Why Does Appending One Element to a Nil Slice in Go Double Its Capacity?

Capacity Expansion in Nil Slices: Appending One Element Increases Capacity by Two

When working with slices in Go, the concept of capacity is crucial. Capacity represents the underlying storage allocated for a slice, providing space for additional elements. When appending an element to an existing slice, the capacity may increase to accommodate the new element.

In the provided scenario, a nil slice s1 with a length and capacity of 0 has an element appended to it, resulting in a new slice s2 with a length of 1 and a capacity of 2. Why does this increase in capacity occur, even though only one element was added?

Go's slice implementation optimizes performance by allocating sufficient capacity to avoid unnecessary reallocations. When appending to a nil slice, the compiler allocates a small initial capacity, typically 2, to reduce the overhead of repeated allocations and copying.

Additionally, slices in Go have an implicit upper index bound defined by their capacity. This means that even though the length of s2 is 1, the upper index bound is 2, allowing elements to be read or written at that index. However, these values are not considered part of the actual slice data and may not be visible when using printing functions like fmt.Printf().

In practice, it's essential to focus on the length of a slice rather than its capacity. Capacity is mainly used for performance optimizations and should not be directly accessed or relied upon.

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