The Go Programming Language Specification says:
As with selectors, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: pt.Mv is equivalent to (*pt).Mv.
and:
As with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: t.Mp is equivalent to (&t).Mp.
So, what is non-interface method in the given context?
Interface method means the method you refer to (you call) is a call on an interface value (whose method set contains the method). Similarly, non-interface method means the method you refer to (you call) is not a call on an interface value (but on a concrete type).
For example:
var r io.Reader = os.Stdin
r.Read(nil) // Interface method: type of r is an interface (io.Reader)
var p image.Point = image.Point{}
p.String() // Non-interface method, p is a concrete type (image.Point)
To demonstrate the auto-dereferencing and address taking, see this example:
type myint int
func (m myint) ValueInt() int { return int(m) }
func (m *myint) PtrInt() int { return int(*m) }
func main() {
var m myint = myint(1)
fmt.Println(m.ValueInt()) // Normal
fmt.Println(m.PtrInt()) // (&m).PtrInt()
var p *myint = new(myint)
*p = myint(2)
fmt.Println(p.ValueInt()) // (*p).ValueInt()
fmt.Println(p.PtrInt()) // Normal
}
It outputs (try it on the Go Playground):
1
1
2
2
type T struct {}
func (t *T) f() {}
func main() {
x := T{}
x.f()
}
Above, x.f is a non-interface method.
Related
There are already several Q&As on this "X does not implement Y (... method has a pointer receiver)" thing, but to me, they seems to be talking about different things, and not applying to my specific case.
So, instead of making the question very specific, I'm making it broad and abstract -- Seems like there are several different cases that can make this error happen, can someone summary it up please?
I.e., how to avoid the problem, and if it occurs, what are the possibilities? Thx.
This compile-time error arises when you try to assign or pass (or convert) a concrete type to an interface type; and the type itself does not implement the interface, only a pointer to the type.
Short summary: An assignment to a variable of interface type is valid if the value being assigned implements the interface it is assigned to. It implements it if its method set is a superset of the interface. The method set of pointer types includes methods with both pointer and non-pointer receiver. The method set of non-pointer types only includes methods with non-pointer receiver.
Let's see an example:
type Stringer interface {
String() string
}
type MyType struct {
value string
}
func (m *MyType) String() string { return m.value }
The Stringer interface type has one method only: String(). Any value that is stored in an interface value Stringer must have this method. We also created a MyType, and we created a method MyType.String() with pointer receiver. This means the String() method is in the method set of the *MyType type, but not in that of MyType.
When we attempt to assign a value of MyType to a variable of type Stringer, we get the error in question:
m := MyType{value: "something"}
var s Stringer
s = m // cannot use m (type MyType) as type Stringer in assignment:
// MyType does not implement Stringer (String method has pointer receiver)
But everything is ok if we try to assign a value of type *MyType to Stringer:
s = &m
fmt.Println(s)
And we get the expected outcome (try it on the Go Playground):
something
So the requirements to get this compile-time error:
A value of non-pointer concrete type being assigned (or passed or converted)
An interface type being assigned to (or passed to, or converted to)
The concrete type has the required method of the interface, but with a pointer receiver
Possibilities to resolve the issue:
A pointer to the value must be used, whose method set will include the method with the pointer receiver
Or the receiver type must be changed to non-pointer, so the method set of the non-pointer concrete type will also contain the method (and thus satisfy the interface). This may or may not be viable, as if the method has to modify the value, a non-pointer receiver is not an option.
Structs and embedding
When using structs and embedding, often it's not "you" that implement an interface (provide a method implementation), but a type you embed in your struct. Like in this example:
type MyType2 struct {
MyType
}
m := MyType{value: "something"}
m2 := MyType2{MyType: m}
var s Stringer
s = m2 // Compile-time error again
Again, compile-time error, because the method set of MyType2 does not contain the String() method of the embedded MyType, only the method set of *MyType2, so the following works (try it on the Go Playground):
var s Stringer
s = &m2
We can also make it work, if we embed *MyType and using only a non-pointer MyType2 (try it on the Go Playground):
type MyType2 struct {
*MyType
}
m := MyType{value: "something"}
m2 := MyType2{MyType: &m}
var s Stringer
s = m2
Also, whatever we embed (either MyType or *MyType), if we use a pointer *MyType2, it will always work (try it on the Go Playground):
type MyType2 struct {
*MyType
}
m := MyType{value: "something"}
m2 := MyType2{MyType: &m}
var s Stringer
s = &m2
Relevant section from the spec (from section Struct types):
Given a struct type S and a type named T, promoted methods are included in the method set of the struct as follows:
If S contains an anonymous field T, the method sets of S and *S both include promoted methods with receiver T. The method set of *S also includes promoted methods with receiver *T.
