在地图中访问结构（无复制）

Question

Assuming the following

type User struct {
    name string
}

users := make(map[int]User)

users[5] = User{"Steve"}

Why isn't it possible to access the struct instance now stored in the map?

users[5].name = "Mark"

Can anyone shed some light into how to access the map-stored struct, or the logic behind why it's not possible?

Notes

I know that you can achieve this by making a copy of the struct, changing the copy, and copying back into the map -- but that's a costly copy operation.

I also know this can be done by storing struct pointers in my map, but I don't want to do that either.

Answer 1

The fundamental problem is that you can't take the address of something in a map. You might think the compile would re-arrange users[5].name = "Mark" into this

(&users[5]).name = "Mark"

But that doesn't compile, giving this error

cannot take the address of users[5]

This is to allow maps the freedom to re-order things at will to use memory efficiently.

The only way to change something actually in a map is to assign to it, i.e.

t := users[5]
t.name = "Mark"
users[5] = t

So I think you either have to live with the copy above, or live with storing pointers in your map. Storing pointers has the disadvantage of using more memory and more memory allocations which may outweigh the copying in the above - only you and your application can tell that.

A third alternative is to use a slice - your original syntax works perfectly if you change users := make(map[int]User) to users := make([]User, 10)