Why Does Golang Throw an Overflow Error When Calculating Constants?

Understanding Golang's Constant Overflow with uint64

Problem:

Consider the following Golang code:

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

When attempting to compile this code, an error is encountered:

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

Analysis:

The crux of this issue lies in the untyped constant ^(0xFFFF << 48). This constant represents a very large value due to the shift operation. When the untyped constant is converted to an int due to the bitwise operation with userid, the compiler raises an overflow error because the constant exceeds the maximum value that can be represented by an int.

Solution:

To resolve this issue, one can explicitly cast the untyped constant to a type that can accommodate its large magnitude. For instance, one can replace the problematic expression with the following:

<code class="go">did := (userid & (1<<48 - 1))</code>

In this modified expression, 1<<48 - 1 results in the constant 0x0000ffffffffffff, which can be represented by an int64 data type. Alternatively, one can explicitly use int64 in the code to ensure compatibility with both 32-bit and 64-bit architectures:

<code class="go">func main() {
  const userID int64 = 12345
  did := userID & (1<<48 - 1)
  println(did)
}</code>

By adhering to these suggestions, you can effectively handle constant overflows in Golang and maintain code portability across different architectures.

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