Why Does My Go Code Throw a Constant Overflow Error When Using Bitwise Operations with Untyped Constants?

Understanding Constant Overflows in Go

The Go programming language offers a comprehensive type system, including the concept of constants. However, when working with constants, it's essential to be aware of potential overflows. One such case involves using an untyped constant to perform bitwise operations on a typed variable.

Issue Description

Consider the following code snippet:

<code class="go">userid := 12345
did := (userid & ^(0xFFFF << 48))</code>

While attempting to compile this code, you may encounter the error message:

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

Explanation

The expression ^(0xFFFF << 48) represents an untyped constant. In Go, untyped constants can assume arbitrarily large values. In this case, the result of the negation operation is -0xffff000000000001, which is too large to fit into an int.

When you assign the result of the bitwise operation to the variable did, which is of type int, the compiler attempts to convert the untyped constant to an int, resulting in an overflow error.

Solution

To resolve this issue, you can use an alternative constant that fits within the int type. For instance, you can replace the problematic expression with 1<<48 - 1. This calculation produces the constant 0x0000ffffffffffff, which is compatible with int64 on 64-bit systems.

Additional Tip

If you're aiming for portability, it's recommended to use int64 instead of int in your code. This ensures that the mentioned code will work correctly even on systems where int is 32-bits.

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