If S contains an anonymous field *T, the method sets of S and *S both include promoted methods with receiver T or *T.
So in other words: if we embed a non-pointer type, the method set of the non-pointer embedder only gets the methods with non-pointer receivers (from the embedded type).
If we embed a pointer type, the method set of the non-pointer embedder gets methods with both pointer and non-pointer receivers (from the embedded type).
If we use a pointer value to the embedder, regardless of whether the embedded type is pointer or not, the method set of the pointer to the embedder always gets methods with both the pointer and non-pointer receivers (from the embedded type).
Note:
There is a very similar case, namely when you have an interface value which wraps a value of MyType, and you try to type assert another interface value from it, Stringer. In this case the assertion will not hold for the reasons described above, but we get a slightly different runtime-error:
m := MyType{value: "something"}
var i interface{} = m
fmt.Println(i.(Stringer))
Runtime panic (try it on the Go Playground):
panic: interface conversion: main.MyType is not main.Stringer:
missing method String
Attempting to convert instead of type assert, we get the compile-time error we're talking about:
m := MyType{value: "something"}
fmt.Println(Stringer(m))
To keep it short and simple, let say you have a Loader interface and a WebLoader that implements this interface.
package main
import "fmt"
// Loader defines a content loader
type Loader interface {
load(src string) string
}
// WebLoader is a web content loader
type WebLoader struct{}
// load loads the content of a page
func (w *WebLoader) load(src string) string {
return fmt.Sprintf("I loaded this page %s", src)
}
func main() {
webLoader := WebLoader{}
loadContent(webLoader)
}
func loadContent(loader Loader) {
loader.load("google.com")
}
The above code will give you this compile time error
./main.go:20:13: cannot use webLoader (type WebLoader) as type Loader
in argument to loadContent:
WebLoader does not implement Loader (Load method has pointer receiver)
To fix it you only need to change webLoader := WebLoader{} to following:
webLoader := &WebLoader{}
Why this will fix the issue? Because you defined this function func (w *WebLoader) Load to accept a pointer receiver. For more explanation please read #icza and #karora answers
Another case when I have seen this kind of thing happening is if I want to create an interface where some methods will modify an internal value and others will not.
type GetterSetter interface {
GetVal() int
SetVal(x int) int
}
Something that then implements this interface could be like:
type MyTypeA struct {
a int
}
func (m MyTypeA) GetVal() int {
return a
}
func (m *MyTypeA) SetVal(newVal int) int {
int oldVal = m.a
m.a = newVal
return oldVal
}
So the implementing type will likely have some methods which are pointer receivers and some which are not and since I have quite a variety of these various things that are GetterSetters I'd like to check in my tests that they are all doing the expected.
If I were to do something like this:
myTypeInstance := MyType{ 7 }
... maybe some code doing other stuff ...
var f interface{} = myTypeInstance
_, ok := f.(GetterSetter)
if !ok {
t.Fail()
}
Then I won't get the aforementioned "X does not implement Y (Z method has pointer receiver)" error (since it is a compile-time error) but I will have a bad day chasing down exactly why my test is failing...
Instead I have to make sure I do the type check using a pointer, such as:
var f interface{} = new(&MyTypeA)
...
Or:
myTypeInstance := MyType{ 7 }
var f interface{} = &myTypeInstance
...
Then all is happy with the tests!
But wait! In my code, perhaps I have methods which accept a GetterSetter somewhere:
func SomeStuff(g GetterSetter, x int) int {
if x > 10 {
return g.GetVal() + 1
}
return g.GetVal()
}
If I call these methods from inside another type method, this will generate the error:
func (m MyTypeA) OtherThing(x int) {
SomeStuff(m, x)
}
Either of the following calls will work:
func (m *MyTypeA) OtherThing(x int) {
SomeStuff(m, x)
}
func (m MyTypeA) OtherThing(x int) {
SomeStuff(&m, x)
}
Extend from above answers (Thanks for all of your answers)
I think it would be more instinctive to show all the methods of pointer / non pointer struct.
