我正在学习 Go 并决定重写一个我最初用 Python 编写的 MQTT 编排器。最基本的部分工作正常:

package main

import (
    "fmt"
    "time"
    "os"

    MQTT "github.com/eclipse/paho.mqtt.golang"
    log "github.com/sirupsen/logrus"
)

// definitions for a switch

type Switch struct {
    topic string
    state int
}

func allEvents(client MQTT.Client, msg MQTT.Message) {
    log.WithFields(log.Fields{"topic": msg.Topic(), "payload": fmt.Sprintf("%s", msg.Payload())}).Info()
}


func initMQTT() MQTT.Client {
    opts := MQTT.NewClientOptions()
    opts.AddBroker("tcp://mqtt.example.com:1883")
    opts.SetClientID("go-dispatcher")
    opts.SetCleanSession(true)
    client := MQTT.NewClient(opts)
    if token := client.Connect(); token.Wait() && token.Error() != nil {
        panic(token.Error())
    }
    log.Info("connected to MQTT broker")
    return client
}

func main() {
    // this is that part I want to modify later in the question
    c := initMQTT()
    if token := c.Subscribe("#", 0, allEvents); token.Wait() && token.Error() != nil {
        fmt.Println(token.Error())
        os.Exit(1)
    }

    time.Sleep(100000 * time.Hour)
}

在很久以前用 C 使用过指针，我想修改程序以在初始化部分通过引用传递客户端(更多的是作为学习经验，第一个代码对我来说看起来更好)

package main

import (
    "fmt"
    "time"
    "os"

    MQTT "github.com/eclipse/paho.mqtt.golang"
    log "github.com/sirupsen/logrus"
)

// definitions for a switch

type Switch struct {
    topic string
    state int
}

func allEvents(client MQTT.Client, msg MQTT.Message) {
    log.WithFields(log.Fields{"topic": msg.Topic(), "payload": fmt.Sprintf("%s", msg.Payload())}).Info()
}

// this time we get a pointer and do not return anything
func initMQTT(client *MQTT.Client) {
    opts := MQTT.NewClientOptions()
    opts.AddBroker("tcp://mqtt.example.com:1883")
    opts.SetClientID("go-dispatcher")
    opts.SetCleanSession(true)
    client = MQTT.NewClient(opts)
    if token := *client.Connect(); token.Wait() && token.Error() != nil {
        panic(token.Error())
    }
    log.Info("connected to MQTT broker")
}

func main() {
    // the client is defined in main()
    var c MQTT.Client
    // and passed by reference so that the function modifies it
    initMQTT(&c)
    if token := c.Subscribe("#", 0, allEvents); token.Wait() && token.Error() != nil {
        fmt.Println(token.Error())
        os.Exit(1)
    }

    time.Sleep(100000 * time.Hour)
}

编译失败

# command-line-arguments
.\main.go:29:9: cannot use mqtt.NewClient(opts) (type mqtt.Client) as type *mqtt.Client in assignment:
    *mqtt.Client is pointer to interface, not interface
.\main.go:30:21: client.Connect undefined (type *mqtt.Client is pointer to interface, not interface)

以下建议表another question , 我试图删除 & 和 * (盲目地开始，说实话)，并得到运行时错误

time="2018-05-26T21:02:20+02:00" level=info msg="connected to MQTT broker"
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x604486]

goroutine 1 [running]:
main.main()
    D:/Seafile/dev/Go/src/perso/domotique.dispatcher/main.go:39 +0x36

虽然我定义了 c，但我可以将它作为引用传递，但显然 C-way 在这里不正确？(通常是在我读过的例子中)

最佳答案

这是一个常见的误解。因为您已将 c 定义为一个值而不是指针类型，所以如果不重新定义它就无法修改它。

您已经传递了它的内存地址这一事实并不能使您修改它。这与许多其他语言不同。

关于pointers - "pointer to interface, not interface"，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50546169/