Agfdhyk

I have an interface Model, which is implemented by struct Person.

To get a model instance, I have the following helper functions:

func newModel(c string) Model {

    switch c {

    case "person":

        return newPerson()

    }

    return nil

}



func newPerson() *Person {

    return &Person{}

}

The above approach allows me to return a properly typed Person instance (can easily add new models later with same approach).

When I attempted to do something similar for returning a slice of models, I get an error. Code:

func newModels(c string) Model {

    switch c {

    case "person":

        return newPersons()

    }

    return nil

}



func newPersons() *Person {

    var models Person

    return &models

}

Go complains with: cannot use newPersons() (type Person) as type Model in return argument

My goal is to return a slice of whatever model type is requested (whether Person, FutureModel, Terminator2000, w/e). What am I missing, and how can I properly implement such a solution?

This is very similar to a question I just answered: https://stackoverflow.com/a/12990540/727643

The short answer is that you are correct. A slice of structs is not equal to a slice of an interface the struct implements.

A Person and a Model have different memory layouts. This is because the types they are slices of have different memory layouts. A Model is an interface value which means that in memory it is two words in size. One word for the type information, the other for the data. A Person is a struct whose size depends on the fields it contains. In order to convert from a Person to a Model, you will need to loop over the array and do a type conversion for each element.

Since this conversion is an O(n) operation and would result in a new slice being created, Go refuses to do it implicitly. You can do it explicitly with the following code.

models := make(Model, len(persons))

for i, v := range persons {

    models[i] = Model(v)

}

return models

And as dskinner pointed out, you most likely want a slice of pointers and not a pointer to a slice. A pointer to a slice is not normally needed.

*Person        // pointer to slice

*Person        // slice of pointers

Maybe this is an issue with your return type *Person, where it should actually be *Person so to reference that each index of the slice is a reference to a Person, and where a slice is in itself a reference to an array.

Check out the following example:

package main



import (

    "fmt"

)



type Model interface {

    Name() string

}



type Person struct {}



func (p *Person) Name() string {

    return "Me"

}



func NewPersons() (models *Person) {

    return models

}



func main() {

    var p Model

    p = new(Person)

    fmt.Println(p.Name())



    arr := NewPersons()

    arr = append(arr, new(Person))

    fmt.Println(arr[0].Name())

}

As Stephen already answered the question and you're a beginner I emphasize on giving advises.

A better way of working with go's interfaces is not to have a constructor returning
the interface as you might be used to from other languages, like java, but to have
a constructor for each object independently, as they implement the interface implicitly.

Instead of

newModel(type string) Model { ... }

you should do

newPerson() *Person { ... }

newPolitician() *Politician { ... }

with Person and Politician both implementing the methods of Model.
You can still use Person or Politician everywhere where a Model
is accepted, but you can also implement other interfaces.

With your method you would be limited to Model until you do a manual conversion to
another interface type.

Suppose I have a Person which implements the method Walk() and a Model implements ShowOff(), the following would not work straight forward:

newModel("person").ShowOff()

newModel("person").Walk() // Does not compile, Model has no method Walk

However this would:

newPerson().ShowOff()

newPerson().Walk()

Types T and T are distinct types and distinct are their methods as well, even when satisfying the same interface. IOW, every type satisfying Model must implement all of the Model's methods by itself - the method receiver can be only one specific type.