Search code examples
c++casting

Is "(unsigned)value" cast undefined behaviour?

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?

Solution

  • 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.