[英]substrings and the Go garbage collector
When taking a substring of a string in Go, no new memory is allocated. 在Go中获取字符串的子字符串时,不会分配新的内存。 Instead, the underlying representation of the substring contains a Data pointer that is an offset of the original string's Data pointer. 相反,substring的底层表示包含一个Data指针,它是原始字符串的Data指针的偏移量。
This means that if I have a large string and wish to keep track of a small substring, the garbage collector will be unable to free any of the large string until I release all references to the shorter substring. 这意味着如果我有一个大字符串并希望跟踪一个小的子字符串,垃圾收集器将无法释放任何大字符串,直到我释放对较短子字符串的所有引用。
Slices have a similar problem, but you can get around it by making a copy of the subslice using copy(). 切片有类似的问题,但您可以通过使用copy()制作子切片的副本来绕过它。 I am unaware of any similar copy operation for strings. 我不知道字符串的任何类似的复制操作。 What is the idiomatic and fastest way to make a "copy" of a substring? 制作子串的“副本”的惯用和最快方法是什么?
For example, 例如,
package main
import (
"fmt"
"unsafe"
)
type String struct {
str *byte
len int
}
func main() {
str := "abc"
substr := string([]byte(str[1:]))
fmt.Println(str, substr)
fmt.Println(*(*String)(unsafe.Pointer(&str)), *(*String)(unsafe.Pointer(&substr)))
}
Output: 输出:
abc bc
{0x4c0640 3} {0xc21000c940 2}
I know this is an old question, but there are a couple ways you can do this without creating two copies of the data you want. 我知道这是一个老问题,但有几种方法可以做到这一点,而无需创建所需数据的两个副本。
First is to create the []byte
of the substring, then simply coerce it to a string
using unsafe.Pointer
. 首先是创建子字符串的[]byte
,然后使用unsafe.Pointer
将其强制转换为string
。 This works because the header for a []byte
is the same as that for a string
, except that the []byte
has an extra Cap
field at the end, so it just gets truncated. 这是因为[]byte
的标题与string
的标题相同,除了[]byte
末尾有一个额外的Cap
字段,因此它只是被截断。
package main
import (
"fmt"
"unsafe"
)
func main() {
str := "foobar"
byt := []byte(str[3:])
sub := *(*string)(unsafe.Pointer(&byt))
fmt.Println(str, sub)
}
The second way is to use reflect.StringHeader
and reflect.SliceHeader
to do a more explicit header transfer. 第二种方法是使用reflect.StringHeader
和reflect.SliceHeader
进行更明确的标头传输。
package main
import (
"fmt"
"unsafe"
"reflect"
)
func main() {
str := "foobar"
byt := []byte(str[3:])
bytPtr := (*reflect.SliceHeader)(unsafe.Pointer(&byt)).Data
strHdr := reflect.StringHeader{Data: bytPtr, Len: len(byt)}
sub := *(*string)(unsafe.Pointer(&strHdr))
fmt.Println(str, sub)
}
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