如何在具有类型接口的情况下向syncmap.Map添加条目

Question

In the example, I'm trying to store 1 at the key "xxxxxxx" in the syncmap called value. The type of value is interface{}. So, I have a type assertion to make it a syncmap.Map, which is mm. Then, I add the new entry to mm. Unfortunately the new entry, doesn't get added to value, only to mm. Looks like mm is a copy or something. I'm not sure how to make this work.

To be specific, I figure I need to do the type assertion to add the entry. But, I think the type assertion is making a copy. Can you advise on how to do this so that the entry ("xxxxxxx", 1) is actually added into the syncmap called value?

func Test(name string, m syncmap.Map) {
        log.Print(name, " ")
        m.Range(func(key, value interface{}) bool {
                log.Println(">>", value)
                mm := value.(syncmap.Map)
                mm.Store("xxxxxxx", 1)
                PrintMap(">>>>>>>>>>>>> " + key.(string), value.(syncmap.Map))
                return true
        })
        log.Println()
}

Answer 1

You are correct about the inner map being a copy. To make persistent changes to the maps, use pointers.

func Test(name string, m *sync.Map) { // make parameter a pointer
    log.Print(name, " ")
    m.Range(func(key, value interface{}) bool {
        mm, ok := value.(*sync.Map) // outer map must also store inner maps as pointers
        mm.Store("xxxxxxx", 1)
        return true
    })
    log.Println()
}

Update the part of your code that declares the main outer map to store pointers to sync.Map type. Example:

var m sync.Map // outer map
m.Store("A", &sync.Map{})
m.Store("B", &sync.Map{})
m.Store("C", &sync.Map{})

a, _ := m.Load("A")
am, _ := a.(*sync.Map)
am.Store("xxxx", 1)

Note that I've used sync package instead of syncmap since syncmap.Map is now a part of standard library.

Working example: https://play.golang.org/p/ib-dfXjPDy