Why Are Underscores Used in Go Struct Tags for Qt Bindings?

Go Struct Tags with Underscore Prefix Function Names

Question:

In a custom Go struct utilized for Qt bindings, some fields are observed to have a leading underscore character. This practice seems unusual. What is the purpose of these underscores in the context of struct tags?

type CustomLabel struct {
    core.QObject

    _ func() `constructor:"init"`
    _ string `property:"text"`
}

Answer:

The leading underscores here denote what are known as "blank fields" or "anonymous fields." These fields utilize the blank identifier as their field names, which effectively renders them unaddressable. However, they do participate in the struct's memory layout.

These blank fields are commonly utilized for padding purposes. By aligning subsequent fields to byte or memory positions that correspond to the layout of data from external systems, blank fields enable seamless and efficient dumping or reading of struct values.

Specific Use Case:

In the provided example, the underscores are specifically used for generating constructor and property-setting methods for the Qt bindings.

Alternative Approach:

While anonymous fields are commonly used to define type annotations, it is crucial to use them sparingly due to their associated overhead. Since these fields cannot be referenced, they still consume memory. This potential performance impact should be considered when utilizing anonymous fields.

Instead, consider using a type whose size is 0, such as struct{}, for preserving memory efficiency. However, this approach may introduce other performance drawbacks, as it may require reflection to access the type information.

An alternative method involves utilizing 0-sized arrays with the gewünschte type, providing both zero-size footprint and type accessibility.

type CustomLabel struct {
    _ [0]func() `constructor:"init"`
    _ [0]string `property:"text"`
}

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