Why Can't I Directly Modify Struct Fields in Go Maps?

Map Values in Go: Unveiling the Mystery Behind Indirect Modification

When working with Go maps that map integer keys to structs, a curious behavior arises: you cannot directly modify a field in the map value. Instead, you must read the struct, modify it, and then rewrite it back to the map.

Why is this workaround necessary? Does modifying struct fields in maps or slices incur unforeseen hidden costs?

The answer lies in the way Go handles value types. When you store a struct by value in a map, you are essentially creating a copy of the struct. Any modifications made to this copy will not affect the original struct stored in the map.

To rectify this, you can instead store a pointer to the struct in the map. This allows you to directly modify the struct referenced by the pointer. In code, this would translate to using map[whatever]*struct instead of map[whatever]struct.

The choice of using a value type or a pointer type depends on the specific requirements of your application. However, it is important to understand the nuances of value types and pointer types to avoid unexpected behavior when working with maps, slices, and other data structures in Go.

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