从地图值附加切片不会影响地图

Question

mp := map[int][]int{}
slice := make([]int, 0, 1)
fmt.Printf("slice address:%p
", slice)
mp[0] = slice
slice = append(slice, 1)
fmt.Println("after append")
fmt.Printf("slice address:%p
", slice)
fmt.Println("slice:", slice)
fmt.Println("mp[0]:", mp[0])
fmt.Printf("mp[0] address:%p
", mp[0])  

output:

slice address:0xc042008f78  
after append  
slice address:0xc042008f78  
slice: [1]  
mp[0]: []  
mp[0] address:0xc042008f78

The address of the slice does not change as its cap is large enough during append. So why the map value does not take effect?

Answer 1

This is caused by the fact, that multiple slices can be backed by the same data but use different "sections" of the data. This means, that yes, an element is added to the data backing mp[0], but the length of the slice in mp is not changed. You can do that manually:

fmt.Println(mp[0][:1])

which does print [1].

You can grow any slice to it's capacity without changing the underlying data by using slice[:cap(slice)]. slice[:n] will panic if cap(slice) < n. slice[n] on the other hand will panic when len(slice) <= n. I assume that the former is possible to allow the growing of slices without changing the underlying data (as far as that is possible). The latter, I would say, is "normal" behavior.

This also explains why mp[0][:2] panics, as cap(mp[0]) is 1.

For more details you might want to read this official blog post, as suggested by Flimzy.