Go Slices vs. Maps: Why Do Modifications to Map Parameters Reflect, But Slice Ones Don't?

Slice vs Map as Parameters: Understanding the Behavioral Differences

Slices and maps in Go both belong to reference types. A key difference arises when adding new elements to these types when used as arguments to functions. While map elements are automatically reflected in the original map, additions to slices are not visible in the caller.

Underlying Implementation:

The disparity stems from the underlying implementation of these types. Maps are implemented as pointers to hidden hash map structures. When editing a map, the pointer remains unchanged, referencing the same map structure.

Slices, on the other hand, are compact structs holding the pointer to the backing array, its length, and capacity. When an element is modified, the backing array is directly affected, but the slice header (the struct) remains unaltered.

Pass-by-Value Semantics:

Go implements pass-by-value semantics. Therefore, a copy of a slice header is created when passed as an argument. Subsequent changes to the copy, such as appending elements, do not affect the original slice because the slice header remains unmodified.

Implications:

This implementation distinction has implications for the behavior of slices and maps used in parameter passing:

• Maps: Editing a map as an argument automatically propagates changes to the original map.
• Slices: Elements added to a slice within a function are not visible in the original slice.

Addressing the Asymmetry:

To make slices and maps behave uniformly, consider the following:

• Treat slices as pointers and operate on the pointed value, or
• Return a new slice from the function to reflect the modifications.

Understanding the underlying implementation and pass-by-value semantics helps developers navigate the nuances of using slices and maps as function parameters effectively.

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