I would like to append to a slice using only reflection. But I can't figure out how to "replace" the value of a with the new slice.
func main() {
fmt.Println("Hello, playground")
a := []string {"a","b","c"}
values := []string {"d","e"}
v := reflect.ValueOf(a)
fmt.Printf("%t\n\n", v.Type())
fmt.Printf("%t\n\n", v.Type().Elem().Kind())
for _, val := range values {
v.Set(reflect.Append(v, reflect.ValueOf(val)))
}
fmt.Printf("%t - %v", a, a)
}
This code is available for fiddling at https://play.golang.org/p/cDlyH3jBDS.
You can't modify the value wrapped in reflect.Value if it originates from a non-pointer. If it would be allowed, you could only modify a copy and would cause more confusion. A slice value is a header containing a pointer to a backing array, a length and a capacity. When you pass a to reflect.ValueOf(), a copy of this header is made and passed, and any modification you could do on it could only modify this header-copy. Adding elements (and thus changing its length and potentially the pointer and capacity) would not be observed by the original slice header, the original would still point to the same array, and would still contain the same length and capacity values. For details see Are Golang function parameter passed as copy-on-write?; and Golang passing arrays to the function and modifying it.
You have to start from a pointer, and you may use Value.Elem() to obtain the reflect.Value descriptor of the pointed, dereferenced value. But you must start from a pointer.
Changing this single line in your code makes it work:
v := reflect.ValueOf(&a).Elem()
And also to print the type of a value, use the %T verb (%t is for bool values):
fmt.Printf("%T\n\n", v.Type())
fmt.Printf("%T\n\n", v.Type().Elem().Kind())
// ...
fmt.Printf("%T - %v", a, a)
Output (try it on the Go Playground):
Hello, playground
*reflect.rtype
reflect.Kind
[]string - [a b c d e]
For a deeper understanding of Go's reflection, read the blog post: The Laws of Reflection
And read related questions+answers:
Assigning a value to struct member through reflection in Go
Changing pointer type and value under interface with reflection
Using reflection SetString
Related
I have a variable that needs to be either a string or map[string]string (will be deserializing from JSON). So I declare it as interface{}. How can I check that the value is map[string]string?
This question How to check interface is a map[string]string in golang almost answers my question. But the accepted answer only works if the variable is declared as a map[string]string not if the variable is interface{}.
package main
import (
"fmt"
)
func main() {
var myMap interface{}
myMap = map[string]interface{}{
"foo": "bar",
}
_, ok := myMap.(map[string]string)
if !ok {
fmt.Println("This will be printed")
}
}
See https://play.golang.org/p/mA-CVk7bdb9
I can use two type assertions though. One on the map and one on the map value.
package main
import (
"fmt"
)
func main() {
var myMap interface{}
myMap = map[string]interface{}{
"foo": "bar",
}
valueMap, ok := myMap.(map[string]interface{})
if !ok {
fmt.Println("will not be printed")
}
for _, v := range valueMap {
if _, ok := v.(string); !ok {
fmt.Println("will not be printed")
}
}
}
See https://play.golang.org/p/hCl8eBcKSqE
Question: is there a better way?
If you declare a variable as type interface{}, it is type interface{}. It is not, ever, some map[keytype]valuetype value. But a variable of type interface{} can hold a value that has some other concrete type. When it does so, it does so—that's all there is to it. It still is type interface{}, but it holds a value of some other type.
An interface value has two parts
The key distinction here is between what an interface{} variable is, and what it holds. Any interface variable actually has two slots inside it: one to hold what type is stored in it, and one to hold what value is stored in it. Any time you—or anyone—assign a value to the variable, the compiler fills in both slots: the type, from the type of the value you used, and the value, from the value you used.1 The interface variable compares equal to nil if it has nil in both slots; and that's also the default zero value.
Hence, your runtime test:
valueMap, ok := myMap.(map[string]interface{})
is a sensible thing to do: if myMap holds a value that has type map[string]interface, ok gets set to true and valueMap contains the value (which has that type). If myMap holds a value with some other type, ok gets set to false and valueMap gets set to the zero-value of type map[string]interface{}. In other words, at runtime, the code checks the type-slot first, then either copies the value-slot across to valueMap and sets ok to true, or sets valueMap to nil and sets ok to false.
