I just started learning Go a few days ago.Today, we were debugging a piece of code for a while when we found something that seem counterintuitive of Go to do.
First we defined an interface and a data structure that implements it.
type Executer interface {
Execute()
}
type whatever struct {
name string
}
func (this *whatever) Execute() {
log.Println(this.name)
}
Now consider that I have a nil pointer to whatever and I try to call the method Execute. In other object-oriented languages I have used so far, this would call a null pointer error at the point of calling the method (i.e. w.Execute()) since the object pointer is null. Interestingly, in Go, the method is invoked, the null pointer error occurs at the Execute method when I try to dereference this.name. Why not at the point of calling the method?
func main() {
var w *whatever
w.Execute()
}
So, what I'm seeking to understand now is how is this possible? Does this mean that Go only does early method binding at compile time and at runtime there is no binding of the method with a specific object?
The receiver is just an "ordinary" argument to the function. Ordinary parameters may be of pointer types. When then are, you are allowed to pass nil as the argument, which is perfectly valid. All you need to keep in mind is not to dereference nil pointer arguments. The same applies to the special receiver parameter too. If it's a pointer, it may be nil, you just must not dereference it.
Spec: Method declarations:
The receiver is specified via an extra parameter section preceding the method name.
... The method is said to be bound to its receiver base type and the method name is visible only within selectors for type T or *T.
Allowing nil receiver values is not just something not forbidden, it has practical uses. For an example, see Test for nil values in nested stucts.
In Java you can call static methods on null objects too. It's true you can't do the same in Go, because Go does not have modifiers like static, public, private etc. In Go there are only exported and non-exported methods (implied by the first latter of their name).
But Go offers something similar too: Method expressions and Method values. If you have a method m with T receiver type, the expression T.m will be a function value whose signature contains the parameters and result types of m "prefixed" with the receiver type.
For example:
type Foo int
func (f Foo) Bar(s string) string { return fmt.Sprint(s, f) }
func main() {
fooBar := Foo.Bar // A function of type: func(Foo, string) string
res := fooBar(1, "Go")
fmt.Println(res)
}
Foo.Bar will be a function with type func (Foo, string) string, and you can call it like any other ordinary function; and you also have to pass the receiver as the first argument. The above app outputs (try it on the Go Playground):
Go1
Going a little "forward", we are not required to store Foo.Bar in a variable, we can directly call Foo.Bar:
fmt.Println(Foo.Bar(1, "Go"))
Which outputs the same (try it on the Go Playground). Now this almost looks like a static method call in Java.
And as the "final" step, when we use the above expression on a value of Foo itself (instead of the type identifier), we arrive at the method values, which saves the receiver and so the type of a method value does not include the receiver, its signature will be that of the method, and we can call it without having to pass a receiver:
var f Foo = Foo(1)
bar := f.Bar
fmt.Println(bar("Go"))
This again will output the same, try it on the Go Playground.
See related questions:
Pass method argument to function
golang function alias on method receiver
Related
As generics have been released in Go 1.18 pretty recently, I've started learning them. I generally get the concept, because I have some Java experience from the past. But I don't get some implementation specifics.
For instance: when it's more suitable to use any instead of interface{}? Here's an example:
func printInterface(foo interface{}) {
fmt.Printf("%v\n", foo)
}
func printAny[T any](foo T) {
fmt.Printf("%v\n", foo)
}
func (suite *TestSuite) TestString() {
printInterface("foo")
printAny("foo")
}
Both implementations work. However, if I try to print nil with any-version, I'll get a compile-time error:
cannot infer T.
https://go.dev/play/p/0gmU4rhhaOP
And I won't get this error if I try to print nil with interface{}-version.
So what's the use-case for any? When and which benefits does it bring, compared to simply using interface{}?
I'm asking to provide a specific example, where one implementation is objectively more suitable than another and/or where there is a specific benefit that can be evaluated.
Beside any and interface{} being type aliases — hence, equivalent in usage —, there is a practical difference between any as type parameter and any as regular function argument, as in your example.
The difference is that in printAny[T any](foo T) the type of foo is not any/interface{}, but it's T. And T after instantiation is a concrete type, that may or may not be an interface itself. You can then only pass arguments to an instantiated printAny that can be assigned to that concrete type.