Here is the playground code.
https://play.golang.org/p/jkYrqF4KyIf
To summarize all the example.
Pointer struct type would include all non pointer / pointer receiver methods
Non pointer struct type would only include non pointer receiver methods.
For embedded struct
non pointer outer struct + non pointer embedded struct => only non pointer receiver methods.
non pointer outer struct + pointer embedded struct / pointer outer struct + non pointer embedded struct / pointer outer struct + pointer embedded struct => all embedded methods
The following is the source code to try the embed type.
Modify function is defined as func (f *F) Modify(f2 F). Could anyone explain why the Modify function is not shown in the first reflection loop? But in the second reflection loop, both Modify and Validate can be got from *s.
package main
import "fmt"
import "reflect"
type F func(int) bool
func (f F) Validate(n int) bool {
return f(n)
}
func (f *F) Modify(f2 F) {
*f = f2
}
type B bool
func (b B) IsTrue() bool {
return bool(b)
}
func (pb *B) Invert() {
*pb = !*pb
}
type I interface {
Load()
Save()
}
func PrintTypeMethods(t reflect.Type) {
fmt.Println(t, "has", t.NumMethod(), "methods:")
for i := 0; i < t.NumMethod(); i++ {
fmt.Print(" method#", i, ": ",
t.Method(i).Name, "\n")
}
}
func main() {
var s struct {
F
*B
I
}
PrintTypeMethods(reflect.TypeOf(s))
fmt.Println()
PrintTypeMethods(reflect.TypeOf(&s))
}
output:
struct { main.F; *main.B; main.I } has 5 methods:
method#0: Invert
method#1: IsTrue
method#2: Load
method#3: Save
method#4: Validate
*struct { main.F; *main.B; main.I } has 6 methods:
method#0: Invert
method#1: IsTrue
method#2: Load
method#3: Modify
method#4: Save
method#5: Validate
Method sets
A type may have a method set associated with it. The method set of an
interface type is its interface. The method set of any other type T
consists of all methods declared with receiver type T. The method set
of the corresponding pointer type *T is the set of all methods
declared with receiver *T or T (that is, it also contains the method
set of T). Further rules apply to structs containing embedded fields,
as described in the section on struct types. Any other type has an
empty method set. In a method set, each method must have a unique
non-blank method name.
Struct types
A field or method f of an embedded field in a struct x is called
promoted if x.f is a legal selector that denotes that field or method
f.
...
Given a struct type S and a defined type T, promoted methods are
included in the method set of the struct as follows:
If S contains an embedded field T, the method sets of S and *S both include promoted methods with receiver T. The method set of *S also includes promoted methods with receiver *T.
If S contains an embedded field *T, the method sets of S and *S both include promoted methods with receiver T or *T.
If a method m is defined for a type T, that method is available for both T and *T:
type T struct {}
func (t T) m() {}
func main() {
t:=T{}
tp:=&T{}
t.m() // valid: m defined for T
tp.m() // valid: m defined for *T
}
If a method is defined with a pointer receiver, it is only defined for *T and not for T':
func (t *T) n() {}
func main() {
t:=T{}
tp:=&TP{
t.n() // Valid: &t is passed to n
tp.b // valid
mp:=map[int]T{1:t}
mp[1].n() // not valid. mp[1] is not addressable
pp:=map[int]*T{1:&t}
pp[1].n() // valid: pp[1] is *T
}
The reason for this is simple: it prevents unintentionally modifying the copy instead of the indented object. If a method with pointer receiver was available for the value type as well, with the following code:
mp[1].n()
n, taking a pointer receiver, would have modified a copy of the value of mp[1], and not the value stored at mp[1]. The fact that methods with pointer receivers are not available for value types prevents that, and this becomes a compile error, because n is not defined for T, and mp[1] is not addressable, preventing the value to be converted to a pointer by the compiler.
Assuming we have an understanding that,
For explicit method definition for type X, GO compiler implicitly defines the same method for type *X and vice versa, if I declare,
func (c Cat) foo(){
//do stuff_
}
and declare,
func (c *Cat) foo(){
// do stuff_
}
then GO compiler gives error,
Compile error: method re-declared
which indicates that, pointer method is implicitly defined and vice versa
In the below code,
package main
type X interface{
foo();
bar();
}
type Cat struct{
}
func (c Cat) foo(){
// do stuff_
}
func (c *Cat) bar(){
// do stuff_
}
func main() {
var c Cat
var p *Cat
var x X
x = p // OK; *Cat has explicit method bar() and implicit method foo()
x = c //compile error: Cat has explicit method foo() and implicit method bar()
}
GO compiler gives error,
cannot use c (type Cat) as type X in assignment:
Cat does not implement X (bar method has pointer receiver)
at x = c, because, implicit pointer methods satisfy interfaces, but implicit non-pointer methods do not.
