如何在C中的初始化程序中使用static_assert？

Question

信不信由你,我想static_assert在宏扩展到指定的初始化器:

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

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

我想要一个错误INIT(42),但取消注释static_assert是一个语法错误.AFAIK static_assert在语法上是一种声明.我如何在这个例子中使用它？

Answer 1

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

......我不确定自己是如何结束这种憎恶的.但是嘿,它适用于(for 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友好的版本(只需添加然后减去1所以数组的正大小):

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