Fastest way to allocate a large string in Go?

Question

I need to create a string in Go that is 1048577 characters (1MB + 1 byte). The content of the string is totally unimportant. Is there a way to allocate this directly without concatenating or using buffers?

Also, it's worth noting that the value of string will not change. It's for a unit test to verify that strings that are too long will return an error.

Answer 1

Use strings.Builder to allocate a string without using extra buffers.

var b strings.Builder
b.Grow(1048577)
for i := 0; i < 1048577; i++ {
  b.WriteByte(0)
}
s := b.String()

The call to the Grow method allocates a slice with capacity 1048577. The WriteByte calls fill the slice to capacity. The String() method uses unsafe to convert that slice to a string.

The cost of the loop can be reduced by writing chunks of N bytes at a time and filling single bytes at the end.

If you are not opposed to using the unsafe package, then use this:

p := make([]byte, 1048577)
s := *(*string)(unsafe.Pointer(&p))

If you are asking about how to do this with the simplest code, then use the following:

s := string(make([]byte, 1048577)

This approach does not meet the requirements set forth in the question. It uses an extra buffer instead of allocating the string directly.

Answer 2

I ended up using this:

string(make([]byte, 1048577))

https://play.golang.org/p/afPukPc1Esr