How this impacts your code is most evident with multiple arguments. If we change the function signatures a bit:
func printInterface(foo, bar any) {
fmt.Println(foo, bar)
}
func printAny[T any](foo, bar T) {
fmt.Println(foo, bar)
}
After instantiation:
the function printAny accepts any two arguments of the same type — whichever is used to instantiate T
printInterface, which is equivalent to printInterface(foo, bar interface{}) can still accept two arguments of different types, since both would be individually assignable to any/interface{}.
printInterface(12.5, 0.1) // ok
printInterface(12.5, "blah") // ok, int and string individually assignable to any
printAny(10, 20) // ok, T inferred to int, 20 assignable to int
printAny(10, "k") // compiler error, T inferred to int, "k" not assignable to int
printAny[any](10, "k") // ok, T explicitly instantiated to any, int and string assignable to any
printAny(nil, nil) // compiler error, no way to infer T
printAny[any](nil, nil) // ok, T explicitly instantiated to any, nil assignable to any
A playground: https://go.dev/play/p/pDjP986cj96
Note: the generic version cannot be called with nil without explicit type arguments simply because nil alone doesn't carry type information, so the compiler can't infer T. However nil can be normally assigned to variables of interface type.
any is an alias for interface{}. Spec: Interface types:
For convenience, the predeclared type any is an alias for the empty interface.
Since it is an alias, it doesn't matter which one you use. They are one and the same. They are interchangeable. You can replace one with the other, the code will mean the same.
any is shorter and clearer, but only works from Go 1.18.
Since they are interchangeable, this also works:
func printInterface(foo any) {
fmt.Printf("%v\n", foo)
}
The reason why printAny() doesn't work is due to it being a generic function with a type parameter. To use it, it must be instantiated (its type parameter must be assigned with a known type). Trying to call it with nil carries no type information, so instantiation cannot happen, type inference won't work.
If you call it with a nil value that carries type info, it'll work, or if you specify the type param explicitly (try it on the Go Playground):
printAny((*int)(nil))
printAny[*int](nil)
// Or
var r io.Reader
printAny(r)
And as said, any is interchangeable with interface{}, so you'll have the same code if you swap both occurrences (try this one on the Go Playground):
func printInterface(foo any) {
fmt.Printf("%v\n", foo)
}
func printAny[T interface{}](foo T) {
fmt.Printf("%v\n", foo)
}
Your issue is not related to the usage of any/interface{} — whose difference is purely cosmetic — but it is type inference. As you can see from this playground, if you instantiate your function with an explicit type, like printAny[any](nil) it will work.
If you have a function with generic type you need to specify the types. However the go compiler is very smart and can infer some types for you. But nil alone is impossible to infer.
I was just messing around and wrote the below piece of code,
package main
import (
"fmt"
)
type Person struct {
name string
}
func (p Person) printName() {
fmt.Println(p.name)
}
type Man struct {
name string
f func()
}
func main() {
p := Person{name: "John"}
m := Man{name: "Adam"}
m.f = p.printName
p.printName()
m.f()
}
The above code results in the following output. This works across packages too.
John
John
So, here are my questions.
Why does this work?
Struct methods require receivers of the same type. How is the function still able to access members of the Person struct?
What happens when m.f = p.printName is executed in the above example?
This question deals mostly with receivers and may extend a bit to embedding.
From the relevant section of the spec:
A method is a function with a receiver. A method declaration binds an
identifier, the method name, to a method, and associates the method
with the receiver's base type.
MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
Receiver = Parameters .
The receiver is specified via an extra parameter section preceding the
method name. That parameter section must declare a single non-variadic
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 defined in the same package as the
method. The method is said to be bound to the base type and the method
name is visible only within selectors for type T or *T.
A non-blank receiver identifier must be unique in the method
signature. If the receiver's value is not referenced inside the body
of the method, its identifier may be omitted in the declaration. The
same applies in general to parameters of functions and methods.
For a base type, the non-blank names of methods bound to it must be
unique. If the base type is a struct type, the non-blank method and
field names must be distinct.
Given type Point, the declarations
func (p *Point) Length() float64 {
return math.Sqrt(p.x * p.x + p.y *p.y)
}
func (p *Point) Scale(factor float64) {
p.x *= factor
p.y *= factor
}
bind the methods Length and Scale, with receiver type *Point, to the
base type Point.
The type of a method is the type of a function with the receiver as
first argument.
For instance, the method Scale has type
func(p *Point, factor float64)
However, a function declared this way is not a method.
Man has a field named f that is a function that takes no arguments and returns nothing.
