Consider the following example:
type House struct{}
func main() {
house1 := new(House)
house2 := &House{}
fmt.Printf("%T | %T\n", house1, house2)
}
Output: *main.House | *main.House
Both assignments produce a pointer to type House (from package main).
From the go docu of new:
// The new built-in function allocates memory. The first argument is a type,
// not a value, and the value returned is a pointer to a newly
// allocated zero value of that type.
Is memory allocation in both assignments technically the same? Is there a best practice?
Is memory allocation in both assignments technically the same?
I'm not going to talk about implementation details, since those may or may not change.
From the point of view of the language specifications, there is a difference.
Allocating with new(T) follows the rules of allocation (my own note in [italics]):
The built-in function new takes a type T, allocates storage for a variable of that type at run time, and returns a value of type *T pointing to it. The variable is initialized as described in the section on initial values. [its zero value]
Allocating with a composite literal as &T{} follows the rules of composite literals and addressing:
Composite literals construct values for structs, arrays, slices, and maps and create a new value each time they are evaluated.
Taking the address of a composite literal generates a pointer to a unique variable initialized with the literal's value.
So both new(T) and &T{} allocate storage and yield values of type *T. However:
new(T) accepts any type, including predeclared identifiers as int and bool, which may come in handy if you just need to initialize such vars with a one-liner:
n := new(int) // n is type *int and points to 0
b := new(bool) // b is type *bool and points to false
new (quoting Effective Go) "does not initialize1 that memory, it only zeros it". I.e. the memory location will have the type's zero value.
composite literals can be used only for structs, arrays, slices and maps
composite literals can construct non-zero values by explicitly initializing struct fields or array/slice/map items
Bottom line: as Effective Go points out (same paragraph as above):
if a composite literal contains no fields at all, it creates a zero value for the type. The expressions new(T) and &T{} are equivalent.
[1]: there's an inconsistency in the use of the term "initialization" in the specs and Effective Go. The take-away is that with new you can only yield a zero-value, whereas a composite literal allows you to construct non-zero values. And obviously the specs are the source of truth.
I just want to confirm if my understanding is correct about interface{}{}
Does interface{}{} mean a composite literal of interface type?
So, if I wanted to pass a composite type, lets say []byte as a interface{}, I should assign it to a variable of type interface{}{} and pass in that variable, whereas if I wanted to pass a non composite type, such as a int as a interface{}, I should assign it to a variable of type interface{} and pass in that variable.
Is my understanding correct on this?
interface{}{} is invalid syntax.
It is not an empty interface composite literal — there's no such thing. The following don't compile:
var a interface{}{} = "foo" // not a type
b := interface{}{} // not a value
From the specs Composite Literals:
Composite literals construct values for structs, arrays, slices, and maps and create a new value each time they are evaluated.
The valid syntax is interface{}, which is the empty interface, i.e. an interface with no name and empty method set.
If you compare it to a named interface, you will see that the extra set of curly braces at the end makes no sense:
type Fooer interface {
Foo() error
}{} // ???
You can instantiate an empty interface{} by converting nil:
a := (interface{})(nil)
Or you can declare a var of unnamed interface type:
type A struct {}
func (a A) Foo() string {
return "foo"
}
var a interface{ Foo() string } = A{}
To answer your question:
So, if I wanted to pass a composite type [...]
you can assign any value to a variable of type interface{}, that's about it. Whether the value is composite or not is irrelevant because all types satisfy the empty interface:
var a interface{} = []byte{0x01, 0x02}
var b interface{} = "string_literal"
Playground: https://play.golang.org/p/w-l1dU-6Hb5
The empty interface interface{} essentially says "I don't care". Anything can be passed as an empty interface. So for your examples they all could be stored as the empty interface.
Possibly a more interesting question is why you want to avoid types in the first place, and what you're trying to achieve. But for the purposes of using interface{} you can pass anything to it, even a "nil".
interface{} type of empty interface.
you assign []byte or int to any empty variable of empty interface (interface{} type) or you can pass it directly to function that exepts interface values.
I'm trying to understand this piece code of an ORM library, but I cannot understand what is meaning of (*User)(nil)? First parenthesis is a pointer to User struct, then what is 2nd parenthesis stand for?
type User struct {
Id int64
Name string
Emails []string
}
for _, model := range []interface{}{(*User)(nil), (*Story)(nil)} {
err := db.CreateTable(model, &orm.CreateTableOptions{
// ....
}
}
In Go, nils can by typed, so that a nil of the type *User is different from a nil of the type *Story. So (*User)(nil) is actually a type conversion that makes a typed nil pointer. Further more, the typed pointer is then implicitly converted to interface{} according to the CreateTable signature. Interfaces always store their type along with the data, and that information can be accessed in runtime using reflect, like it happens in ORM.
Because in Go types are not "first-class citizens", i.e. you can't just pass a type around as a value, they use a typed nil pointer idiom that essentially allows providing the desired model type as an argument to CreateTable without creating an instance of it or messing with reflect in the client code.
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?
