go - 无法分配给 mapMeasures[ts].Delta

Question

这个问题在这里已经有了答案:





Accessing struct fields inside a map value (without copying)

(2 个回答)


1年前关闭。




我有一个类型:

type Measure struct {
    Timestamp     time.Time
    Delta         float64
}

我正在将一组度量值转换为以时间戳为键的 map :

func MeasuresToMap(measures []models.Measure) map[string]models.Measure {
    mapMeasures := make(map[string]models.Measure, len(measures)) //
    for _, measure := range measures {
        mapMeasures[measure.Timestamp.Format(time.RFC3339)] = measure
    }
    return mapMeasures
}

我这样做:

mapMeasures := misc.MeasuresToMap(myMeasures)
for ts, _ := range mapMeasures {
        mapMeasures[ts].Delta = 0
    }

我得到:

Cannot assign to mapMeasures[ts].Delta

但我能做的:

for ts, measure := range mapMeasures {
        measure.Delta = 0
        mapMeasures[ts] = measure
    }

这两个选项有什么区别，为什么我不能直接更新 Delta 字段？

Answer 1

你不能分配给你 的东西无法获取地址的。
mapMeasures[ts].Delta = 0 就是这种情况。 - 你不能取Measure的地址.以下是 Go Programming Language (A. Donovan, B. Kernighan) 书中关于获取 map 元素地址的内容:

One reason that we can’t take the address of a map element is that growing a map might cause rehashing of existing elements into new storage locations, thus potentially invalidating the address.

另一方面，当您执行 for ts, measure := range mapMeasures {...}您创建一个显式变量 measure因此您可以获取它的地址并分配给它。