As we saw above golang internally treats
func (p Person) printName()
as
func printName(p Person)
and this can be considered as a function of no arguments when it acts on a Person struct (and it does because p is a Person and p.printName acts on p). Therefore it is allowed to be assigned to Man.f
So the moment you assigned the f field on the Man struct to a function that has captured the Person instance with name "John" and reads the name from that, therefore you get effect of the second "John" being printed.
The Man.name field has never come into play on that one.
I suspect that what you would expect as normal behaviour can be achieved with struct embedding of Person into a Man.
here is a playground link to demonstrate that
A method, in Go, is basically a function with a receiver. That receiver is wraped by the compiler, and beyond that, there is nothing more different from a normal function. That means, at anywhere, the method always gets the receiver which it is bound to, no matter how you call it, assign it to another variable or anythiing else.
In your code, f is not a method of type Man. It is merely a field of type func(). You can set to anything that match the signature, and the function will know nothing about Man or its instance. That means, m.f has no knowledge of m and no access to m.name or any other field of m.
And a note, you can call methods like: Person.PrintName(p) where p is of type Person.
function in Golang is also a value, it can be passed as parameter or assigned by other function too, method in Golang is also a function, with one different, it has receiver in it's value, so when use assign a method to a function (which have same signature) it's refer to the receiver and executing code in that method to destination function
I'm working on unit tests on a Go project, and I'm new to Go. So to start I wanted to test something easy. And I started with this function:
func (this *Service) InList(idPerson string, personsId []string) bool {
for _, personsId := range personsId {
if id == idPerson {
return true
}
}
return false
}
Service is a struct defined on top of the class.
This is the test I wrote:
func TestValidatePersonID(t *testing.T) {
personID := "12345"
personIDs := []string{"12345", "123456t", "1234567a"}
ok := *Service.InList(personID, personIDs)
if !ok {
t.Errorf("Id %v not found", personID)
}
}
If i try to Call Service without * I get the error:
invalid method expresion (needs pointer reciever)
If i try to call the function (*Service).inList, it says I'm missing an argument. I'm new to Go if anyone could point to me what I'm doing wrong and how Could I get a pointer receiver of that Service in my test?. I would appreciatte it.
The correct syntax for the method expression is:
ok := (*Service).InList(nil, personID, personIDs)
This snippet adds nil as the receiver argument and uses parentheses to specify the type correctly.
The approached used in the question is not idiomatic. Either call a method on a value
s := Service{}
ok := s.InList(personID, personIDs)
or convert the method to a function.
You have to call a method on an instance of its receiver type. So, for a method defined on *Service, you must call it on an instance of *Service:
var foo *Service
foo = &Service{}
foo.InList(personID, personIDs)
However, in your case, there's no reason for this to be a method; it doesn't seem to have anything at all to do with its receiver (it never references it), so it could just be a regular function. Also note that it's unidiomatic to name the receiver this in Go.
I also highly recommend at least taking the Go tour, which covers writing methods in detail, with interactive examples.
If you do not reference the receiver object, then you should not have one, keep your code as simple as possible.
There are three ways of writing a method or function, with each its own purpose.
without receiver, when no receiver is referenced in the function ( we call this a function )
a value receiver, the receiver is referenced, but not changed in the method ( we call this a method )
a pointer receiver, something in the receiver will be changed in the method
The Go FAQ answers a question regarding the choice of by-value vs. by-pointer receiver definition in methods. One of the statements in that answer is:
If some of the methods of the type must have pointer receivers, the rest should too, so the method set is consistent regardless of how the type is used.
This implies that if I have for a data type a few methods that mutate the data, thus require by-pointer receiver, I should use by-pointer receiver for all the methods defined for that data type.
On the other hand, the "fmt" package invokes the String() method as defined in the Stringer interface by value. If one defines the String() method with a receiver by-pointer it would not be invoked when the associated data type is given as a parameter to fmt.Println (or other fmt formatting methods). This leaves one no choice but to implement the String() method with a receiver by value.
How can one be consistent with the choice of by-value vs. by-pointer, as the FAQ suggests, while fulfilling fmt requirements for the Stringer interface?
EDIT:
In order to emphasize the essence of the problem I mention, consider a case where one has a data type with a set of methods defined with receiver by-value (including String()). Then one wishes to add an additional method that mutates that data type - so he defines it with receiver by-pointer, and (in order to be consistent, per FAQ answer) he also updates all the other methods of that data type to use by-pointer receiver. This change has zero impact on any code that uses the methods of this data type - BUT for invocations of fmt formatting functions (that now require passing a pointer to a variable instead of its value, as before the change). So consistency requirements are only problematic in the context of fmt. The need to adjust the manner one provides a variable to fmt.Println (or similar function) based on the receiver type breaks the capability to easily refactor one's package.