If and when ok has been set to true, each valueMap[k] value is type interface{}. As before, for myMap itself, each of these interface{} variables can—but do not have to—hold a value of type string, and you must use some sort of "what is the actual type-and-value" run-time test to tease them apart.
When you use json.Unmarshal to stuff decoded JSON into a variable of type interface{}, it is capable of deserializing any of these documented JSON types. The list then tells you what type gets stuffed into the interface variable:
bool, for JSON booleans
float64, for JSON numbers
string, for JSON strings
[]interface{}, for JSON arrays
map[string]interface{}, for JSON objects
nil for JSON null
So after doing json.Unmarshal into a variable of type interface{}, you should check what type got put into the type-slot of the variable. You can do this with an assertion and an ok boolean, or you can, if you prefer, use a type switch to decode it:
var i interface
if err := json.Unmarshal(data, &i); err != nil {
panic(err)
}
switch v := i.(type) {
case string:
... code ...
case map[string]interface{}:
... code ...
... add some or all of the types listed ...
}
The thing is, no matter what you do in code here, you did have json.Unmarshal put something into an interface{}, and interface{} is the type of i. You must test at runtime what type and value pair the interface holds.
Your other option is to inspect your JSON strings manually and decide what type of variable to provide to json.Unmarshal. That gives you less code to write after the Unmarshal, but more code to write before it.
There's a more complete example here, on the Go playground, of using type switches to inspect the result from a json.Unmarshal. It's deliberately incomplete but, I think, has enough input and output cases to let you work out how to handle everything, given the quote above about what json.Unmarshal writes into a variable of type interface{}.
1Of course, if you assign one interface{} from some other interface{}:
var i1, i2 interface{}
... set i1 from some actual value ...
// more code, then:
i2 = i1
the compiler just copies both slots from i1 into i2. The two-separate-slots thing becomes clearer when you do:
var f float64
... code that sets f to, say, 1.5 ...
i2 = f
for instance, as that writes float64 into the type-slot, and the value 1.5 into the value-slot. The compiler knows that f is float64 so the type-setting just means "stick a constant in it". The compiler doesn't necessarily know the value of f so the value-setting is a copy of whatever the actual value is.
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.
I started learning golang a couple of days ago and found reflect.Valueof() and Value.Elem() quite confusing. What is the difference between this two function/methods and how to use them correctly?
Both function/methods return a Value, and according to the go doc
ValueOf returns a new Value initialized to the concrete value stored in the interface i. ValueOf(nil) returns the zero Value.
Elem returns the value that the interface v contains or that the pointer v points to. It panics if v's Kind is not Interface or Ptr. It returns the zero Value if v is nil.
I found this code from a post on stackoverflow but still don't understand when to use .Elem()
func SetField(obj interface{}, name string, value interface{}) error {
// won't work if I remove .Elem()
structValue := reflect.ValueOf(obj).Elem()
structFieldValue := structValue.FieldByName(name)
if !structFieldValue.IsValid() {
return fmt.Errorf("No such field: %s in obj", name)
}
if !structFieldValue.CanSet() {
return fmt.Errorf("Cannot set %s field value", name)
}
structFieldType := structFieldValue.Type()
// won't work either if I add .Elem() to the end
val := reflect.ValueOf(value)
if structFieldType != val.Type() {
return fmt.Errorf("Provided value %v type %v didn't match obj field type %v",val,val.Type(),structFieldType)
}
structFieldValue.Set(val)
return nil
}
reflect.ValueOf() is a function, think of it as the entry point to reflection. When you have a "non-reflection" value, such as a string or int, you can use reflect.ValueOf() to get a reflect.Value descriptor of it.
Value.Elem() is a method of reflect.Value. So you can only use this if you already have a reflect.Value. You may use Value.Elem() to get the value (reflect.Value) pointed by the value wrapped by the original reflect.Value. Note that you may also use reflect.Indirect() for this. There's another "use case" for Value.Elem(), but it's more "advanced", we return to it at the end of the answer.
To "leave" reflection, you may use the general Value.Interface() method, which returns you the wrapped value as an interface{}.
