Go Slices vs. Maps: Why Do They Behave Differently When Adding Elements?

Slice vs. Map Parameters: Understanding the Differences

In Go, both slices and maps serve as reference types. Modifying elements within these structures propagates the changes to all referencing variables. However, differences arise when adding new elements.

Maps, being pointers to their underlying data structure, retain the same reference address when new elements are added. This means that any changes made through multiple references are reflected in all instances.

In contrast, slices are represented by lightweight structs containing pointers to the backing array, length, and capacity. Adding elements requires creating a new slice header with updated length and potentially a new backing array. This change is not propagated to the original slice header, leading to the noted observation.

The root cause for this disparity lies in the underlying implementation. Maps are pointers, while slices are structs. As a result, pointer manipulations in maps allow changes to be broadcast, while slice headers must be recreated when appending elements.

To align their behaviors, slices could be implemented as pointers, but this is rarely employed due to limited language support and the prevalence of returning new slices instead.

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