(x)

// Catch `size_t` as `size_t`    
// If `size_t` is distinctive, apply the size_t function
// If `size_t` is not distinctive, others will be caught here too
#define bar(x) _Generic((x), \
  size_t: foo_size_t(x), \
  default: foo(x) \
  )

int main(void) {
  foo((float)1);  // What happens when type matching nothing
  foo(1lu);
  foo((size_t)1);
  bar((float)1);  // What happens when type matching nothing
  bar((size_t)1);
  bar(1lu);
}

输出

TBD
unsigned long
unsigned long  // Overlap when size_t matches a type, else TBD
TBD
size_t // expect this as unsigned long if size_t was distinctive.
size_t  

请注意，创造性地使用_Generic可能会导致问题，因为对 C 的扩展仍处于萌芽阶段。