String Internals

Why string values are immutable?

A string is a data structure that points to a read-only backing array.

String Header stores where a string value starts and ends in computer memory.

String Header

Pointer	57	-> where it starts
Length	5	-> where it ends

Backing Array

h	e	l	l	o

String values which are exactly the same share the same backing array for memory efficiency.

Slicing a string also reuses the same backing array in memory which makes it very efficient.

To make a change to a string, we'll need to convert to a []byte which allocates a new backing array and copies the string values.

Here is what the string header looks like in Go.

// StringHeader is used by a string value
// In practice, you should use: reflect.Header
type StringHeader struct {
	// points to a backing array's item
	pointer uintptr // where it starts
	length  int     // where it ends
}

// dump prints the string header of a string value
func dump(s string) {
	ptr := *(*StringHeader)(unsafe.Pointer(&s))
	fmt.Printf("%q: %+v\n", s, ptr)
}