Can Go Slices Be Shrunk: How to Reduce a Slice's Capacity?

Shrinking Slice Capacity in Go

Slices in Go are dynamic arrays that automatically resize as needed. However, their capacity, which indicates the maximum number of elements they can hold, typically remains unchanged. This can lead to questions about whether Go offers a way to shrink this capacity after reducing the number of elements in a slice.

In the provided code snippet, a slice a is initialized and populated with 10 million integers. The goal is to reduce its size to just 10 elements. Traditional methods like slicing and deletion do not modify the slice's capacity.

Shrinking Slice Capacity

Go does provide a way to effectively achieve a "realloc" operation:

a = append([]int64(nil), a[:newSize]...)

This code creates a new empty slice and appends the first newSize elements of a to it. The new slice will have a capacity equal to newSize.

Considerations

It's important to note that this operation may involve copying the underlying array. The Go compiler decides whether to perform an in-place resize or a copy, based on optimizations.

Also, this approach is generally a micro-optimization. Memory consumption concerns should be addressed through careful algorithm and data structure selection, rather than solely relying on shrinking slice capacity.

Limitations

The provided code snippet is partially correct. The compiler can't always determine if other pointers reference the slice's backing array. Consequently, the operation is guaranteed to perform a copy. This limitation may be addressed in future Go releases.

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