New features of go1.18: efficient replication, new clone API for strings and bytes standard library

Hello everyone , I'm fried fish .

Go1.18 In a few weeks （3 month ） It's about to be released , Previously, we have updated several issues of new version features , Today I'll bring you a new optimization class , Is with the strings and bytes Standard library .

background

Want to copy faster

In everyday programming , byte []byte It is often copied . You need to write the following code ：

dup := make([]byte, len(data))
copy(dup, data)

@Ilia Choly I think it's too troublesome , After all, I have to write it every time , Or encapsulate it into the following functions ：

// Clone returns a copy of b
func Clone(b []byte) []byte {
  b2 := make([]byte, len(b))
  copy(b2, b)
  return b2
}

To do this, I want to add a shortcut , But this obviously doesn't hold water , Students will find a ready-made way to be familiar with grammar ：

b2 := append([]byte(nil), b...)

One line can achieve the effect , Even faster , Because the allocated slice will not be initialized to zero .

Copying will share memory

many Go developer , When writing an application, copy the slice （Slice） when , You'll find the copied slices s1 And the original s0 There are memory associations , Its essential reason lies in its underlying data structure , This can lead to many hidden problems .

The sample code is as follows ：

import (
    "fmt"
    "reflect"
    "unsafe"
)

func main() {
    s0 := " I have fried fish in my head "
    s1 := s0[:3]
    s0h := (*reflect.StringHeader)(unsafe.Pointer(&s0))
    s1h := (*reflect.StringHeader)(unsafe.Pointer(&s1))

    fmt.Printf("Len is equal: %t\n", s0h.Len == s1h.Len)
    fmt.Printf("Data is equa: %t\n", s0h.Data == s1h.Data)
}

From the above procedure , What do you think of variables s0 and s1 comparison , their Len Is it equal ？Data Is it equal ？

The output is as follows ：

Len is equal: false
Data is equa: true

What do you think , That seems fine .Len It's not equal , After all, it is copied by index . but Data It's equal , Why , yes Go Out BUG Did you? ？

This is actually related to Go stay String and Slice Related to the underlying data structure , for example String The runtime performance of is StringHeader.

His underlying structure is as follows ：

type StringHeader struct {
    Data uintptr
    Len  int
}

Data： Point to the specific underlying array .
Len： Represents the length of the string slice .

The point is Data, A pointer to the underlying data is essentially an address , Therefore, when copying , It will also be copied . Causing unnecessary duplication and “ dirty ” data , Cause all kinds of strange BUG.

This problem , Pain points of new features .

New characteristics

stay Go1.18 Among the new features of ,strings and bytes Added one Clone Method , To solve the problems mentioned earlier 2 There are two problems .

The source code is as follows ：

func Clone(s string) string {
    if len(s) == 0 {
        return ""
    }
    b := make([]byte, len(s))
    copy(b, s)
    return *(*string)(unsafe.Pointer(&b))
}

adopt copy Function to copy the original string , Get a new one []byte data .
adopt *(*string)(unsafe.Pointer(&b)) Pointer operation , Realization byte To string Memory zero conversion .

thus , Two problems in the background are solved in a very clever way . You don't have to write similar code all the time .

summary

all the time ,Go Medium string and slice It is controversial because of the underlying array pointing and capacity expansion mechanism , At the end, we also give some of the official account numbers before. “ pit ”, Interested readers can have a look at .

this 1.18 New method update , It has a certain function , You can also know clearly in your study Clone His role and definition , Keep an eye on it .

Do you have similar code in your project ？

If you have any questions, you are welcome to feedback and exchange in the comments area , The best relationship is mutual achievement , Everyone give the thumbs-up Namely Fried fish The greatest power of creation , Thank you for your support .

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