Named Types vs. Embedding: How Do I Add Methods to an Existing Go Type?

Adding Methods to an Existing Type: Named Types vs. Embedding

When dealing with named types, it's important to understand the distinction between creating a new type and embedding an existing type. Let's explore how to add methods to an existing type while preserving its original methods.

Case 1: Named Types

If you use the type keyword to define a new type, it will be distinct from the underlying type. For instance, in the code snippet provided:

type StartTime time.Time

StartTime is a new type, and it won't have the methods of time.Time, such as Date(). This is because named types do not inherit methods from their underlying types.

Case 2: Embedding

To add methods to an existing type without losing its original methods, use embedding. This involves creating a new struct that embeds the existing type. For example, we could rewrite the above code using embedding:

type StartTime struct {
    time.Time
}

In this case, StartTime embeds time.Time, meaning it includes all its methods and fields.

How Embedding Works

When a struct embeds another struct anonymously (without specifying a field name), the fields and methods of the embedded struct are promoted to the embedding struct. This means they can be used directly on the embedding struct's instance.

Example of Usage

With embedding, you can access the methods of both StartTime and time.Time:

func main() {
    s := StartTime{time.Now()}
    fmt.Println(s.Date()) // Will output the current date
}

Conclusion

If you need to modify the behavior or add methods to an existing type, consider using embedding. It allows you to preserve the original methods while extending the type's functionality.

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