Why Does Slicing a Slice in Go Affect Capacity Differently Depending on Colon Placement?

Dealing with Slice Length and Capacity

When working with slices, it's crucial to understand the impact of slicing operations on both the length and capacity of the slice. The question arises as to why the capacity is decreased when slicing with s[2:], but remains unaffected when using s[:4] or s[:0].

The key to this difference lies in where the slicing parameter, the colon and numeric value (2), is placed. When the number is placed before the colon, as in s[2:], the operation effectively drops the first two elements of the slice. This results in not only reducing the length of the slice but also altering the pointer to its first element. Since the capacity of the slice is determined by the size of the underlying array, changing the pointer affects the capacity as well.

In contrast, when the number is placed after the colon, as in s[:4] or s[:0], the slicing operation adjusts the length of the slice within the existing array without modifying the pointer. This means that the capacity of the slice remains unchanged.

Another point to note is that it's not possible to recover the first two elements that were dropped using s[2:] when reassigning the sliced slice to the same variable. This is because slicing operations do not duplicate the underlying data but instead create a new slice referencing the original array. Modifying a sliced slice modifies the original slice, making it impossible to retrieve the lost elements without additional operations.

To visualize this concept, consider the following representation of a slice in memory:

[]byte{a, b, c, d, e}

If we execute s = s[2:4], the slice is reduced to:

[]byte{c, d}

The capacity of the original slice decreases because the pointer now refers to a different part of the array.

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