Type assertion errors when casting from interface to actual object

Question

Encountering type assertion errors in the example below.

Errors:

49: cannot convert z (type IZoo) to type Zoo: need type assertion

49: cannot assign to Zoo(z).animals

type IAnimal interface {}

type IZoo interface {}

type Zoo struct {
    animals map[string]IAnimal
}

func NewZoo() *Zoo {
    var z IZoo = &Zoo{}

    Zoo(z).animals = map[string]IAnimal{} // cannot convert z (type IZoo) to type Zoo: need type assertion
    
    return z // cannot use z (type IZoo) as type *Zoo in return argument: need type assertion
}

Zoyd · Accepted Answer · 2017-03-11 23:37:40Z

2

The error message says it all: you need a type assertion.

y := z.(Zoo)
y.animals = map[string]IAnimal{}

answered Mar 11, 2017 at 23:37

Zoyd

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5 Comments

user2727195 Over a year ago

couldn't you do it in one line, do we need separate declaration and assignment

zerkms Over a year ago

@user2727195 z.(Zoo).animals = what do you think this expression should do? What it does - it creates a temporary object that is to be thrown away after this statement. Hence, a question: what does it mean to assign a value to an object's field that you would not be able to access? You basically can remove that line and the observed behaviour of a program would not change.

user2727195 Over a year ago

@zerkms is there an explanation on the docs about this temporary object

zerkms Over a year ago

I'm not sure. It's a generic knowledge - structs in Go copy on assignment.

Mr_Pink Over a year ago

to clarify, everything in Go is a copy on assignment.

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