golang got empty slice after initialize - for-loop

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.