If you define your methods with pointer receiver, then you should use and pass pointer values and not non-pointer values. Doing so the passed value does indeed implement Stringer, and the fmt package will have no problem "detecting" and calling your String() method.
Example:
type Person struct {
Name string
}
func (p *Person) String() string {
return fmt.Sprintf("Person[%s]", p.Name)
}
func main() {
p := &Person{Name: "Bob"}
fmt.Println(p)
}
Output (try it on the Go Playground):
Person[Bob]
If you would pass a value of type Person to fmt.Println() instead of a pointer of type *Person, yes, indeed the Person.String() would not be called. But if all methods of Person has pointer receiver, that's a strong indication that you should use the type and its values as pointers (unless you don't intend its methods to be used).
Yes, you have to know whether you have to use Person or *Person. Deal with it. If you want to write correct and efficient programs, you have to know a lot more than just whether to use pointer or non-pointer values, I don't know why this is a big deal for you. Look it up if you don't know, and if you're lazy, use a pointer as the method set of (the type of) a pointer value contains methods with both pointer and non-pointer receiver.
Also the author of Person may provide you a NewPerson() factory function which you can rely on to return the value of the correct type (e.g. Person if methods have value receivers, and *Person if the methods have pointer receivers), and so you won't have to know which to use.
Answer to later adding a method with pointer receiver to a type which previously only had methods with value receiver:
Yes, as you described in the question, that might not break existing code, yet continuing to use a non-pointer value may not profit from the later added method with pointer receiver.
We might ask: is this a problem? When the type was used, the new method you just added didn't exist. So the original code made no assumption about its existence. So it shouldn't be a problem.
Second consideration: the type only had methods with value receiver, so one could easily assume that by their use, the value was immutable as methods with value receiver cannot alter the value. Code that used the type may have built on this, assuming it was not changed by its methods, so using it from multiple goroutines may have omitted certain synchronization rightfully.
So I do think that adding a new method with pointer receiver to a type that previously only had methods with value receiver should not be "opaque", the person who adds this new method has the responsibility to either modify uses of this type to "switch" to pointers and make sure the code remains safe and correct, or deal with the fact that non-pointer values will not have this new method.
Tips:
If there's a chance that a type may have mutator methods in the future, you should start creating it with methods with pointer receivers. Doing so you avoid later having to go through the process described above.
Another tip could be to hide the type entirely, and only publish interfaces. Doing so, the users of this type don't have to know whether the interface wraps a pointer or not, it just doesn't matter. They receive an interface value, and they call methods of the interface. It's the responsibility of the package author to take care of proper method receivers, and return the appropriate type that implements the interface. The clients don't see this and they don't depend on this. All they see and use is the interface.
In order to emphasize the essence of the problem I mention, consider a case where one has a data type with a set of methods defined with receiver by-value (including String()). Then one wishes to add an additional method that mutates that data type - so he defines it with receiver by-pointer, and (in order to be consistent, per FAQ answer) he also updates all the other methods of that data type to use by-pointer receiver. This change has zero impact on any code that uses the methods of this data type - BUT for invocations of fmt formatting functions (that now require passing a pointer to a variable instead of its value, as before the change).
This is not true. All interface of it and some of type assertions will be affected as well - that is why fmt is affected. e.g. :
package main
import (
"fmt"
)
type I interface {
String() string
}
func (t t) String() string { return "" }
func (p *p) String() string { return "" }
type t struct{}
type p struct{}
func S(i I) {}
func main() {
fmt.Println("Hello, playground")
T := t{}
P := p{}
_ = P
S(T)
//S(P) //fail
}
To understand this from the root, you should know that a pointer method and a value method is different from the very base. However, for convenience, like the omit of ;, golang compiler looks for cases using pointer methods without a pointer and change it back.
