Golang初始化后得到了空片

Question

I have 3 struct: Queue,Config,Tasker

type Queue struct {
  Name        string
  Concurrent  int
  Connections []*redis.Client
}

type Config struct {
  Queues    []Queue
  RedisAddr string
  RedisDB   int
}

type Tasker struct {
  Config Config
}

The problem happend in this method, I initialize queue.Connections in for-loop, but I got zero length of queue.Connections outside the for-loop

func (t *Tasker) StartListening() {
  for j := 0; j < len(t.Config.Queues); j++ {
    queue := t.Config.Queues[j]
    queue.Connections = make([]*redis.Client, queue.Concurrent)
    fmt.Println(len(queue.Connections)) //here print correct length, 1 for default queue, 2 for mail queue
  }
  fmt.Println(len(t.Config.Queues[0].Connections)) //but why here print 0?
}

This is my test code

func main() {
  config := Config{
    RedisAddr: "10.1.1.59:6379",
    RedisDB:   8,
    Queues: []Queue{
      Queue{Name: "default", Concurrent: 1},
      Queue{Name: "mail", Concurrent: 2},
    },
  }
  daemon := Tasker{Config: config}
  daemon.StartListening()

}

why fmt.Println(len(t.Config.Queues[0].Connections)) is 0 outside the for-loop?

Answer 1

You are creating a new Queue instead of accessing the one in the Config structure, and this new value prevents modification to the Queue in Config.Queues. Try direct assignment:

// ...

t.Config.Queues[j].Connections = make([]*redis.Client, queue.Concurrent)

// ...

Or if you want to use an auxillary variable, change Config.Queues type to []*Queue:

type Config struct {
  Queues    []*Queue
  RedisAddr string
  RedisDB   int
}

// ...

config := Config{
  RedisAddr: "10.1.1.59:6379",
  RedisDB:   8,
  Queues: []*Queue{
    &Queue{Name: "default", Concurrent: 1},
    &Queue{Name: "mail", Concurrent: 2},
  },
}

Now your original code should work.