I have the following structures:
type Type interface {
getFoo() []byte
}
type Concrete struct {
}
func (this *Concrete) getFoo() []byte {
example := []byte{2, 3, 4}
return example
}
Now I have some array of Type interfaces, e.g.:
var arr []*Type
And I want to create array of concrete structures and initialize the above array with it, e.g.:
var cObjArr []*Concrete
cObj := new(Concrete)
cObjArr = append(cObjArr, cObj)
arr = cObj
But it gives me an error that cannot use type []*Concrete as type []*Type in assignment. What's wrong?
There are a few problems here.
First,
type Type interface{} {
getFoo() []byte
}
should be
type Type interface {
getFoo() []byte
}
I assume that is a result of trying to show a small, reproducible example.
The next is that arr should be a slice of type Type, not *Type. A pointer to an interface is VERY rarely what you actually mean.
So, your arr is now a slice of Type... []Type. For the remainder of the current scope arr will always HAVE TO BE of type []Type.
cObjArr is of type []*Concrete. That's fine, but the value of cObjArr can not be assigned to arr since it is a different type.
You have a couple options here.
Instead of appending to cObjArr, just append to arr
https://play.golang.org/p/m3-83s6R5c
Or iterate through cObjArr and append to arr
https://play.golang.org/p/wvWaChcOWY
First of all, you should declare arr as []Type instead of []*Type. The objects you want to put in the array are Type, not *Type (*Concrete implements the Type interface).
Based on the error, your code must be trying to do arr = cObjArr. You can't do that because the types of the two slice objects are different. Instead, you can append cObj directly to arr.
Related
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.
Not able to figure out how to convert interface{} returned from function into an array of structs
As part of some practise i was trying to create a function which can take 2 slices of some type and concatenates both and returns the slice.
The code can be found here - https://play.golang.org/p/P9pfrf_qTS1
type mystruct struct {
name string
value string
}
func appendarr(array1 interface{}, array2 interface{}) interface{} {
p := reflect.ValueOf(array1)
q := reflect.ValueOf(array2)
r := reflect.AppendSlice(p, q)
return reflect.ValueOf(r).Interface()
}
func main() {
fmt.Println("=======")
array1 := []mystruct{
mystruct{"a1n1", "a1v1"},
mystruct{"a1n2", "a1v2"},
}
array2 := []mystruct{
mystruct{"a2n1", "a2v1"},
mystruct{"a2n2", "a2v2"},
}
arrayOp := appendarr(array1, array2)
fmt.Printf("arr: %#v\n", arrayOp) // this shows all the elements from array1 and 2
val := reflect.ValueOf(arrayOp)
fmt.Println(val) // output is <[]main.mystruct Value>
fmt.Println(val.Interface().([]mystruct)) // exception - interface {} is reflect.Value, not []main.mystruct
}
I may have slices of different types of structs. I want to concatenate them and access the elements individually.
If there is any other way of achieving the same, please do let me know.
reflect.Append() returns a value of type reflect.Value, so you don't have to (you shouldn't) pass that to reflect.ValueOf().
So simply change the return statement to:
return r.Interface()
With this it works and outputs (try it on the Go Playground):
=======
arr: []main.mystruct{main.mystruct{name:"a1n1", value:"a1v1"}, main.mystruct{name:"a1n2", value:"a1v2"}, main.mystruct{name:"a2n1", value:"a2v1"}, main.mystruct{name:"a2n2", value:"a2v2"}}
[{a1n1 a1v1} {a1n2 a1v2} {a2n1 a2v1} {a2n2 a2v2}]
[{a1n1 a1v1} {a1n2 a1v2} {a2n1 a2v1} {a2n2 a2v2}]
You also don't need to do any reflection-kungfu on the result: it's your slice wrapped in interface{}. Wrapping it in reflect.Value and calling Value.Interface() on it is just a redundant cycle. You may simply do:
arrayOp.([]mystruct)
On a side note: you shouldn't create a "generic" append() function that uses reflection under the hood, as this functionality is available as a built-in function append(). The builtin function is generic, it gets help from the compiler so it provides the generic nature at compile-time. Whatever you come up with using reflection will be slower.
var list = []func(*someType) error {
...
