Golang常量到底出了什么问题uint64

Question

userid := 12345
did := (userid & ^(0xFFFF << 48))

when compiling this code, I got:

./xxxx.go:511: constant -18446462598732840961 overflows int

Do you know what is the matter with this and how to solve it ? Thanks.

Answer 1

^(0xFFFF << 48) is an untyped constant, which in go is an arbitrarily large value.

0xffff << 48 is 0xffff000000000000. When you negate it, you get -0xffff000000000001 (since with two's complement, -x = ^x + 1, or ^x = -(x + 1)).

When you write userid := 12345, userid gets the type int. Then when you try to and (&) it with the untyped constant -0xffff000000000001 the compiler figures that this constant needs to be an int. At this point, the compiler complains because the value is too large in magnitude to be an int.

If you're trying to get the constant 0x0000ffffffffffff, then you can use 1<<48 - 1, which (if you've got 64-bit ints), will fit. Since your code will never work if int is 32-bits, then you should probably use int64 in your code rather than int to make it portable.

The blog post https://blog.golang.org/constants explains how constants work, and some background on why they are the way they are.