Why Use Underscores in Go Struct Tags?

Go Struct Tags with Underscore Notation

The Mystery of Underscores

In Go, struct tags are annotations used to provide additional information to the compiler. However, the use of leading underscores in struct tags may seem confusing.

Unveiling the Purpose of Blank Fields

The blank identifier in Go, represented by the underscore (_), creates a field in a struct that cannot be referred to. These blank fields are not visible to the program and are designated solely for internal purposes.

Alignment in QT Bindings

In the provided code, the blank fields marked with underscores serve a specific purpose in Qt bindings. They are used as padding to align the subsequent fields to byte or memory positions that match the layout of data coming from external sources. This alignment optimizes the process of reading or writing data from/to other systems.

Cautions and Alternatives

While blank fields as type annotations can be beneficial, they should be used prudently as they can introduce unnecessary memory overhead. Consider using zero-sized arrays of the desired type instead, which preserve type information without affecting the struct's size.

Reading Blank Field Annotations

Accessing the type information carried by blank fields is possible through reflection. The code snippet below demonstrates how to retrieve the tag and type of the blank field using the Type.Elem() method.

f := reflect.ValueOf(CustomLabel{}).Type().Field(0)
fmt.Println(f.Tag)
fmt.Println(f.Type)
fmt.Println(f.Type.Elem())

For More Insight

To delve deeper into the intricacies of struct tags, explore the following resources:

• [Struct Tags in Go](https://go.dev/blog/tags)
• [Blank Fields in Go](https://go.dev/blog/blank-identifiers)

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