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strings: align Clone output with string([]byte(""))

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Add a comment how strings of length 0 are treated and
that they always will result in the return of a string
equal to the constant string "".

The previous implementation would return a string header that uses
runtime.zerobase as the backing array pointer while the string constant
"" has 0 as pointer value.

Using 0 has the backing array pointer is also the behaviour of
string([]byte(input)) which makes the new behaviour a better drop in
replacement.

Change-Id: Ic5460e9494b6708edbdfa4361e878d50db54ba10
Reviewed-on: https://go-review.googlesource.com/c/go/+/360255
Trust: Martin Möhrmann <martin@golang.org>
Run-TryBot: Martin Möhrmann <martin@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
This commit is contained in:
Martin Möhrmann 2021-11-01 06:42:01 +01:00
parent 4056934e48
commit 82f902ae8e
2 changed files with 15 additions and 1 deletions
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5
src/strings/clone.go
View File

@ -16,7 +16,12 @@ import (
// overuse of Clone can make programs use more memory.
// Clone should typically be used only rarely, and only when
// profiling indicates that it is needed.
// For strings of length zero the string "" will be returned
// and no allocation is made.
func Clone(s string) string {
if len(s) == 0 {
return ""
}
b := make([]byte, len(s))
copy(b, s)
return *(*string)(unsafe.Pointer(&b))

11
src/strings/clone_test.go
View File

@ -11,9 +11,13 @@ import (
"unsafe"
)
var emptyString string
func TestClone(t *testing.T) {
var cloneTests = []string{
"",
strings.Clone(""),
strings.Repeat("a", 42)[:0],
"short",
strings.Repeat("a", 42),
}
@ -25,9 +29,14 @@ func TestClone(t *testing.T) {
inputHeader := (*reflect.StringHeader)(unsafe.Pointer(&input))
cloneHeader := (*reflect.StringHeader)(unsafe.Pointer(&clone))
if inputHeader.Data == cloneHeader.Data {
if len(input) != 0 && cloneHeader.Data == inputHeader.Data {
t.Errorf("Clone(%q) return value should not reference inputs backing memory.", input)
}
emptyHeader := (*reflect.StringHeader)(unsafe.Pointer(&emptyString))
if len(input) == 0 && cloneHeader.Data != emptyHeader.Data {
t.Errorf("Clone(%#v) return value should be equal to empty string.", inputHeader)
}
}
}