Interface Inside Struct
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Tomi Häsä
Mar 5, 2016, 2:29:50 PM3/5/16
to golang-nuts
Placing an interface inside a struct might not be the best option in every case, but I was wondering how do I create a factory function for such a struct? Below is my failed example.
If I remove the comments from the factory function, I get:
invalid type for composite literal: Counter
If I remove the Counter{} argument, I get:
too few values in struct initializer
If I put Counter as an argument, I get:
type Counter is not an expression
Code:
/**********************************************************************
* Interface inside struct *
*********************************************************************/
package main
import "fmt"
type Counter interface {
increase()
decrease()
nothing() // testing
}
type Number struct {
value int
co Counter
}
// factory function
//func NewNumber( x int ) *Number {
// return &Number{ x, Counter{} }
//}
func (nu Number) increase() {
a := nu.value
a++
fmt.Printf( "My value: %d \n", a )
nu.value = a
fmt.Printf( "Increasing myself: %d \n", number.value )
//nu.value = nu.value + 1
}
func (nu Number) decrease() {
//a := nu.value
//a--
//nu.value = a
nu.value--
}
func (nu Number) nothing() {
nu.value = nu.value
}
var (
number Number
)
func main() {
number.value = 0
fmt.Printf( "Number in the beginning: %d \n", number.value )
number.increase()
fmt.Printf( "Increased number: %d \n", number.value )
fmt.Println()
}
If I run this program I get:
Number in the beginning: 0
My value: 1
Increasing myself: 0
Increased number: 0
Justin Israel
Mar 5, 2016, 2:44:13 PM3/5/16
to Tomi Häsä, golang-nuts
To fix the error in your factory function, try replacing Counter{} with nil.
Counter is an interface and not a concrete type, so you cannot construct instances of an interface like that.
Although I am not clear why your Number struct implements your Counter interface, as well as wanting to store a Counter interface field. That aspect has no relation to your desire to have a factory function and have a Counter interface field on your struct.
Justin
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Nico
Mar 6, 2016, 2:17:41 AM3/6/16
to golan...@googlegroups.com
On 05/03/16 19:29, Tomi Häsä wrote:
> Placing an interface inside a struct might not be the best option in every case, but I was wondering how do I create a factory function for such a struct? Below is my failed example.
Just, use nil (see the updated code below).
> Placing an interface inside a struct might not be the best option in every case, but I was wondering how do I create a factory function for such a struct? Below is my failed example.
---
I would also read this FAQ entry https://golang.org/doc/faq#methods_on_values_or_pointers .
---
https://play.golang.org/p/CId4GQWg6u
package main
import "fmt"
type Counter interface {
increase()
decrease()
nothing() // testing
}
type Number struct {
value int
co Counter
}
return &Number{value, nil}
}
func (nu *Number) increase() {
a := nu.value
a++
fmt.Printf("My value: %d \n", a)
nu.value = a
fmt.Printf("Increasing myself: %d \n", number.value)
a++
fmt.Printf("My value: %d \n", a)
nu.value = a
fmt.Printf("Increasing myself: %d \n", number.value)
nu.value = nu.value + 1
}
func (nu *Number) decrease() {
}
nu.value--
}
func (nu *Number) nothing() {
Krzysztof Kowalczyk
Mar 6, 2016, 11:48:07 PM3/6/16
to golang-nuts
To reiterate what Justin said, embedding Counter inside Number does nothing in this particular example and probably isn't what you meant.
Type (Number) conforms to interface (Counter) by implementing interface methods. What you probably meant is just this:
package main
import "fmt"
type Counter interface {
increase()
decrease()
nothing() // testing
}
type Number struct {
value int
}
func NewNumber(value int) *Number {
return &Number{value}
}
func (nu *Number) increase() {
a := nu.value
a++
fmt.Printf("My value: %d \n", a)
nu.value = a
fmt.Printf("Increasing myself: %d \n", number.value)
nu.value = nu.value + 1
}
func (nu *Number) decrease() {
nu.value--
}
func (nu *Number) nothing() {
nu.value = nu.value
}
var (
number Number
)
func main() {
number.value = 0
fmt.Printf("Number in the beginning: %d \n", number.value)
number.increase()
fmt.Printf("Increased number: %d \n", number.value)
fmt.Println()
}
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