builtin: append will change addr of element in slice ?

[zhuliquan]

Go version

go version go1.23.2 linux/amd64

Output of go env in your module/workspace:

GO111MODULE='on'
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/zhuliquan/.cache/go-build'
GOENV='/home/zhuliquan/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/data/home/zhuliquan/workspace/go_workspace/pkg/mod'
GOOS='linux'
GOPATH='/data/home/zhuliquan/workspace/go_workspace'
GOPROXY='https://goproxy.cn,direct'
GOROOT='/data/home/zhuliquan/infra/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/data/home/zhuliquan/infra/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.2'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/zhuliquan/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2218099817=/tmp/go-build -gno-record-gcc-switches'

What did you do?

I write a function to extract element from a slice, this function will return a element and a slice which contains other element.

func partSlice[T any](l []T, idx int) (T, []T) {
    return l[idx], append(l[:idx], l[idx+1:]...)
}

What did you see happen?

I write below case to invoke partSlice func.

package main

import (
        "fmt"
)

func main() {
        l := []int{1, 2}
        fmt.Printf("l addr: %p\n", l)
        k, s := partSlice(l, 0)
        fmt.Printf("k: %d, s: %v, l: %v\n", k, s, l)
        fmt.Printf("s addr: %p\n", s)
        fmt.Printf("l addr: %p\n", s)
}

func partSlice[T any](l []T, idx int) (T, []T) {
        return l[idx], append(l[:idx], l[idx+1:]...)
}

the result is

l addr: 0xc000012110
k: 2, s: [2], l: [2 2]
s addr: 0xc000012110
l addr: 0xc000012110

I am confuse that k is 2, I expect it is 1. And l is replace to [2 2]!
According to addr of s and l. append(l[idx], l[idx+1]...) make 1th and 2nd elment have same addr?

What did you expect to see?

expect got result is 1 [2]

[randall77]

This is behaving as expected.
The append is happening before the evaluation of l[idx]. (They could happen in either order according to https://go.dev/ref/spec#Order_of_evaluation.)

append will modify the existing slice's backing store - it does not allocate a new slice. You might want to read https://go.dev/blog/slices-intro if you haven't already.

[gabyhelp]

Related Issues

• appending two slices with default ends modifies base slice #37604 (closed)
• Unexpected different behavior with append on a byte slice vs int slice. #45436 (closed)
• bug: look like to watch same address when assign to another variable from sub-slice. #56617 (closed)
• Slice has unpredictable result #68010 (closed)
• main: chained appends change contents of previous variable #65034 (closed)
• container/list: the last Element is changed when use array append #31404 (closed)
• It is a GC issue? #44317 (closed)
• cmd/compile: incosistent behaviour when assigning slices from a base slice with append #56999 (closed)
• bug report : builtin function "append" Malfunction #32127 (closed)

Related Discussions

• why the append built-in function returns a new address?

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[zigo101]

see #27821

And read https://go101.org/article/evaluation-orders.html