On the book The Go Programming Language Phrasebook said:
If you require performance, then you can use statically
typed definitions and avoid the dynamic lookup.
If you require flexibility, then you can use the
late binding mechanism of interfaces
Can someone explain me what "statically typed definitions" and "dynamic lookup" are for methods and functions in Go?
Imagine that we have the following code:
type A struct {}
func (a A) Foo() {
fmt.Println("Foo called")
}
type I interface {
Foo()
}
I can now create a variable of type A and call this method:
a := A{}
a.Foo()
The compiler knows the static type of the variable, so knows that the method call refers to the A.Foo method. So it can compile the above code to use a direct call to A.Foo, which will be as fast as a normal function call.
If instead we use a variable of type I, things are different:
var i I = A{}
i.Foo()
The variable i can hold any type that has a Foo method. In this particular case it is holding an A value, but won't necessarily know this at compile time. So instead the compiler generates code to check the dynamic type of i, look up the associated Foo method and finally call that method. This form of dispatch is slower than the first, but has the benefit the code will work for any type implementing the interface.
This is similar to C++'s distinction between virtual and non-virtual methods, except rather than the type of dispatch being fixed for a method at its definition, it depends on the type of variable you use in Go.
What the book refers to when it says using static types is using non interface types:
func Foo(v int, s string) { ... }
The other option is using interface:
func Bar(a interface{}, b interface{}) { ... }
Because with the first option, Go will know at compile time what type of value the function will retrieve (int and string in this case), it will compile the code specifically for those types.
With the second option, you will have to use reflection at runtime in order to know the values contained in the interfaces. This is a bit of overhead, but it allows you to pass different types of values as parameters to the function, thus being more dynamic.
Further reading: Laws of Reflection in Go
Related
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.
when I generate go code from a protocol buffer file, I noticed that each of the generated structs implement the Message interface, https://github.com/golang/protobuf/blob/master/proto/lib.go#L277
For an example of the code that is generated, see https://github.com/google/go-genproto/blob/master/googleapis/rpc/status/status.pb.go#L97
Clearly, the other methods on Message interface, String() and Reset(), have an obvious purpose and the concrete implementation example makes that clear. However, I don't understand the purpose of the ProtoMessage() method. The method takes no parameters and returns no arguments, so why is it there?
Quoting from official doc: Protocol Buffers: Go Generated Code:
Given a simple message declaration:
message Foo {}
the protocol buffer compiler generates a struct called Foo. A *Foo implements the Message interface. See the inline comments for more information.
type Foo struct {
}
// Reset sets the proto's state to default values.
func (m *Foo) Reset() { *m = Foo{} }
// String returns a string representation of the proto.
func (m *Foo) String() string { return proto.CompactTextString(m) }
// ProtoMessage acts as a tag to make sure no one accidentally implements the
// proto.Message interface.
func (*Foo) ProtoMessage() {}
Note that all of these members are always present; the optimize_for option does not affect the output of the Go code generator.
This is a (similar) technique described in Go official FAQ: How can I guarantee my type satisfies an interface?
If you wish the users of an interface to explicitly declare that they implement it, you can add a method with a descriptive name to the interface's method set. For example:
type Fooer interface {
Foo()
ImplementsFooer()
}
A type must then implement the ImplementsFooer method to be a Fooer, clearly documenting the fact and announcing it in godoc's output.
type Bar struct{}
func (b Bar) ImplementsFooer() {}
func (b Bar) Foo() {}
Most code doesn't make use of such constraints, since they limit the utility of the interface idea. Sometimes, though, they're necessary to resolve ambiguities among similar interfaces.
So the ProtoMessage() method basically has 2 purposes:
The main purpose is as documented in the generated code: to make sure you don't pass a value for protobuf serialization / deserialization which otherwise (without this method) would implement the proto.Message interface but is not a "real" protobuf message value:
ProtoMessage acts as a tag to make sure no one accidentally implements the proto.Message interface.
And it also documents the fact that the type is a (implements) Message. This may not have a real value at first as the code is generated and you should not bother much with it, but code analyzers and Go IDE's may make good use of it; and it also appears in generated docs.