As explained here: https://tour.golang.org/methods/6
So back to the orignal question: consistency of pointer methods. If you read the faq more carefully, you will find it is the very last part of considering to use a value or pointer methods. And you can find counter-example in standard lib examples, in container/heap :
// A PriorityQueue implements heap.Interface and holds Items.
type PriorityQueue []*Item
func (pq PriorityQueue) Len() int { return len(pq) }
func (pq PriorityQueue) Less(i, j int) bool {
// We want Pop to give us the highest, not lowest, priority so we use greater than here.
return pq[i].priority > pq[j].priority
}
func (pq PriorityQueue) Swap(i, j int) {
pq[i], pq[j] = pq[j], pq[i]
pq[i].index = i
pq[j].index = j
}
func (pq *PriorityQueue) Push(x interface{}) {
n := len(*pq)
item := x.(*Item)
item.index = n
*pq = append(*pq, item)
}
func (pq *PriorityQueue) Pop() interface{} {
old := *pq
n := len(old)
item := old[n-1]
item.index = -1 // for safety
*pq = old[0 : n-1]
return item
}
// update modifies the priority and value of an Item in the queue.
func (pq *PriorityQueue) update(item *Item, value string, priority int) {
item.value = value
item.priority = priority
heap.Fix(pq, item.index)
}
So, indeed, as the FAQ say, to determine whether to use a pointer methods, take the following consideration in order:
Does the method need to modify the receiver? If yes, use a pointer. If not, there should be a good reason or it makes confusion.
Efficiency. If the receiver is large, a big struct for instance, it will be much cheaper to use a pointer receiver. However, efficiency is not easy to discuss. If you think it is an issue, profile and/or benchmark it before doint so.
Consistency. If some of the methods of the type must have pointer receivers, the rest should too, so the method set is consistent regardless of how the type is used. This, to me, means that if the type shall be used as a pointer (e.g., frequent modify), it should use the method set to mark so. It does not mean one type can only have solely pointer methods or the other way around.
The previous answers here do not address the underlying issue, although the answer from leaf bebop is solid advice.
Given a value, you can in fact invoke either pointer or value receiver methods on it, the compiler will do that for you. However, that does not apply when invoking via interface implementations.
This boils down to this dicussion about interface implementations.
In that discussion the discussion is about implementing interfaces with nil pointers. But the underlying discussion revolves around the same issue: when implementing an interface you must choose the pointer or the value type, and there will be no attempt by the compiler, nor can there be any attempt in golang code, to figure out exactly what type it is, and adjust the interface call accordingly.
So for example, when calling
fmt.Println(object)
you are implementing the arg of type interface{} with object of type X. The fmt code within has no interest in knowing whether the type of object is a pointer type or not. It will not even be able to tell without using reflection. It will simply call String() on whatever type that is.
So if you supplied a value of type X, and there just so happens to be a (*X) String() string method, that does not matter, that method will not be called, it will only type-assert whether that type X implements Stringer, it has no interest if type *X asserts Stringer. Since there is no (X) String() string method, it will move on. It will not attempt to check what X may happen to be, whether it's a pointer type, and if not, whether the associated pointer type implements Stringer, and call that String() method instead.
So this is not really a pointer vs value methods issue, this is an interface implementation issue when implementing interface{} in calls to fmt methods.
This code works fine:
feedService := postgres.FeedService{}
feeds, err := feedService.GetAllRssFeeds()
But this code gives me error:
feeds, err = postgres.FeedService{}.GetAllRssFeeds()
controllers\feed_controller.go:35: cannot call pointer method on
postgres.FeedService literal controllers\feed_controller.go:35: cannot
take the address of postgres.FeedService literal
Why this two pieces of code is not equal ?
Here is a struct declaration:
type FeedService struct {
}
func (s *FeedService) GetAllRssFeeds() ([]*quzx.RssFeed, error) {
Your FeedService.GetAllRssFeeds() method has pointer receiver, so a pointer to FeedService is needed to call this method.
In your first example you use a short variable declaration to store a FeedService struct value in a local variable. Local variables are addressable, so when you write feedService.GetAllRssFeeds() after that, the compiler will automatically take the address of feedService and use that as the receiver value. It is a shorthand for:
feeds, err := (&feedService).GetAllRssFeeds()
It is in Spec: Calls:
If x is addressable and &x's method set contains m, x.m() is shorthand for (&x).m().
In your second example you don't create a local variable, you just use a struct composite literal, but by itself it is not (automatically) addressable, so the compiler cannot obtain a pointer to FeedService value to be used as the receiver, and hence cannot call the method.
Note that it is allowed to take the address of a composite literal explicitly, so the following also works:
feeds, err := (&postgres.FeedService{}).GetAllRssFeeds()
This is in Spec: Composite literals:
Taking the address of a composite literal generates a pointer to a unique variable initialized with the literal's value.
See related questions:
What is the method set of `sync.WaitGroup`?
Calling a method with a pointer receiver by an object instead of a pointer to it?