Why Does Modifying Structs in Go Maps Seem to Create Copies, and How Can I Avoid This?

Go's Maps and Value Modification Puzzle: Demystifying Struct Field Mutations

In Go, when working with maps that contain structs as values, attempting to directly modify a field of a struct within a map value may leave you puzzled. Why does Go enforce this seemingly counterintuitive behavior?

When you assign a struct value to a map, a copy of that struct is created within the map. This means that any changes made to the struct accessed through the map are isolated to the copy and do not affect the original struct in the map.

This value-freezing mechanism has its merits. By default, modifying the struct's fields would effectively create a new struct object in the map, causing potential concurrency issues and unexpected behavior. However, this behavior also raises questions about the underlying costs of modifying struct fields in other data structures like slices.

The answer lies in performance optimizations. While pointer storage incurs a slight overhead compared to value storage, it allows for faster modification. In the specific case of Go, maps are optimized for value storage, and storing pointers would disrupt these optimizations.

To work around this limitation, one can opt for pointer-based storage in the map. By storing a pointer to the struct instead of the struct itself, direct modification becomes possible. This approach, however, comes with its own trade-offs, such as the potential for dangling pointers if the struct is removed from the map.

Therefore, understanding Go's value-freezing behavior in maps is crucial for effective and efficient programming. Whether you choose to store structs by value or pointer ultimately depends on the specific requirements and performance considerations of your application.

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