Why Does My Go Code Experience an Overflow Error When Using Constants?

Cascading Constant Overflows in Go

When working with constants in Go, it's important to understand their type conversions to avoid potential overflows. Consider the following code snippet:

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

Compiling this code results in the following error:

constant -18446462598732840961 overflows int

To resolve this issue, we need to delve into the underlying type conversions that Go performs with constants.

Understanding Untyped Constants and Type Conversion

In the above code, the constant 0xFFFF << 48 is an untyped constant, which means it has no specific type assigned to it. When combining untyped constants with typed values, the untyped constant takes on the type of the other operand.

In our case, we're performing an AND operation (&) between the typed constant userid (which is an int) and the untyped constant 0xFFFF << 48. Therefore, the resulting expression (userid &^ (0xFFFF << 48)) becomes an int.

Overflowing Integer Constants

The problem arises when we negate the untyped constant 0xFFFF << 48, resulting in -0xffff000000000001. Since this value is too large for an int type (which is signed and has a range of -2^31 to 2^31-1), it causes the compilation error.

Using Int64 for Portability

To avoid this issue, we can use a larger integer type, such as int64, which has a higher range and can accommodate larger constant values.

<code class="go">const (
    userid int64 = 12345
    did = (userid &^ (0xFFFF << 48))
)</code>

This change allows us to represent and manipulate constants within the constraints of the available integer types without encountering overflows.

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