“（無符號）值”是否投射未定義的行為？

Question

我正在編寫一些代碼來測試位域中的位，我寫了這個：

return (unsigned)(m_category | category) > 0

我雖然這不會編譯，因為我沒有給出整數類型，但令我驚訝的是它做到了。 所以我想知道，這是做什么的？ 它是未定義的嗎？

Answer 1

unsigned與unsigned int相同。

強制轉換會將有符號整數轉換為無符號整數，如下所示：

https://en.cppreference.com/w/cpp/language/implicit_conversion#Integral_conversions

 If the destination type is unsigned, the resulting value is the smallest unsigned value equal to the source value modulo 2n where n is the number of bits used to represent the destination type. That is, depending on whether the destination type is wider or narrower, signed integers are sign-extended[footnote 1] or truncated and unsigned integers are zero-extended or truncated respectively.