"method requires pointer receiver" in Go Programming Language

Question

I just saw a presentation of the Go programming language and thought I'd try to write a few lines. Everything worked fine until I tried to use an interface in this situation. How do I solve this?

package main

import "fmt"

type entity float32

func (e *entity) inc() {
    *e++
}

type incer interface {
    inc()
}

func doSomething(i incer) {
    i.inc()
}

func main() {
    fmt.Println("Hello, 世界")

    var e entity = 3
    e.inc()
    doSomething(e)
    fmt.Println(e)
}

I get the compiler error:

prog.go:24: cannot use e (type entity) as type incer in function argument:
entity does not implement incer (inc method requires pointer receiver)

I want to use a pointer so that the inc() will affect the enity outside the function. What is the syntax I should use?

/Ricky

newacct · Accepted Answer · 2011-09-16 00:29:11Z

23

I think there is some confusion here. inc is a method of the type *entity, and not of the type entity (while you can call methods on values directly on pointers; you cannot generally call methods on pointers directly on values). What you may be confused about is why you could call e.inc(), instead of having to do (&e).inc(). This is a little-known special case documented at the bottom of the Calls section in the language specification, that says if x is addressable, and &x's method set contains m, then x.m() is shorthand for (&x).m(). This applies to this case because e is a variable, so it is addressable; but other expressions may not be addressable. I would recommend that you not use this shortcut, however, as it causes confusion; it makes you think that e conforms to the interface inter, while it does not.

answered Sep 16, 2011 at 0:29

newacct

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2 Comments

kostix Over a year ago

@Ricky, to clear things up a bit more: actual arguments are always passed to methods by value, so for "accessor" methods the compiler has no troubles converting e.foo() to (&e).foo() but "mutator" methods must obviously change the real value, not their local method copy and hence values of non-reference types have to be passed to such methods using pointers, and the compiler cannot perform the trick being discussed. See "Should I define methods on values or pointers" in golang.org/doc/go_faq.html

David Over a year ago

newacct: great explanation.

Krzysztof Kowalczyk · Accepted Answer · 2011-09-15 23:45:47Z

4

Change it to: doSomething(&e). func (e *entity) inc() satisfies incer interface only for *entity type. There is no inc() for just entity type and that's what's you're passing to doSomething().

answered Sep 15, 2011 at 23:45

Krzysztof Kowalczyk

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