如何在C中的初始化程序中使用static_assert？

Question

信不信由你，我想在擴展為指定初始化程序的宏中使用static_assert ：

#define INIT(N) \
        /* static_assert((N) < 42, "too large"), */ \
        [(N)] = (N)

int array[99] = { INIT(1), INIT(2), INIT(42) };

我希望INIT(42)出現錯誤，但取消注釋static_assert是語法錯誤。 AFAIK static_assert在語法上是一個聲明。 在此示例中如何使用它？

Answer 1

#define INIT(N) \
    [(N)] = (sizeof((struct {_Static_assert((N) < 42, "too large");char c[N];}){{0}}.c))

...我不確定自己如何最終被憎惡。 但是，它可以工作（對於N > 0 ）！

// A struct declaration is a valid place to put a static_assert
        struct {_Static_assert((N) < 42, "too large");          }
// Then we can place that declaration in a compound literal...
       (struct {_Static_assert((N) < 42, "too large");          }){   }
// But we can't just throw it away with `,`: that would yield a non-constant expression.
// So let's add an array of size N to the struct...
       (struct {_Static_assert((N) < 42, "too large");char c[N];}){{0}}
// And pry N out again through sizeof!
sizeof((struct {_Static_assert((N) < 42, "too large");char c[N];}){{0}}.c)

0友好版本（只需將其相加然后減去1以使數組具有正大小）：

#define INIT(N) \
    [(N)] = (sizeof((struct { \
        _Static_assert((N) < 42, "too large"); \
        char c[(N) + 1]; \
    }){{0}}.c) - 1)