Why Does a `sha1.Sum` Slice Cause an \"Invalid Operation: Slice of Unaddressable Value\" Error in Go?

Unexpected Error When Joining Lines

When attempting to modify the following code to join two lines, an erroneous message appears:

Original:

hash := sha1.Sum([]byte(uf.Pwd))
u.Pwhash = hex.EncodeToString(hash[:])

Joined:

u.Pwhash = hex.EncodeToString(sha1.Sum([]byte(uf.Pwd))[:])

The modified code triggers the error:

invalid operation sha1.Sum(([]byte)(uf.Pwd))[:] (slice of unaddressable value)

Understanding the Issue

This error arises because in the modified code, the return value of the function call sha1.Sum is directly sliced. Function return values are not addressable, and thus, cannot be sliced.

Addressability in Go

In Go, only certain entities are addressable, including:

• Variables
• Pointer indirections
• Slice indexing
• Field selectors of addressable structs
• Array indexing using an addressable array (exception: composite literals)

The error occurs because sha1.Sum returns an array, which is only addressable when stored in a local variable as in the original code.

Solution

To resolve the issue, the return value of sha1.Sum should be stored in a local variable first, giving it an addressable reference.

Conclusion

Understanding the concept of addressability is crucial in Go to avoid such errors when slicing arrays. Always ensure that the sliced entity is addressable, either directly or through an intermediate variable assignment.

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