使用iota时，我很困惑

Question

您正在使用哪个版本的Go（go version）？

$ go version
1.13

此问题会在最新版本中重现吗？

是

您正在使用什么操作系统和处理器体系结构（go env？

go env输出

$ go env
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/chezixin/go"
GOPRIVATE=""
GOPROXY="https://goproxy.io"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/chezixin/go/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/mr/_x323k891dq9yvmvlgwqf4f40000gn/T/go-build490011880=/tmp/go-build -gno-record-gcc-switches -fno-common"

您做了什么？

package main

import "fmt"

type T struct {
    Name  string
    Port  int
    State State
}

type State int

const (
    Running State = iota
    Stopped
    Rebooting
    Terminated
)
func (s State) String() string {
    switch s {
    case Running:
        return "Running"
    case Stopped:
        return "Stopped"
    case Rebooting:
        return "Rebooting"
    case Terminated:
        return "Terminated"
    default:
        return "Unknown"
    }
}

func main() {
    t := T{Name: "example", Port: 6666}
    fmt.Printf("t %+v\n", t)
}

结果：

t {Name:example Port:6666 State:Running}

您期望看到什么？

相反，您看到了什么？

Although the string is rewritten, this is related to State.const.
I am printing a T struct in main. The zero value is 0.
But why do you want to display State:Running

When T.State = 0, why does the program think T.State = const.Running?

I understand this now:
String is an interface type, T.State=0, const.Running=0, the two types are the same, the value is the same, the program will think that the two are equal? Doesn't it distinguish between const and struct at the moment?
According to logic, you should distinguish between const and struct, but the program thinks they are equal. Is this a bug? Or am I getting into a misunderstanding?

Can you tell me how to understand it correctly?
thank you very much
Thank you

Answer 1

T.State的类型为State。这与字段T.State是否为结构的一部分无关。类型State的零值为0，也等于Running。因此，当您将t的实例分配为T且未初始化的T.State实例时，T.State会得到零值，即Running。

据我从代码中可以看出，State似乎有一个'Unknown'值，它是除声明的值以外的任何值。如果要为Unknown的零值打印State，请更改常量声明，以使Unknown获得零值（即iota）。