What I'm trying to do:
I have a library package which defines a few types, all implementing a given interface. Throughout the code, there are callbacks involved, and instead of writing the callback type everywhere I defined a type for it.
type Foo interface {
Bar()
}
type MyCallback func(f Foo)
type CoolFoo int
type BadFoo int
func (cf *CoolFoo) Bar(cb MyCallback) {
}
func (bf *BadFoo) Bar(cb MyCallback) {
}
Then later from client code using that library, I want to call using callbacks. If I call it by using the interface type it works:
cf := &CoolFoo{}
cf.Bar(func(f packageName.Foo) {
})
But I would rather have more self documenting code, and proper type hinting in my IDE, so I try to call it using the implementor type, such as this:
cf := &CoolFoo{}
cf.Bar(func(f packageName.CoolFoo) {
})
Which fails to compile, with the error:
cannot use func literal (type func(packageName.CoolFoo)) as type
packageName.MyCallback in argument to cf.Bar
Is it not possible, or am I making some dummy mistake ? I'm not very experienced in go dev, and I've tried looking around, but couldn't find the solution here.
What I've found is passing it as a Foo or an interface{} and then casting in the callback to what I want, but I would like to avoid it as it feels messy
Thanks for any help
The function signature is func(f Foo), so that is exactly what must be passed. The reason is simple: if your method expects a callback function of that signature, it can pass in any type that implements Foo. If you passed in a function that only accepted CoolFoo, the caller could still pass in BadFoo, because the types you've defined allow it. Thus, the compiler requires that the types match precisely. Likewise, if your callback function tries to cast to a concrete type, but that isn't the type that was passed to it, it will fail, because you're making an assumption that isn't supported by the code.
If your callback will only pass in CoolFoo, maybe that should be the type you use in the signature. If CoolFoo and BadFoo will each only pass in their respective types to their callbacks, maybe you need two different callback signatures. If they really are interchangeable, then the case you describe (a function taking CoolFoo specifically) isn't a problem with the language, it's a problem with your design.
package main
import "fmt"
type IA interface {
A()
B()
}
type AbsA struct {
IA
}
type CA struct {
AbsA
}
func (a CA) B() {
fmt.Println("Show B")
}
func (a AbsA) A() {
fmt.Println("Show A")
a.B()
}
func t7() {
var a IA
a = CA{}
a.A()
}
func main() {
t7()
}
Running this will get a nil pointer panic, because when AbsA calls B(), it will use the embedded IA, but IA is an interface.
Is there any way to make this work?
EDIT
I try to make a abstract super class, AbsA only implement part of interface, but it need to use other functions,A is a subclass of AbsA, only implement the rest part of interface.
But this looks bad, is there any other way to do this ? actually , I just want to do something like this
func (a IA)A(){
fmt.Println("Show A")
a.B()
}
but go can not use a interface as a receiver.
I don't know what you're trying to accomplish, but if you're defining structs with embedded interfaces, you need to initialize them.
http://play.golang.org/p/C32yWfNDRp
var a IA
a = CA{
AbsA: AbsA{
IA: CA{},
},
}
a.A()
If you're expecting OOP inheritance behavior, Go doesn't provide that. Embedding is only a form of automatic delegation.
No, at least not as you have it designed here. I'm not entirely sure what you're trying to accomplish but this is a very very poor use of embedding. The reason it doesn't work is because the CA type relies on the embedded AbsA type for method A() which then calls a 'method b' but the only actual implementation of B() has CA as the receiving type so once you've calling into the embedded types implementation of A() it can't call back up to the embedding types implementation of B().
You're treating embedding as if it's a two way street but in reality the embedding type knows all the methods on it's embed and the embed knows nothing about the embeddor. In my personal opinion, this should generate a compiler error. Perhaps in a future release it will (if I can tell at compile time that you will get a run-time error 100% of the time the compiler should able to too, right?).
As it is now you either need to implement B() with a receiver of type Absa or add another implementation of A() with a receiver of type CA. I'm not really sure what you're trying to accomplish so I can't say what the best course of action is. Note that if I were relating this to traditional OOP inheritance I would describe your code as attempting to access properties of a child class from within a parent class, clearly it's not possible. Absa will never be able to call up to an implementation of B() defined for the type embedding it. Just like trying to use a method defined on an inheriting type from the parent would never work or make sense only in most of the OOP languages you would get a compile time error.
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