[英]Is "(unsigned)value" cast undefined behaviour?
I am writing some code to test for a bit in a bit-field and I wrote this:我正在编写一些代码来测试位域中的位,我写了这个:
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.我虽然这不会编译,因为我没有给出整数类型,但令我惊讶的是它做到了。 So I'm wondering, what does this do?
所以我想知道,这是做什么的? Is it undefined?
它是未定义的吗?
unsigned
is the same as unsigned int
. unsigned
与unsigned int
相同。
The cast will convert the signed integer to unsigned as follows:强制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 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.
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