如何在Golang中将功能扩展到导入的类型

Question

I have an imported type

type ExternalType struct {
   quantity int
}


type ExternalArray []*ExternalType

I want to be able to implement the sort interface for ExternalArray for so that I sort it by quantity.

However, I am not sure how I could do that?

A concrete example is this:

https://play.golang.org/p/bEPtJ8NHQK

Answer 1

Define a type in the current package that sorts a slice with the same element type as the imported type:

type byQuantity []*pkg.ExternalType

func (a byQuantity) Len() int           { return len(a) }
func (a byQuantity) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
func (a byQuantity) Less(i, j int) bool { return a[i].Quantity < a[j].Quantity }

Convert the imported slice type value to the type defined above and sort:

a := pkg.ExternalArray{{1}, {3}, {2}}
sort.Sort(byQuantity(a))
// a is now sorted by quantity

Because the original slice and the converted slice share the same backing array, sort on the converted slice also sorts the original slice.

playground example