Why Does Dropping Elements from the Beginning of a Go Slice Change Its Capacity?

Why does the capacity of a slice change when dropping the first n items?

In Go, slices are implemented as structures, resembling the following:

<code class="go">type slice struct {
    array unsafe.Pointer
    len   int
    cap   int
}</code>

If we modify the function used to print the slice to include the pointer to the underlying array, we get the following output:

ptr=0x450000 len=6 cap=6 [2 3 5 7 11 13]
ptr=0x450000 len=4 cap=6 [2 3 5 7]
ptr=0x450008 len=2 cap=4 [5 7]

As you can see, dropping the last two items does not change the pointer, while dropping the first two items does. This is because a slice is essentially a window into the underlying array. Dropping items from the beginning forces the slice to be recreated in a new location with a different pointer, while dropping items from the end simply adjusts the length of the existing slice.

For further information, please refer to the following resources:

• [Go Slices: usage and internals](https://blog.golang.org/slices)
• [Slice types](https://go.dev/ref/spec#Slice_types)
• [Slice expressions](https://go.dev/ref/spec#Slice_expressions)

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