Empty Interface vs. Empty Struct in Go Maps: What are the Memory Implications?

Empty Interface vs. Empty Struct in Map Value: Memory Implications

When implementing custom sets in Go, developers can opt for either an empty interface or an empty struct as the map's value type. However, there are subtle differences between these two approaches that can impact performance.

Empty Interface

An empty interface is a special Go type that is compatible with any other type. It stores a pointer to the underlying data structure and a type descriptor. As a result, empty interfaces can consume significant memory, especially when the underlying data structure is large.

Empty Struct

An empty struct is a type with no fields. Since it contains no data, it has a fixed memory footprint of zero bytes. This makes empty structs particularly useful when optimizing for memory usage.

Example

To illustrate the memory usage difference, consider the following example:

type MyType uint8
var myMap = make(map[MyType]interface{})

In this example, each entry in the map occupies 8 bytes (for a 32-bit architecture) or 16 bytes (for a 64-bit architecture), even though the value is nil.

If we switch to an empty struct as the map value type:

type MyType uint8
var myMap = make(map[MyType]struct{})

Each entry now occupies 0 bytes, effectively reducing the memory overhead.

Conclusion

When implementing sets in Go, the choice between an empty interface and an empty struct depends on the specific requirements of the application. If memory usage is a critical consideration, an empty struct is the more efficient option due to its zero-byte footprint. However, the use of empty interfaces may be preferred when flexibility and compatibility are more important than memory efficiency.

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