How to initialize a list of objects given only an interface sample?

Question

I'm writing a database interface in Google Go. It takes encoding.BinaryMarshaler objects to save and saves them as []byte slices, and it loads data into encoding.BinaryUnmarshaler to return it:

func (db *DB) Get(bucket []byte, key []byte, destination encoding.BinaryUnmarshaler) (encoding.BinaryUnmarshaler, error) {

I want to implement being able to load an arbitrary length slice of encoding.BinaryUnmarshalers in one go (for example "load all data from a bucket X"). I want the function to be able to load any number of data objects without knowing beforehand how many objects are to be loaded, so I don't expect the final user to pass me a slice to be filled. Instead, I take a encoding.BinaryUnmarshaler sample object to know what structures I'm dealing with:

func (db *DB) GetAll(bucket []byte, sample encoding.BinaryUnmarshaler) ([]encoding.BinaryUnmarshaler, error) {

The problem I ran into while coding this, is that I'm not sure how to initialize new instances of a given object, since I don't know what object I am dealing with, only what interface it conforms to. What I tried doing was:

tmp:=new(reflect.TypeOf(sample))

but that just caused an error.

How can I create a new object in go without knowing what structure it is, having an example object instead?

If you "don't know what object ... [you are] dealing with" then your're lost. That's something you need to know: Transmit, Take a look at how gob does it. — Volker
– Volker, Commented Oct 2, 2015 at 22:26
Possible duplicate of New instance of struct from the type at runtime in GO — kenfire
– kenfire, Commented Dec 8, 2017 at 8:50
@kenfire this question was asked a month before the other one ;) — ThePiachu
– ThePiachu, Commented Dec 10, 2017 at 20:40
@ThePiachu Yes but you did not accept your answer so I couldn't do the other way :) — kenfire
– kenfire, Commented Dec 11, 2017 at 8:24

dave · Accepted Answer · 2015-10-05 17:32:11Z

1

You would have to use reflect.New along with reflect.TypeOf:

tmp := reflect.New(reflect.TypeOf(sample))

http://play.golang.org/p/-ujqWtRzaP

edited Oct 5, 2015 at 17:32

answered Oct 2, 2015 at 22:34

dave

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