}
I am new to Go and I am trying to understand what does the syntax mean ?
Is the return of function an array?
This declares and initializes a variable list as a slice whose elements are functions with signature func(*someType) error.
Slices in Go are convenient mechanisms for representing sequences of data of a particular type. They have type []T for any element type T (but remember Go does not have generics). A slice is defined only by the type of the items it contains; its length is not part of its type definition and can change at runtime. (Arrays in Go, by contrast, are of fixed length - their type is [N]T for length N and element type T).
Under the surface, a slice consists of a backing array, a length of the current data and a capacity. The runtime manages the memory allocation of the array to accommodate all data in the slice.
func in go is a type like int,string...
So they are sample syntax:
var listInt := []int{1,2,3}
var listStr := []string{"1","2","3"}
var listFunc := []func(param anyType) anyType {
func(param anyType) anyType { ... return new(anyType) },
func(param anyType) anyType { ... return new(anyType) },
}
I'm writing a test that a JSON list is empty.
{"matches": []}
The object has type map[string]interface{}, and I want to test that the list is empty.
var matches := response["matches"]
if len(matches) != 0 {
t.Errorf("Non-empty match list!")
}
However I'm told at compile time that this is invalid
invalid argument matches (type interface {}) for len
If I try casting to a list type:
matches := response["matches"].([]string)
I get a panic:
panic: interface conversion: interface is []interface {}, not []string [recovered]
What do I want to write here?
JSON parsing with maps in Go uses interfaces everywhere. Imagine you have the following JSON object:
{
"stuff" : [
"stuff1",
"stuff2",
"stuff3",
]
}
The Go JSON library will parse the outer object as a map from keys to values, as you've seen in your code. It maps variable names as keys to the values that correspond to those variable names. However, since it has no way of knowing ahead of time what those values are, the value type of the map is simply interface{}. So let's say you know there's a key called "stuff", and you know that its value is an array. You could do:
arr := myMap["stuff"]
And you know that it's an array type, so you can actually instead do:
arr := myMap["stuff"].([]interface{})
the problem here is that while you're right that it's an array, and the JSON library knows this, it has no way of knowing that every element will be of type string, so there's no way for it to decide that the array type should actually be []string. Imagine if you had done this instead:
{
"stuff" : [
"stuff1",
"stuff2",
3
]
}
Well "stuff" can't now be an array of strings because one of the elements isn't a string. In fact, it can't be an array of anything - there's no single type that would satisfy the types of all of the elements. So that's why the Go JSON library has no choice but to leave it as []interface{}. Luckily, since all you want is the length, you're already done. You can just do:
arr := myMap["stuff"].([]interface{})
l := len(arr)
Now that's all fine and good, but let's say that down the road you want to actually look at one of the elements. You could now take out an element and, knowing that it's a string, do:
arr := myMap["stuff"].([]interface{})
iv := arr[0] // interface value
sv := iv.(string) // string value
NOTE
When I say "array," I mean array in the JSON sense - these are JSON arrays. The data structure that represents them in Go is called a "slice" (Go has arrays too, but they're a separate thing - if you're used to arrays in languages like C or Java, Go slices are the closest analogue).
When dealing with JSON, you can add type declarations for array and object, then add methods as needed to help with conversion:
package main
import "encoding/json"
type (
array []interface{}
object map[string]interface{}
)
func (o object) a(s string) array {
return o[s].([]interface{})
}
func main() {
data := []byte(`{"matches": []}`)
var response object
json.Unmarshal(data, &response)
matches := response.a("matches")
mLen := len(matches)
println(mLen == 0)
}
https://golang.org/ref/spec#Type_declarations
I have an interface Model, which is implemented by struct Person.
To get a model instance, I have the following helper functions:
func newModel(c string) Model {
switch c {
case "person":
return newPerson()
}
return nil
}
func newPerson() *Person {
return &Person{}
}
The above approach allows me to return a properly typed Person instance (can easily add new models later with same approach).