I'm new to interfaces and trying to do SOAP request by github
I don't understand the meaning of
Msg interface{}
in this code:
type Envelope struct {
Body `xml:"soap:"`
}
type Body struct {
Msg interface{}
}
I've observed the same syntax in
fmt.Println
but don't understand what's being achieved by
interface{}
Note: Go 1.18 (Q1 2022) does rename interface{} to any (alias for interface{}).
See issue 49884, CL 368254 and commit 2580d0e.
See the last part of this answer.
You can refer to the article "How to use interfaces in Go" (based on "Russ Cox’s description of interfaces"):
What is an interface?
An interface is two things:
it is a set of methods,
but it is also a type
The interface{} type (or any with Go 1.18+), the empty interface is the interface that has no methods.
Since there is no implements keyword, all types implement at least zero methods, and satisfying an interface is done automatically, all types satisfy the empty interface.
That means that if you write a function that takes an interface{} value as a parameter, you can supply that function with any value.
(That is what Msg represents in your question: any value)
func DoSomething(v interface{}) {
// ...
}
func DoSomething(v any) {
// ...
}
Here’s where it gets confusing:
inside of the DoSomething function, what is v's type?
Beginner gophers are led to believe that “v is of any type”, but that is wrong.
v is not of any type; it is of interface{} type.
When passing a value into the DoSomething function, the Go runtime will perform a type conversion (if necessary), and convert the value to an interface{} value.
All values have exactly one type at runtime, and v's one static type is interface{} (or any with Go 1.18+).
An interface value is constructed of two words of data:
one word is used to point to a method table for the value’s underlying type,
and the other word is used to point to the actual data being held by that value.
Addendum: This is were Russ's article is quite complete regarding an interface structure:
type Stringer interface {
String() string
}
Interface values are represented as a two-word pair giving a pointer to information about the type stored in the interface and a pointer to the associated data.
Assigning b to an interface value of type Stringer sets both words of the interface value.
The first word in the interface value points at what I call an interface table or itable (pronounced i-table; in the runtime sources, the C implementation name is Itab).
The itable begins with some metadata about the types involved and then becomes a list of function pointers.
Note that the itable corresponds to the interface type, not the dynamic type.
In terms of our example, the itable for Stringer holding type Binary lists the methods used to satisfy Stringer, which is just String: Binary's other methods (Get) make no appearance in the itable.
The second word in the interface value points at the actual data, in this case a copy of b.
The assignment var s Stringer = b makes a copy of b rather than point at b for the same reason that var c uint64 = b makes a copy: if b later changes, s and c are supposed to have the original value, not the new one.
Values stored in interfaces might be arbitrarily large, but only one word is dedicated to holding the value in the interface structure, so the assignment allocates a chunk of memory on the heap and records the pointer in the one-word slot.
Issue 33232 seems to point out to any as an alias to interface{} in Go 1.18 (Q1 2022)
Russ Cox explains:
'any' being only for constraints is a detail that will be in every writeup of generics - books, blog posts, and so on.
If we think we are likely to allow it eventually, it makes sense to allow it from the start and avoid invalidating all that written material.
'any' being only for constraints is an unexpected cut-out that reduces generality and orthogonality of concepts.
It's easy to say "let's just wait and see", but prescribing uses tends to create much more jagged features than full generality. We saw this with type aliases as well (and resisted almost all the proposed cut-outs, thankfully).
If 'any' is allowed in generics but not non-generic code, then it might encourage people to overuse generics simply because 'any' is nicer to write than 'interface{}', when the decision about generics or not should really be made by considering other factors.
If we allow 'any' for ordinary non-generic usage too, then seeing interface{} in code could serve as a kind of signal that the code predates generics and has not yet been reconsidered in the post-generics world.
Some code using interface{} should use generics. Other code should continue to use interfaces.
Rewriting it one way or another to remove the text 'interface{}' would give people a clear way to see what they'd updated and hadn't. (Of course, some code that might be better with generics must still use interface{} for backwards-compatibility reasons, but it can still be updated to confirm that the decision was considered and made.)
That thread also includes an explanation about interface{}:
It's not a special design, but a logical consequence of Go's type declaration syntax.
You can use anonymous interfaces with more than zero methods:
func f(a interface{Foo(); Bar()}) {
a.Foo()
a.Bar()
}
Analogous to how you can use anonymous structs anywhere a type is expected:
func f(a struct{Foo int; Bar string}) {
fmt.Println(a.Foo)
fmt.Println(a.Bar)
}
An empty interface just happens to match all types because all types have at least zero methods.
Removing interface{} would mean removing all interface functionality from the language if you want to stay consistent / don't want to introduce a special case.
interface{} means you can put value of any type, including your own custom type. All types in Go satisfy an empty interface (interface{} is an empty interface).
In your example, Msg field can have value of any type.
Example:
package main
import (
"fmt"
)
type Body struct {
Msg interface{}
}
func main() {
b := Body{}
b.Msg = "5"
fmt.Printf("%#v %T \n", b.Msg, b.Msg) // Output: "5" string
b.Msg = 5
fmt.Printf("%#v %T", b.Msg, b.Msg) //Output: 5 int
}
Go Playground
There are already good answers here. Let me add my own too for others who want to understand it intuitively:
Interface
Here's an interface with one method:
type Runner interface {
Run()
}
So any type that has a Run() method satisfies the Runner interface:
type Program struct {
/* fields */
}
func (p Program) Run() {
/* running */
}
func (p Program) Stop() {
/* stopping */
}
Although the Program type has also a Stop method, it still satisfies the Runner interface because all that is needed is to have all of the methods of an interface to satisfy it.
