How to Handle Shared Fields in Different Structs with Generics in Go?

Handling Shared Fields in Different Structs with Generics

When working with external packages, it can be challenging to define generic functions that operate on structs containing common members. This article explores a problem where an attempt was made to create a generic function to modify shared fields of two distinct Firebase message structs, Message and MulticastMessage.

Despite having similar Android configuration fields, these structs have no explicit relationship, and attempts to access their common properties directly using type parameters resulted in errors. The reason for this is that Go 1.18 does not yet support accessing common fields or methods of type parameters, as explained in the linked threads.

Solution Options

To address this issue, several solutions are proposed:

1. Type Switch

If the number of types involved is limited, a type switch statement can be used to access the shared fields:

<code class="go">func highPriority[T firebaseMessage](message T) T {
    switch m := any(message).(type) {
    case *messaging.Message:
        setConfig(m.Android)
    case *messaging.MulticastMessage:
        setConfig(m.Android)
    }
    return message
}</code>

2. Wrapper with Method

This method involves creating a wrapper struct with the additional method that you want to use in the generic function:

<code class="go">type MessageWrapper struct {
    messaging.Message
}

func (w *MessageWrapper) SetConfig(cfg messaging.Android) {
    *w.Android = cfg
}</code>

The generic function can then access this common method:

<code class="go">func highPriority[T wrappedMessage](message T) T {
    message.SetConfig(messaging.Android{"some-value"})
    return message
}</code>

3. Reflection

If the number of structs is large, reflection can be used to access the shared fields dynamically:

<code class="go">func highPriority[T firebaseMessage](message T) T {
    cfg := &messaging.Android{}
    reflect.ValueOf(message).Elem().FieldByName("Android").Set(reflect.ValueOf(cfg))
    return message
}</code>

Notes

It's important to note that these solutions have their limitations:

• Type switch and wrapper methods are less suitable for large numbers of types.
• Reflection requires addressable structs and fields.

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