What is the point in setting a slice's capacity?

Question

In Golang, we can use the builtin make() function to create a slice with a given initial length and capacity.

Consider the following lines, the slice's length is set to 1, and its capacity 3:

func main() {
    var slice = make([]int, 1, 3)
    slice[0] = 1
    slice = append(slice, 6, 0, 2, 4, 3, 1)
    fmt.Println(slice)
}

I was surprised to see that this program prints:

[1 6 0 2 4 3 1]

This got me wondering- what is the point of initially defining a slice's capacity if append() can simply blow past it? Are there performance gains for setting a sufficiently large capacity?

Answer 1

A slice is really just a fancy way to manage an underlying array. It automatically tracks size, and re-allocates new space as needed.

As you append to a slice, the runtime doubles its capacity every time it exceeds its current capacity. It has to copy all of the elements to do that. If you know how big it will be before you start, you can avoid a few copy operations and memory allocations by grabbing it all up front.

When you make a slice providing capacity, you set tht initial capacity, not any kind of limit.

See this blog post on slices for some interesting internal details of slices.