Why Do Go's Maps and Slices Behave Differently When Passed as Function Parameters?

Why Slice and Map Behave Differently as Parameters

In Go, slice and map reference types exhibit contrasting behavior when passed as function parameters. Maps, being pointers to data structures, reflect any modifications made to their elements within the function. However, new elements added to slices, which are structs that store an array pointer, length, and capacity, are not reflected in the argument.

This difference stems from the underlying implementation:

• Maps: Maps are pointers to a data structure, meaning that when a map is passed to a function, it provides a pointer to the actual map. Any modifications to the map's elements are reflected in the original map, as the pointer remains unchanged.
• Slices: Slices, on the other hand, are structs containing an array pointer, length, and capacity. When a slice is passed to a function, the compiler creates a copy of the slice header (i.e., the struct). If changes are made to the slice, these changes only affect the copy. The function does not know about the original slice, and any new elements added to the slice are not reflected in the original.

This behavior is further influenced by Go's pass-by-value semantics, where copies of values are passed to functions. Maps, being pointers, are still passed by value, but the copy still points to the same underlying data structure. This allows modifications to be reflected in the original. However, when a slice is passed, a copy of the slice header is made, and any changes to that copy are not propagated back to the original.

To achieve similar behavior for slices, one would need to pass pointers to slices, effectively making them hidden pointers. However, this practice is rarely used, and instead, the more common approach is to return a new slice with the desired modifications.

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