When I attempted to do something similar for returning a slice of models, I get an error. Code:
func newModels(c string) []Model {
switch c {
case "person":
return newPersons()
}
return nil
}
func newPersons() *[]Person {
var models []Person
return &models
}
Go complains with: cannot use newPersons() (type []Person) as type []Model in return argument
My goal is to return a slice of whatever model type is requested (whether []Person, []FutureModel, []Terminator2000, w/e). What am I missing, and how can I properly implement such a solution?
This is very similar to a question I just answered: https://stackoverflow.com/a/12990540/727643
The short answer is that you are correct. A slice of structs is not equal to a slice of an interface the struct implements.
A []Person and a []Model have different memory layouts. This is because the types they are slices of have different memory layouts. A Model is an interface value which means that in memory it is two words in size. One word for the type information, the other for the data. A Person is a struct whose size depends on the fields it contains. In order to convert from a []Person to a []Model, you will need to loop over the array and do a type conversion for each element.
Since this conversion is an O(n) operation and would result in a new slice being created, Go refuses to do it implicitly. You can do it explicitly with the following code.
models := make([]Model, len(persons))
for i, v := range persons {
models[i] = Model(v)
}
return models
And as dskinner pointed out, you most likely want a slice of pointers and not a pointer to a slice. A pointer to a slice is not normally needed.
*[]Person // pointer to slice
[]*Person // slice of pointers
Maybe this is an issue with your return type *[]Person, where it should actually be []*Person so to reference that each index of the slice is a reference to a Person, and where a slice [] is in itself a reference to an array.
Check out the following example:
package main
import (
"fmt"
)
type Model interface {
Name() string
}
type Person struct {}
func (p *Person) Name() string {
return "Me"
}
func NewPersons() (models []*Person) {
return models
}
func main() {
var p Model
p = new(Person)
fmt.Println(p.Name())
arr := NewPersons()
arr = append(arr, new(Person))
fmt.Println(arr[0].Name())
}
As Stephen already answered the question and you're a beginner I emphasize on giving advises.
A better way of working with go's interfaces is not to have a constructor returning
the interface as you might be used to from other languages, like java, but to have
a constructor for each object independently, as they implement the interface implicitly.
Instead of
newModel(type string) Model { ... }
you should do
newPerson() *Person { ... }
newPolitician() *Politician { ... }
with Person and Politician both implementing the methods of Model.
You can still use Person or Politician everywhere where a Model
is accepted, but you can also implement other interfaces.
With your method you would be limited to Model until you do a manual conversion to
another interface type.
Suppose I have a Person which implements the method Walk() and a Model implements ShowOff(), the following would not work straight forward:
newModel("person").ShowOff()
newModel("person").Walk() // Does not compile, Model has no method Walk
However this would:
newPerson().ShowOff()
newPerson().Walk()
As others have already answered, []T is a distinct type. I'd just like to add that a simple utility can be used to convert them generically.
import "reflect"
// Convert a slice or array of a specific type to array of interface{}
func ToIntf(s interface{}) []interface{} {
v := reflect.ValueOf(s)
// There is no need to check, we want to panic if it's not slice or array
intf := make([]interface{}, v.Len())
for i := 0; i < v.Len(); i++ {
intf[i] = v.Index(i).Interface()
}
return intf
}
Now, you can use it like this:
ToIntf([]int{1,2,3})
Types T and []T are distinct types and distinct are their methods as well, even when satisfying the same interface. IOW, every type satisfying Model must implement all of the Model's methods by itself - the method receiver can be only one specific type.
Even if Go's implementation allowed this, it's unfortunately unsound: You can't assign a []Person to a variable of type []Model because a []Model has different capabilities. For example, suppose we also have Animal which implements Model:
var people []Person = ...
var models []Model = people // not allowed in real Go
models[0] = Animal{..} // ???
var person Person = people[0] // !!!
If we allow line 2, then line 3 should also work because models can perfectly well store an Animal. And line 4 should still work because people stores Persons. But then we end up with a variable of type Person holding an Animal!
Java actually allows the equivalent of line 2, and it's widely considered a mistake. (The error is caught at run time; line 3 would throw an ArrayStoreException.)