Why Does `copy()` Return an Empty Slice When Copying to an Empty Destination Slice in Go?

Why Copying an Empty Slice with copy() Results in an Empty Slice

When working with slices in Go, the copy() function is often used to create a copy of an existing slice. However, if the destination slice is empty, the copy operation behaves unexpectedly.

According to the documentation for copy():

The copy built-in function copies elements from a source slice into a destination slice. (As a special case, it also will copy bytes from a string to a slice of bytes.) The source and destination may overlap. Copy returns the number of elements copied, which will be the minimum of len(src) and len(dst).

This suggests that the number of elements copied should be the minimum of the lengths of the source and destination slices. However, when the destination slice is empty (as in your example where tmp is initialized as an empty slice), no elements are copied.

The reason for this behavior lies in the implementation of copy(). The function iterates over the source slice and copies elements to the destination slice until it reaches the end of either the source or destination slice. If the destination slice is empty, the loop will end immediately since there are no elements to copy.

To create a non-empty copy of a slice, the destination slice must be initialized with a non-zero length. This can be achieved using the make() function, as shown in the following example:

arr := []int{1, 2, 3}
tmp := make([]int, len(arr))
copy(tmp, arr)

fmt.Println(tmp) // Output: [1 2 3]

By explicitly setting the length of the destination slice, we ensure that copy() has enough space to copy all the elements from the source slice. This behavior is also documented in the Go Language Specification, which states that the number of elements copied is the minimum of the lengths of the source and destination slices.

Therefore, when attempting to copy an empty slice using copy(), it is important to initialize the destination slice with a non-zero length to ensure that the copy operation succeeds.

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