“（无符号）值”是否强制转换为未定义行为？

Question

我正在编写一些代码来测试位字段中的位，我写了这样的：

return (unsigned)(m_category | category) > 0

我虽然这不会编译，因为我没有给出整数类型，但令我惊讶的是它确实编译了。所以我想知道，这是做什么的？是未定义的吗？

Answer 1

unsigned是相同的unsigned int。

转换会将有符号整数转换为无符号整数，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.