For example:
var i int = 3
var p *int = &i
fmt.Println(p, i)
v := reflect.ValueOf(p)
fmt.Println(v.Interface()) // This is the p pointer
v2 := v.Elem()
fmt.Println(v2.Interface()) // This is i's value: 3
This will output (try it on the Go Playground):
0x414020 3
0x414020
3
For a great introduction to Go's reflection, read The Go Blog: The Laws of Reflection. Although if you're just starting with Go, I'd focus on other things and leave reflection for a later adventure.
Another use case for Value.Elem()
This is kind of an advanced topic, so don't freak out if you don't understand it. You don't need to.
We saw how Value.Elem() can be used to "navigate" when a pointer is wrapped in the reflect.Value. Doc of Value.Elem() says:
Elem returns the value that the interface v contains or that the pointer v points to.
So if reflect.Value wraps an interface value, Value.Elem() may also be used to get the concrete value wrapped in that interface value.
Interfaces in Go is its own topic, for the internals, you may read Go Data Structures: Interfaces by Russ Cox. Again, not necessarily a topic for Go starters.
Basically whatever value you pass to reflect.ValueOf(), if it's not already an interface value, it will be wrapped in an interface{} implicitly. If the passed value is already an interface value, then the concrete value stored in it will be passed as a interface{}. This second "use case" surfaces if you pass a pointer to interface (which is otherwise very rare in Go!).
So if you pass a pointer to interface, this pointer will be wrapped in an interface{} value. You may use Value.Elem() to get the pointed value, which will be an interface value (not a concrete value), and using Value.Elem() again on this will give you the concrete value.
This example illustrates it:
var r io.Reader = os.Stdin // os.Stdin is of type *os.File which implements io.Reader
v := reflect.ValueOf(r) // r is interface wrapping *os.File value
fmt.Println(v.Type()) // *os.File
v2 := reflect.ValueOf(&r) // pointer passed, will be wrapped in interface{}
fmt.Println(v2.Type()) // *io.Reader
fmt.Println(v2.Elem().Type()) // navigate to pointed: io.Reader (interface type)
fmt.Println(v2.Elem().Elem().Type()) // 2nd Elem(): get concrete value in interface: *os.File
Try it on the Go Playground.
I have a struct type with a *int64 field.
type SomeType struct {
SomeField *int64
}
At some point in my code, I want to declare a literal of this (say, when I know said value should be 0, or pointing to a 0, you know what I mean)
instance := SomeType{
SomeField: &0,
}
...except this doesn't work
./main.go:xx: cannot use &0 (type *int) as type *int64 in field value
So I try this
instance := SomeType{
SomeField: &int64(0),
}
...but this also doesn't work
./main.go:xx: cannot take the address of int64(0)
How do I do this? The only solution I can come up with is using a placeholder variable
var placeholder int64
placeholder = 0
instance := SomeType{
SomeField: &placeholder,
}
Note: the &0 syntax works fine when it's a *int instead of an *int64. Edit: no it does not. Sorry about this.
Edit:
Aparently there was too much ambiguity to my question. I'm looking for a way to literally state a *int64. This could be used inside a constructor, or to state literal struct values, or even as arguments to other functions. But helper functions or using a different type are not solutions I'm looking for.
The Go Language Specification (Address operators) does not allow to take the address of a numeric constant (not of an untyped nor of a typed constant).
The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, x [in the expression of &x] may also be a (possibly parenthesized) composite literal.
For reasoning why this isn't allowed, see related question: Find address of constant in go. A similar question (similarly not allowed to take its address): How can I store reference to the result of an operation in Go?
0) Generic solution (from Go 1.18)
Generics are added in Go 1.18. This means we can create a single, generic Ptr() function that returns a pointer to whatever value we pass to it. Hopefully it'll get added to the standard library. Until then, you can use github.com/icza/gog, the gog.Ptr() function (disclosure: I'm the author).
