c++ - 意外的宏观评估

Question

为什么打印 ffffffff？

#define MYMACRO(n) (((uint16_t)0 - 1) >> (16 - (n)))
std::cout << std::hex << MYMACRO(1);

当打印 1 时？

#define MYMACRO(n) (((uint32_t)0 - 1) >> (32 - (n)))
std::cout << std::hex << MYMACRO(1);

我用 GCC 和 cpp.sh 试过了。

Answer 1

解释在 type-promotion-in-c 中.

(§6.3.1.1 Boolean, characters, and integers):

If an int can represent all values of the original type, the value is converted to an int; ... These are called the integer promotions. All other types are unchanged by the integer promotions.

因此对于您的第一种情况，((uint16_t)0 - 1) 被转换为 int 以满足减法的需要。所以右移操作是算术右移而不是逻辑右移。