Question:
Why implicit non-pointer methods do not satisfy interfaces?
Let's look into the language specification:
A type may have a method set associated with it. The method set
of an interface type is its interface. The method set of any other
type T consists of all methods declared with receiver type T. The
method set of the corresponding pointer type *T is the set of all
methods declared with receiver *T or T (that is, it also contains
the method set of T).
In your example, the method set of the interface type x is [foo(), bar()]. The method set of the type Cat is [foo()], and the method set of the type *Cat is [foo()] + [bar()] = [foo(), bar()].
This explains, why variable p satisfies the interface x, but variable c doesn't.
Method set
Following the spec:
The method set of any other named type T consists of all methods with receiver type T. The method set of the corresponding pointer type *T is the set of all methods with receiver *T or T (that is, it also contains the method set of T).
Method set definition sounds weird until you follow addressable and not addressable types concept.
Addressable and not addressable types
It is possible to call a pointer receiver method on a value if the value is of addressable type.
As with selectors, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: pt.Mv is equivalent to (*pt).Mv.
As with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: t.Mp is equivalent to (&t).Mp.
It is ok to call pointer receiver methods on values till you are dealing with addressable types (struct is addressable):
type Cat struct {}
func (c *Cat) bar() string { return "Mew" }
func main() {
var c Cat
c.bar()
}
Variables of interface type are not addressable
But not all Go types are addressable. Also variables referenced through interfaces are not addressable.
It is impossible to call pointer receiver on values of not addressable types:
type X interface {
bar() string
}
type Cat struct{}
func (c *Cat) bar() string { return "Mew" }
/* Note `cat` variable is not a `struct` type value but
it is type of `X` interface therefor it is not addressable. */
func CatBar(cat X) {
fmt.Print(cat.bar())
}
func main() {
var c Cat
CatBar(c)
}
So with the following error Go runtime prevents segment fault:
cannot use c (type Cat) as type X in assignment:
Cat does not implement X (bar method has pointer receiver)
Add a little to dev.bmax's answer.
type Cat struct{
}
func (c Cat) foo(){
// do stuff_
}
func (c *Cat) bar(){
// do stuff_
}
you can do
var c cat
c.bar() // ok to call bar(), since c is a variable.
but not
cat{}.bar() // not ok to call bar(), c is not a variable.
It's legal to call a *T method on an argument of type T so long as the argument is a variable; the compiler implicitly takes its address. But this is mere syntactic sugar: a value of type T does not posses all methods that a *T pointer does, and as a result it might satisfy fewer interfaces.
On the other hand, you can always call foo() with Cat or *Cat.
How about this?
package main
import (
"fmt"
)
type Growler interface{
Growl() bool
}
type Cat struct{
Name string
Age int
}
// *Cat is good for both objects and "references" (pointers to objects)
func (c *Cat) Speak() bool{
fmt.Println("Meow!")
return true
}
func (c *Cat) Growl() bool{
fmt.Println("Grrr!")
return true
}
func main() {
var felix Cat // is not a pointer
felix.Speak() // works :-)
felix.Growl() // works :-)
var ginger *Cat = new(Cat)
ginger.Speak() // works :-)
ginger.Growl() // works :-)
}
In the documentation for effective Go it states:
As we saw with ByteSize, methods can be defined for any named type
(except a pointer ...
type ByteSlice []byte
func (slice ByteSlice) Append(data []byte) []byte {
// Body exactly the same as above
}
It then goes on to provide an example with a pointer as receiver:
func (p *ByteSlice) Append(data []byte) {
slice := *p
// Body as above, without the return.
*p = slice
}
Doesn't that contradict ? Or does it mean that this isn't valid:
type ByteSlice []byte
type Pb *ByteSlice
func (p Pb) Append(data []byte) []byte {
}
Though it looks just like a typedef!
