What Is Struct Embedding in Go (Composition Over Inheritance)

Struct embedding in Go is a form of composition where a struct includes another struct as an unexported field, automatically promoting the embedded struct's methods and fields to the outer struct. This allows you to reuse behavior and data without inheritance, creating a "has-a" relationship rather than an "is-a" one.

type Logger struct {
    prefix string
}

func (l *Logger) Log(msg string) {
    fmt.Println(l.prefix + ": " + msg)
}

type Service struct {
    Logger // Embedded struct
    name   string
}

// Service now has a Log method promoted from Logger
func main() {
    s := &Service{Logger: Logger{prefix: "INFO"}, name: "API"}
    s.Log("Started") // Output: INFO: Started
}

Struct embedding lets you build complex types by combining simpler ones, like adding a battery and a screen to a phone case to make a phone. Instead of copying code or using inheritance, you just include the other type directly, and its features become available on your new type. This keeps your code modular and easier to maintain.