This is how it can look like:
func Ptr[T any](v T) *T {
return &v
}
Testing it:
i := Ptr(2)
log.Printf("%T %v", i, *i)
s := Ptr("abc")
log.Printf("%T %v", s, *s)
x := Ptr[any](nil)
log.Printf("%T %v", x, *x)
Which will output (try it on the Go Playground):
2009/11/10 23:00:00 *int 2
2009/11/10 23:00:00 *string abc
2009/11/10 23:00:00 *interface {} <nil>
Your other options (prior to Go 1.18) (try all on the Go Playground):
1) With new()
You can simply use the builtin new() function to allocate a new zero-valued int64 and get its address:
instance := SomeType{
SomeField: new(int64),
}
But note that this can only be used to allocate and obtain a pointer to the zero value of any type.
2) With helper variable
Simplest and recommended for non-zero elements is to use a helper variable whose address can be taken:
helper := int64(2)
instance2 := SomeType{
SomeField: &helper,
}
3) With helper function
Note: Helper functions to acquire a pointer to a non-zero value are available in my github.com/icza/gox library, in the gox package, so you don't have to add these to all your projects where you need it.
Or if you need this many times, you can create a helper function which allocates and returns an *int64:
func create(x int64) *int64 {
return &x
}
And using it:
instance3 := SomeType{
SomeField: create(3),
}
Note that we actually didn't allocate anything, the Go compiler did that when we returned the address of the function argument. The Go compiler performs escape analysis, and allocates local variables on the heap (instead of the stack) if they may escape the function. For details, see Is returning a slice of a local array in a Go function safe?
4) With a one-liner anonymous function
instance4 := SomeType{
SomeField: func() *int64 { i := int64(4); return &i }(),
}
Or as a (shorter) alternative:
instance4 := SomeType{
SomeField: func(i int64) *int64 { return &i }(4),
}
5) With slice literal, indexing and taking address
If you would want *SomeField to be other than 0, then you need something addressable.
You can still do that, but that's ugly:
instance5 := SomeType{
SomeField: &[]int64{5}[0],
}
fmt.Println(*instance2.SomeField) // Prints 5
What happens here is an []int64 slice is created with a literal, having one element (5). And it is indexed (0th element) and the address of the 0th element is taken. In the background an array of [1]int64 will also be allocated and used as the backing array for the slice. So there is a lot of boilerplate here.
6) With a helper struct literal
Let's examine the exception to the addressability requirements:
As an exception to the addressability requirement, x [in the expression of &x] may also be a (possibly parenthesized) composite literal.
This means that taking the address of a composite literal, e.g. a struct literal is ok. If we do so, we will have the struct value allocated and a pointer obtained to it. But if so, another requirement will become available to us: "field selector of an addressable struct operand". So if the struct literal contains a field of type int64, we can also take the address of that field!
Let's see this option in action. We will use this wrapper struct type:
type intwrapper struct {
x int64
}
And now we can do:
instance6 := SomeType{
SomeField: &(&intwrapper{6}).x,
}
Note that this
&(&intwrapper{6}).x
means the following:
& ( (&intwrapper{6}).x )
But we can omit the "outer" parenthesis as the address operator & is applied to the result of the selector expression.
Also note that in the background the following will happen (this is also a valid syntax):
&(*(&intwrapper{6})).x
7) With helper anonymous struct literal
The principle is the same as with case #6, but we can also use an anonymous struct literal, so no helper/wrapper struct type definition needed:
instance7 := SomeType{
SomeField: &(&struct{ x int64 }{7}).x,
}
Use a function which return an address of an int64 variable to solve the problem.
In the below code we use function f which accepts an integer and
returns a pointer value which holds the address of the integer. By using this method we can easily solve the above problem.
type myStr struct {
url *int64
}
func main() {
f := func(s int64) *int64 {
return &s
}
myStr{
url: f(12345),
}
}
There is another elegant way to achieve this which doesn't produce much boilerplate code and doesn't look ugly in my opinion. In case I need a struct with pointers to primitives instead of values, to make sure that zero-valued struct members aren't used across the project, I will create a function with those primitives as arguments.
You can define a function which creates your struct and then pass primitives to this function and then use pointers to function arguments.
type Config struct {
Code *uint8
Name *string
}
func NewConfig(code uint8, name string) *Config {
return &Config{
Code: &code,
Name: &name,
}
}
func UseConfig() {
config := NewConfig(1, "test")
// ...
