How Does Go\'s `append` Function Enlarges Slices?

Enlarging Slices in Go's "append" Function

The Go language's "append" function allows for convenient modification of slices. When the append operation requires an expansion of the slice, an intriguing question arises: how does this enlargement occur? Is the capacity of the slice always doubled as some may speculate?

To unravel this mystery, it is necessary to delve into the Go source code. The implementation responsible for slice expansion resides in the "cmd/compile" directory of the Go project. Specifically, the file "list.go" contains the function "growslice," which is responsible for this operation.

The algorithm employed by "growslice" for slice enlargement follows these rules:

1. If appending to the slice would more than double its length, the new capacity is simply set to the new length.
2. Otherwise, a more nuanced approach is taken. If the current slice length is less than 1024, the capacity is doubled. If it exceeds 1024, the capacity is increased by 25%. This step is repeatedly applied until the new capacity accommodates the required length.

It is important to note that this implementation is subject to change in the future. The Go development team has indicated that the heuristics used for slice enlargement can be adjusted as necessary. For the latest information on the implementation, consult the master branch of the Go project on GitHub.

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