Appending to a slice in a struct in a map [duplicate] - go

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I'm trying to append to a slice within a map:
type MyOuterStruct struct{
name string
inners map[string]MyInnerStruct
}
type MyInnerStruct struct{
name string
list []int
}
func main() {
ms := MyOuterStruct{
name: "outer",
inners: map[string]MyInnerStruct{
"a": {name: "inner"},
},
}
ms.inners["a"].list = append(ms.inners["a"].list, 4, 5, 6)
fmt.Println(ms.inners["a"].list)
//cannot assign to struct field ms.inners["a"].list in map
}
I know the issue is I'm assigning to an "unaddressable" field, but I'm not sure how to structure this properly.
I've tried the following
myCurrentList := ms.inners["a"].list
myCurrentList = append(myCurrentList, 4, 5, 6)
ms.inners["a"].list = myCurrentList
But this wasn't correct either.
How should I structure the MyInnerStruct so that I can have access to a dynamic slice list than I can append to?

Answer 1

The problem is that ms.inners["a"] returns a copy of the value stored for that key. So when you modify a member of that copy, it is not reflected on the copy stored in the map.
The easiest solution is to define the map as:
map[string]*MyInnerStruct
This way, ms.inners["a"] returns a pointer to the value stored in the map. Another solution is:
x:=ms.inners["a"]
x.list = myCurrentList
ms.inners["a"]=x
This copies the struct stored in the map to x, modifies it, and copies it back.