How to Preallocate Slices for Performance in Go

Preallocate slices in Go by specifying the capacity in the make function to avoid memory reallocations during growth. This reduces CPU overhead from copying data to larger memory blocks as the slice expands.

// Preallocate a slice with a known capacity of 100
items := make([]string, 0, 100)

// If you know the exact final size, set length and capacity
items = make([]string, 100)

In the standard library's archive/zip package, this is done safely by validating the expected count against the file size before allocation to prevent malicious archives from causing out-of-memory errors:

// From archive/zip/reader.go: init()
if end.directorySize < uint64(size) && (uint64(size)-end.directorySize)/30 >= end.directoryRecords {
    r.File = make([]*File, 0, end.directoryRecords)
}

Preallocating a slice is like reserving a specific number of seats at a table before guests arrive. Instead of moving everyone to a bigger table every time a new guest shows up, you prepare the space upfront. This saves time and effort, making your program run faster when you know roughly how much data you will handle.