So, it has a Run method and it satisfies the Runner interface.
Empty Interface
Here's a named empty interface without any methods:
type Empty interface {
/* it has no methods */
}
So any type satisfies this interface. Because, no method is needed to satisfy this interface. For example:
// Because, Empty interface has no methods, following types satisfy the Empty interface
var a Empty
a = 5
a = 6.5
a = "hello"
But, does the Program type above satisfy it? Yes:
a = Program{} // ok
interface{} is equal to the Empty interface above.
var b interface{}
// true: a == b
b = a
b = 9
b = "bye"
As you see, there's nothing mysterious about it but it's very easy to abuse. Stay away from it as much as you can.
https://play.golang.org/p/A-vwTddWJ7G
It's called the empty interface and is implemented by all types, which means you can put anything in the Msg field.
Example :
body := Body{3}
fmt.Printf("%#v\n", body) // -> main.Body{Msg:3}
body = Body{"anything"}
fmt.Printf("%#v\n", body) // -> main.Body{Msg:"anything"}
body = Body{body}
fmt.Printf("%#v\n", body) // -> main.Body{Msg:main.Body{Msg:"anything"}}
This is the logical extension of the fact that a type implements an interface as soon as it has all methods of the interface.
From the Golang Specifications:
An interface type specifies a method set called its interface. A
variable of interface type can store a value of any type with a method
set that is any superset of the interface. Such a type is said to
implement the interface. The value of an uninitialized variable of
interface type is nil.
A type implements any interface comprising any subset of its methods
and may therefore implement several distinct interfaces. For instance,
all types implement the empty interface:
interface{}
The concepts to graps are:
Everything has a Type. You can define a new type, let's call it T. Let's say now our Type T has 3 methods: A, B, C.
The set of methods specified for a type is called the "interface type". Let's call it in our example: T_interface. Is equal to T_interface = (A, B, C)
You can create an "interface type" by defining the signature of the methods. MyInterface = (A, )
When you specify a variable of type, "interface type", you can assign to it only types which have an interface that is a superset of your interface.
That means that all the methods contained in MyInterface have to be contained inside T_interface
You can deduce that all the "interface types" of all the types are a superset of the empty interface.
An example that extends the excellent answer by #VonC and the comment by #NickCraig-Wood. interface{} can point to anything and you need a cast/type assertion to use it.
package main
import (
. "fmt"
"strconv"
)
var c = cat("Fish")
var d = dog("Bone")
func main() {
var i interface{} = c
switch i.(type) {
case cat:
c.Eat() // Fish
}
i = d
switch i.(type) {
case dog:
d.Eat() // Bone
}
i = "4.3"
Printf("%T %v\n", i, i) // string 4.3
s, _ := i.(string) // type assertion
f, _ := strconv.ParseFloat(s, 64)
n := int(f) // type conversion
Printf("%T %v\n", n, n) // int 4
}
type cat string
type dog string
func (c cat) Eat() { Println(c) }
func (d dog) Eat() { Println(d) }
i is a variable of an empty interface with a value cat("Fish"). It is legal to create a method value from a value of interface type. See https://golang.org/ref/spec#Interface_types.
A type switch confirms i interface type is cat("Fish") . See https://golang.org/doc/effective_go.html#type_switch. i is then reassigned to dog("Bone"). A type switch confirms that i interface’s type has changed to dog("Bone") .
You can also ask the compiler to check that the type T implements the interface I by attempting an assignment: var _ I = T{}. See https://golang.org/doc/faq#guarantee_satisfies_interface and https://stackoverflow.com/a/60663003/12817546.
All types implement the empty interface interface{}. See https://talks.golang.org/2012/goforc.slide#44 and https://golang.org/ref/spec#Interface_types . In this example, i is reassigned, this time to a string "4.3".i is then assigned to a new string variable s with i.(string) before s is converted to a float64 type f using strconv. Finally f is converted to n an int type equal to 4. See What is the difference between type conversion and type assertion?
Go's built-in maps and slices, plus the ability to use the empty interface to construct containers (with explicit unboxing) mean in many cases it is possible to write code that does what generics would enable, if less smoothly. See https://golang.org/doc/faq#generics.
Interface is a type which is unknown at compile time
It is a contract between object and the struct type to satisfy with common functionality
or
common functionality acting on different types of struct objects
for example in the below code PrintDetails is a common functionality acting on different types of structs as Engineer,Manager,
Seniorhead
please find the example code
interface examplehttps://play.golang.org/p/QnAqEYGiiF7
A method can bind to any type(int, string, pointer, and so on) in GO
Interface is a way of declear what method one type should have, as long as A type has implement those methods, this can be assigned to this interface.
Interface{} just has no declear of method, so it can accept any type