Named pointer types could lead to ambiguities like so:
type T int
func (t *T) Get() T {
return *t + 1
}
type P *T
func (p P) Get() T {
return *p + 2
}
func F() {
var v1 T
var v2 = &v1
var v3 P = &v1
fmt.Println(v1.Get(), v2.Get(), v3.Get())
}
In the last case (esp. v3) per the current spec, it's ambiguous which Get() method should be called. Yes, there's no reason that the method resolution couldn't be defined, but it's one more detail to add to the language for something that already has a solution.
A named type that is a pointer isn't the same as a pointer to a named type.
This is what the language reference says:
That parameter section must declare a single parameter, the receiver.
Its type must be of the form T or *T (possibly using parentheses)
where T is a type name. The type denoted by T is called the receiver
base type; it must not be a pointer or interface type and it must be
declared in the same package as the method.
That's clear enough: if T denotes a pointer type, then you can't use it as a method receiver.
Note the careful language of the reference here: the phrase "Its type must be of the form T or T*" refers to the syntax-level specification of the method receiver (that is, the "form" of the type). That's different from the phrase "The type denoted by T", where it's talking about the type that T names (that is, what the type "denotes").
In Go, you can pass functions as parameters like callFunction(fn func). For example:
package main
import "fmt"
func example() {
fmt.Println("hello from example")
}
func callFunction(fn func) {
fn()
}
func main() {
callFunction(example)
}
But is it possible to call a function when it's a member of a struct? The following code would fail, but gives you an example of what I'm talking about:
package main
import "fmt"
type Example struct {
x int
y int
}
var example Example
func (e Example) StructFunction() {
fmt.Println("hello from example")
}
func callFunction(fn func) {
fn()
}
func main() {
callFunction(example.StructFunction)
}
(I know what I'm trying to do in that example is a little odd. The exact problem I have doesn't scale down to a simple example very well, but that's the essence of my problem. However I'm also intrigued about this from an academic perspective)
Methods (which are not "members of a struct" but methods of any named type, not only structs) are first class values. Go 1.0.3 didn't yet implemented method values but the tip version (as in the comming Go 1.1) has support method values. Quoting the full section here:
Method values
If the expression x has static type T and M is in the method set of type T, x.M is called a method value. The method value x.M is a function value that is callable with the same arguments as a method call of x.M. The expression x is evaluated and saved during the evaluation of the method value; the saved copy is then used as the receiver in any calls, which may be executed later.
The type T may be an interface or non-interface type.
As in the discussion of method expressions above, consider a struct type T with two methods, Mv, whose receiver is of type T, and Mp, whose receiver is of type *T.
type T struct {
a int
}
func (tv T) Mv(a int) int { return 0 } // value receiver
func (tp *T) Mp(f float32) float32 { return 1 } // pointer receiver
var t T
var pt *T
func makeT() T
The expression
t.Mv
yields a function value of type
func(int) int
These two invocations are equivalent:
t.Mv(7)
f := t.Mv; f(7)
Similarly, the expression
pt.Mp
yields a function value of type
func(float32) float32
As with selectors, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: pt.Mv is equivalent to (*pt).Mv.
As with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: t.Mv is equivalent to (&t).Mv.
f := t.Mv; f(7) // like t.Mv(7)
f := pt.Mp; f(7) // like pt.Mp(7)
f := pt.Mv; f(7) // like (*pt).Mv(7)
f := t.Mp; f(7) // like (&t).Mp(7)
f := makeT().Mp // invalid: result of makeT() is not addressable
Although the examples above use non-interface types, it is also legal to create a method value from a value of interface type.
var i interface { M(int) } = myVal
f := i.M; f(7) // like i.M(7)
Go 1.0 does not support the use of bound methods as function values. It will be supported in Go 1.1, but until then you can get similar behaviour through a closure. For example:
func main() {
callFunction(func() { example.StructFunction() })
}
It isn't quite as convenient, since you end up duplicating the function prototype but should do the trick.
I fixed your compile errors.
package main
import "fmt"
type Example struct {
x, y float64
}
var example Example
func (e Example) StructFunction() {
fmt.Println("hello from example")
}
func callFunction(fn func()) {
fn()
}
func main() {
callFunction(example.StructFunction)
}
Output:
hello from example
To add to #zzzz great answer (and the one given at https://golang.org/ref/spec#Method_values) here is an example that creates a method value from a value of an interface type.
package main
import "fmt"
type T struct{}
func (T) M(i int) { fmt.Println(i) }
func main() {
myVal := T{}
var i interface{ M(int) } = myVal
f := i.M
f(7) // like i.M(7)
}