Example
Composition provides an alternative to inheritance. A struct may include another type by name in its declaration:
type Request struct {
Resource string
}
type AuthenticatedRequest struct {
Request
Username, Password string
}
In the example above, AuthenticatedRequest will contain four public members: Resource, Request, Username, and Password.
Composite structs can be instantiated and used the same way as normal structs:
func main() {
ar := new(AuthenticatedRequest)
ar.Resource = "example.com/request"
ar.Username = "bob"
ar.Password = "P@ssw0rd"
fmt.Printf("%#v", ar)
}
Embedding
In the previous example, Request is an embedded field. Composition can also be achieved by embedding a different type. This is useful, for example, to decorate a Struct with more functionality. For example, continuing with the Resource example, we want a function that formats the content of the Resource field to prefix it with http:// or https://. We have two options: create the new methods on AuthenticatedRequest or embed it from a different struct:
type ResourceFormatter struct {}
func(r *ResourceFormatter) FormatHTTP(resource string) string {
return fmt.Sprintf("http://%s", resource)
}
func(r *ResourceFormatter) FormatHTTPS(resource string) string {
return fmt.Sprintf("https://%s", resource)
}
type AuthenticatedRequest struct {
Request
Username, Password string
ResourceFormatter
}
And now the main function could do the following:
func main() {
ar := new(AuthenticatedRequest)
ar.Resource = "www.example.com/request"
ar.Username = "bob"
ar.Password = "P@ssw0rd"
println(ar.FormatHTTP(ar.Resource))
println(ar.FormatHTTPS(ar.Resource))
fmt.Printf("%#v", ar)
}
Look that the AuthenticatedRequest that has a ResourceFormatter embedded struct.
But the downside of it is that you cannot access objects outside of your composition. So ResourceFormatter cannot access members from AuthenticatedRequest.