}
// in case there are many values, modern IDE will highlight argument names for you, so you don't have to remember
func UseConfig2() {
config := NewConfig(
1,
"test",
)
// ...
}
If you don't mind using third party libraries, there's the lo package which uses generics (go 1.18+) which has the .ToPtr() function
ptr := lo.ToPtr("hello world")
// *string{"hello world"}
I want to swap two numbers using interface but the interface concept is so confusing to me.
http://play.golang.org/p/qhwyxMRj-c
This is the code and playground. How do I use interface and swap two input numbers? Do I need to define two structures?
type num struct {
value interface{}
}
type numbers struct {
b *num
c *num
}
func (a *num) SwapNum(var1, var2 interface{}) {
var a num
temp := var1
var1 = var2
var2 = temp
}
func main() {
a := 1
b := 2
c := 3.5
d := 5.5
SwapNum(a, b)
fmt.Println(a, b) // 2 1
SwapNum(c, d)
fmt.Println(c, d) // 5.5 3.5
}
First of all, the interface{} type is simply a type which accepts all values as it is an interface with an empty method set and every type can satisfy that. int for example does not have any methods, neither does interface{}.
For a method which swaps the values of two variables you first need to make sure these variables are actually modifiable. Values passed to a function are always copied (except reference types like slices and maps but that is not our concern at the moment). You can achieve modifiable parameter by using a pointer to the variable.
So with that knowledge you can go on and define SwapNum like this:
func SwapNum(a interface{}, b interface{})
Now SwapNum is a function that accepts two parameters of any type.
You can't write
func SwapNum(a *interface{}, b *interface{})
as this would only accept parameters of type *interface{} and not just any type.
(Try it for yourself here).
So we have a signature, the only thing left is swapping the values.
func SwapNum(a interface{}, b interface{}) {
*a, *b = *b, *a
}
No, this will not work that way. By using interface{} we must do runtime type assertions to check whether we're doing the right thing or not. So the code must be expanded using the reflect package. This article might get you started if you don't know about reflection.
Basically we will need this function:
func SwapNum(a interface{}, b interface{}) {
ra := reflect.ValueOf(a).Elem()
rb := reflect.ValueOf(b).Elem()
tmp := ra.Interface()
ra.Set(rb)
rb.Set(reflect.ValueOf(tmp))
}
This code makes a reflection of a and b using reflect.ValueOf() so that we can
inspect it. In the same line we're assuming that we've got pointer values and dereference
them by calling .Elem() on them.
This basically translates to ra := *a and rb := *b.
After that, we're making a copy of *a by requesting the value using .Interface()
and assigning it (effectively making a copy).
Finally, we set the value of a to b with [ra.Set(rb)]5, which translates to *a = *b
and then assigning b to a, which we stored in the temp. variable before. For this,
we need to convert tmp back to a reflection of itself so that rb.Set() can be used
(it takes a reflect.Value as parameter).
Can we do better?
Yes! We can make the code more type safe, or better, make the definition of Swap type safe
by using reflect.MakeFunc. In the doc (follow the link) is an example which is very
like what you're trying. Essentially you can fill a function prototype with content
by using reflection. As you supplied the prototype (the signature) of the function the
compiler can check the types, which it can't when the value is reduced to interface{}.
Example usage:
var intSwap func(*int, *int)
a,b := 1, 0
makeSwap(&intSwap)
intSwap(&a, &b)
// a is now 0, b is now 1
The code behind this:
swap := func(in []reflect.Value) []reflect.Value {
ra := in[0].Elem()
rb := in[1].Elem()
tmp := ra.Interface()
ra.Set(rb)
rb.Set(reflect.ValueOf(tmp))
return nil
}
makeSwap := func(fptr interface{}) {
fn := reflect.ValueOf(fptr).Elem()
v := reflect.MakeFunc(fn.Type(), swap)
fn.Set(v)
}
The code of swap is basically the same as that of SwapNum. makeSwap is the same
as the one used in the docs where it is explained pretty well.
Disclaimer: The code above makes a lot of assumptions about what is given and
what the values look like. Normally you need to check, for example, that the given
values to SwapNum actually are pointer values and so forth. I left that out for
reasons of clarity.