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

Slices vs. Maps in Parameter Passing: A Deeper Dive

In Go, slices and maps share the trait of being reference types. However, their behaviors differ when it comes to adding new elements in parameters. While new elements added to maps are automatically reflected in the argument, new elements added to slices are "discarded" in the argument.

Implementation Differences

This discrepancy stems from how these types are implemented. Maps are implemented as pointers to internal hash map data structures. When a new element is added to a map, the hash map data structure is updated, but the underlying pointer remains unchanged. This ensures that all references to the map point to the same underlying data structure.

Slices, on the other hand, are implemented as structs that store a pointer to the backing array, along with the slice length and capacity. When a new element is added to a slice, a new slice header needs to be created with updated length and potentially a new backing array if needed. This new slice header is assigned to the variable pointing to it, but the original slice header remains unchanged.

Passing by Value

Another factor contributing to the observed behavior is Go's pass-by-value semantics. When a map is passed to a function, the function receives a copy of the map pointer. Any modifications made to the map through this copy will also affect the original map because they both point to the same underlying data structure.

When a slice is passed to a function, the function receives a copy of the slice header. Modifications to the slice will create a new slice header with updated length and capacity, but the original slice header will not be affected. As a result, the argument will not see the changes made within the function.

Implications for API Consistency

The differing behaviors of slices and maps in this context can lead to potential pitfalls for developers, particularly those new to Go. The API for these reference types appears inconsistent, as one behaves as expected for value modification but the other does not.

Possible Solution

To achieve consistency in API behavior, one could make slices behave like pointers to underlying data structures, similar to maps. However, this approach is rarely used and lacks language support. Instead, the common practice is to return a new slice when adding elements. This ensures that the caller receives the updated version of the slice.

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