I am writing some code to test for a bit in a bit-field and I wrote this:

return (unsigned)(m_category | category) > 0

I though this wouldn't compile, since I didn't give the integer type, but to my surprise it did. So I'm wondering, what does this do? Is it undefined?

CodePudding user response：

unsigned is the same as unsigned int.

The cast will convert the signed integer to unsigned as follows:

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.