duanhan3067 2018-06-27 18:55
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嵌套地图的情况下,map.LoadOrStore返回的地址值是否与输入相同?

I'm doing a nested sync.Map but I wonder if I can save some few lines of code if the value returned by LoadOrStore is the same as the input in case of a map, I mean this:

var mapa sync.Map
mapaInterFace, ok := sessiones.LoadOrStore(userID,mapa)
if ok {
    mapa,ok=mapaInterFace.(sync.Map)
    if !ok{
        return errors.New("type assertion")
    }
}

If mapa were the same as the returned by LoadOrStore when the value is stored, I can immediately use it, but if not I have to add after the previous code, the type assertion:

mapa,ok=mapaInterFace.(sync.Map)
    if !ok{
        return errors.New("type assertion")
    }

and doing some often it can make some ugly code

Update: sessiones is type sync.Map

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  • duanjuelian4640 2018-06-27 20:09
    关注

    As I explain later, you should use pointers for the sync.Map types. Therefore, we can simplify to:

    var mapa, mapb = new(sync.Map), new(sync.Map)
    var key string
    
    if actual, loaded := mapa.LoadOrStore(key, mapb); loaded {
        if maps, ok := actual.(*sync.Map); ok {
            mapb = maps
        } else {
            // handle loaded value type assertion error
        }
    }
    

    Now the assignments are cheap because we are assigning pointers (*sync.Map) not structs (sync.Map).


    Package sync

    import "sync" 
    

    type Map

    Map is like a Go map[interface{}]interface{} but is safe for concurrent use by multiple goroutines without additional locking or coordination. Loads, stores, and deletes run in amortized constant time.

    The Map type is specialized. Most code should use a plain Go map instead, with separate locking or coordination, for better type safety and to make it easier to maintain other invariants along with the map content.

    The Map type is optimized for two common use cases: (1) when the entry for a given key is only ever written once but read many times, as in caches that only grow, or (2) when multiple goroutines read, write, and overwrite entries for disjoint sets of keys. In these two cases, use of a Map may significantly reduce lock contention compared to a Go map paired with a separate Mutex or RWMutex.

    The zero Map is empty and ready for use. A Map must not be copied after first use.

    type Map struct {
            // contains filtered or unexported fields
    }
    

    func (*Map) LoadOrStore

    func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool)
    

    LoadOrStore returns the existing value for the key if present. Otherwise, it stores and returns the given value. The loaded result is true if the value was loaded, false if stored.


    A sync.Map must not be copied after first use.

    In Go, all arguments and receivers are passed by value, as if by assignment (a copy). For example, go vet reports a sync.Map copy error,

    // go vet: variable declaration copies lock value to arg: sync.Map contains sync.Mutex
    var m sync.Map
    var arg interface{} = m
    

    and

    var map1, map2 sync.Map
    // go vet: call of map1.LoadOrStore copies lock value: sync.Map contains sync.Mutex
    map1.LoadOrStore("key", map2)
    

    Use pointers. For example,

        var m sync.Map
        var arg interface{} = &m
    

    and

        var map1, map2 sync.Map
        map1.LoadOrStore("key", &map2)
    
    本回答被题主选为最佳回答 , 对您是否有帮助呢?
    评论

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