go compile_error ai_generated true

cannot assign to struct field in map

ID: go/cannot-assign-to-struct-field

Also available as: JSON · Markdown

92%Fix Rate

92%Confidence

50Evidence

2023-01-01First Seen

Version Compatibility

Version	Status	Introduced	Deprecated	Notes
1	active	—	—	—

Can't modify a struct field directly in a map. Go maps return copies, not references.

generic

95% success Use a map of pointers: map[string]*MyStruct
```
m := map[string]*MyStruct{"key": &MyStruct{Field: 1}}
m["key"].Field = 2  // works!
```
Sources: https://go.dev/blog/maps
90% success Copy, modify, reassign: tmp := m[k]; tmp.Field = val; m[k] = tmp
```
tmp := m[k]; tmp.Field = val; m[k] = tmp
```
Sources: https://go.dev/ref/spec#Assignments

Common approaches that don't work:

Use unsafe to get a pointer to the map value 95% fail
Unsafe and undefined behavior — map values can move in memory
Use a different data structure 55% fail
Maps are fine — just use pointer values

Preceded by: