Is there a way to resolve or introspect a variable identified from a string of its name?
For instance, if I have a string value strVal := "s" that corresponds to the name of a variable such as var s string which is in the same function
's scope, can I dynamically look up the value of the s variable?
I know if these were fields of a struct, I could use reflection, but for my use case, I am dealing with locally scoped variables that are not part of a struct or map.
It seems like you cloud extract this information from a struct.
package main
import (
"fmt"
"reflect"
)
type Article struct {
Id int
Title string
Price float32
Authors []string
}
func main() {
article := Article{}
e := reflect.ValueOf(&article).Elem()
for i := 0; i < e.NumField(); i++ {
name := e.Type().Field(i).Name
varType := e.Type().Field(i).Type
value := e.Field(i).Interface()
fmt.Printf("%v %v %v\n", name, varType, value)
}
}
Example:
https://play.golang.org/p/vWkRzpvWKYn
If you want to analyze while runtime a variable then you could use a debugger like delve
Ref: https://github.com/go-delve/delve
[I]f I have a string value strVal := "s" that corresponds to the name of a var s string in the same scope, can I dynamically look up the value of s?
No. A dead simple No.
Related
I'm trying to access to a struct properties with a variable who contains the property key.
For example, i got this struct :
type person struct {
name string
age int
}
I have a variable "property" who contain a string value "age".
I would like to access to the age with something like person.property ?
Do you think it is possile in golang ?
If you're looking for something like person[property] like you do in Python or JavaScript, the answer is NO, Golang does not support dynamic field/method selection at runtime.
But you can do it with reflect:
import (
"fmt"
"reflect"
)
func main() {
type person struct {
name string
age int
}
v := reflect.ValueOf(person{"Golang", 10})
property := "age"
f := v.FieldByName(property)
fmt.Printf("Person Age: %d\n", f.Int())
}
Iterate through the fields of a struct in Go
I have read above thread and now I'm trying to extend it by processing multiple items at a time
package main
import (
"fmt"
"reflect"
)
type BaseStats struct {
value1 int
value2 byte
value3 int
value4 byte
}
func StatsCreate(stats BaseStats) {
v := reflect.ValueOf(stats)
val := make([]interface{}, v.NumField())
for i := 0; i< v.NumField(); i+=2 {
val[i+0] = v.Field(i+0).Interface().(int)
val[i+1] = v.Field(i+1).Interface().(byte)
fmt.Printf("%v %v", val[i+0], val[i+1])
}
}
func main() {
StatsCreate(BaseStats{20, '%', 400, '$'})
}
Error I had before: panic: reflect.Value.Interface: cannot return value obtained from unexported field or method
Go requires you to make the first letter of any variable or function that you wish to use outside of the package (aka EXPORTED). Because you're trying to pass variables to another package, in this case the reflect package, but they're not exportable. It's very common that all struct field names are capitalized as best practice.
The capitalized->exported paradigm is one of the few things I don't like about the language, but that's the way it is.
I really want a way to print the string representation of a field name in go. It has several use cases, but here is an example:
lets say I have a struct
type Test struct {
Field string `bson:"Field" json:"field"`
OtherField int `bson:"OtherField" json:"otherField"`
}
and, for example, I want to do a mongo find:
collection.Find(bson.M{"OtherField": someValue})
I don't like that I have to put the string "OtherField" in there. It seems brittle and easy to either misstype or have the struct change and then my query fails without me knowing it.
Is there any way to get the string "OtherField" without having to either declare a const or something like that? I know I can use reflection to a get a list of field names from a struct, but I'd really like to do something along the lines of
fieldName := nameOf(Test{}.OtherField)
collection.Find(bson.M{fieldName: someValue})
is there any way to do this in Go?? C# 6 has the built in nameof, but digging through reflection I can't find any way to do this in Go.
I don't really think there is. You may be able to load a set of types via reflection and generate a set of constants for the field names. So:
type Test struct {
Field string `bson:"Field" json:"field"`
OtherField int `bson:"OtherField" json:"otherField"`
}
Could generate something like:
var TestFields = struct{
Field string
OtherField string
}{"Field","OtherField"}
and you could use TestFields.Field as a constant.
Unfortunately, I don't know of any existing tool that does anything like that. Would be fairly simple to do, and wire up to go generate though.
EDIT:
How I'd generate it:
Make a package that accepts an array of reflect.Type or interface{} and spits out a code file.
Make a generate.go somewhere in my repo with main function:
func main(){
var text = mygenerator.Gen(Test{}, OtherStruct{}, ...)
// write text to constants.go or something
}
Add //go:generate go run scripts/generate.go to my main app and run go generate
Here is a function that will return a []string with the struct field names. I think it comes in the order they are defined.
WARNING: Reordering the fields in the struct definition will change the order in which they appear
https://play.golang.org/p/dNATzNn47S
package main
import (
"fmt"
"strings"
"regexp"
)
type Test struct {
Field string `bson:"Field" json:"field"`
OtherField int `bson:"OtherField" json:"otherField"`
}
func main() {
fields, err := GetFieldNames(Test{})
if err != nil {
fmt.Println(err)
return
}
fmt.Println(fields)
}
func GetFieldNames(i interface{}) ([]string, error) {
// regular expression to find the unquoted json
reg := regexp.MustCompile(`(\s*?{\s*?|\s*?,\s*?)(['"])?(?P<Field>[a-zA-Z0-9]+)(['"])?:`)
// print struct in almost json form (fields unquoted)
raw := fmt.Sprintf("%#v", i)
// remove the struct name so string begins with "{"
fjs := raw[strings.Index(raw,"{"):]
// find and grab submatch 3
matches := reg.FindAllStringSubmatch(fjs,-1)
// collect
fields := []string{}
for _, v := range matches {
if len(v) >= 3 && v[3] != "" {
fields = append(fields, v[3])
}
}
return fields, nil
}
Here is a piece of code play.google.org that runs without any problem:
package main
import (
"fmt"
)
func PrintAnonymous(v struct {
i int
s string
}) {
fmt.Printf("%d: %s\n", v.i, v.s)
}
func PrintAnonymous2(v struct{}) {
fmt.Println("Whatever")
}
func main() {
value := struct {
i int
s string
}{
0, "Hello, world!",
}
PrintAnonymous(value)
PrintAnonymous2(struct{}{})
}
However, if the PrintAnonymous() function exists in another package (let's say, temp), the code will not work:
cannot use value (type struct { i int; s string })
as type struct { i int; s string } in argument to temp.PrintAnonymous
My question are:
Is there a way to call a (public) function with anonymous struct as a parameter (a.k.a. PrintAnonymous() above)?
A function with empty struct as a parameter (a.k.a. PrintAnonymous2() above) can be called even if it exists in another package. Is this a special case?
Well, I know that I can always name the struct to solve the problem, I'm just curious about this, and wonder why it seems that this is not allowed.
The fields of your anonymous struct type are unexported. This means you cannot create values of this struct and specify values for the fields from another package. Spec: Composite literals:
It is an error to specify an element for a non-exported field of a struct belonging to a different package.
If you change the struct definition to export the fields, then it will work because all fields can be assigned to by other packages (see Siu Ching Pong -Asuka Kenji-'s answer).
This is the case with the empty struct (with no fields) too: the empty struct has no fields, thus it has no unexported fields, so you're allowed to pass a value of that.
You can call the function with unmodified struct (with unexported fields) via reflection though. You can obtain the reflect.Type of the PrintAnonymous() function, and you can use Type.In() to get the reflect.Type of its first parameter. That is the anonymous struct we want to pass a value for. And you can construct a value of that type using reflect.New(). This will be a reflect.Value, wrapping a pointer to the zero value of the anonymous struct. Sorry, you can't have a struct value with fields having non-zero values (for reason mentioned above).
This is how it could look like:
v := reflect.ValueOf(somepackage.PrintAnonymous)
paramt := v.Type().In(0)
v.Call([]reflect.Value{reflect.New(paramt).Elem()})
This will print:
0:
0 is zero value for int, and "" empty string for string.
For deeper inside into the type system and structs with unexported fields, see related questions:
Identify non builtin-types using reflect
How to clone a structure with unexported field?
Interestingly (this is a bug, see linked issue below), using reflection, you can use a value of your own anonymous struct type (with matching, unexported fields), and in this case we can use values other than the zero value of the struct fields:
value := struct {
i int
s string
}{
1, "Hello, world!",
}
v.Call([]reflect.Value{reflect.ValueOf(value)})
Above runs (without panic):
1: Hello, world!
The reason why this is allowed is due to a bug in the compiler. See the example code below:
s := struct{ i int }{2}
t := reflect.TypeOf(s)
fmt.Printf("Name: %q, PkgPath: %q\n", t.Name(), t.PkgPath())
fmt.Printf("Name: %q, PkgPath: %q\n", t.Field(0).Name, t.Field(0).PkgPath)
t2 := reflect.TypeOf(subplay.PrintAnonymous).In(0)
fmt.Printf("Name: %q, PkgPath: %q\n", t2.Name(), t2.PkgPath())
fmt.Printf("Name: %q, PkgPath: %q\n", t2.Field(0).Name, t2.Field(0).PkgPath)
Output is:
Name: "", PkgPath: ""
Name: "i", PkgPath: "main"
Name: "", PkgPath: ""
Name: "i", PkgPath: "main"
As you can see the unexported field i in both anonymous struct types (in main package and in somepackage as parameter to PrintAnonymous() function) –falsely– report the same package, and thus their type will be equal:
fmt.Println(t == t2) // Prints true
Note: I consider this a flaw: if this is allowed using reflection, then this should be possible without using reflection too. If without reflection the compile-time error is justified, then using reflection should result in runtime panic. I opened an issue for this, you can follow it here: issue #16616. Fix currently aims Go 1.8.
Oh! I just realized that the field names are in lowercase, and thus not public! Changing the first letter of the field names to uppercase solves the problem:
package main
import (
"temp"
)
func main() {
value := struct {
I int
S string
}{
0, "Hello, world!",
}
temp.PrintAnonymous(value)
temp.PrintAnonymous2(struct{}{})
}
package temp
import (
"fmt"
)
func PrintAnonymous(v struct{I int; S string}) {
fmt.Printf("%d: %s\n", v.I, v.S)
}
func PrintAnonymous2(v struct{}) {
fmt.Println("Whatever")
}
I have some code that looks to be working but does nothing in the end:
http://play.golang.org/p/TfAWWy4-R8
Have a struct that has fields of type struct. The inner struct has all string fields.
Using reflect in a loop, want to get all struct fields from outer struct. Next, populate all string values in inner struct. the example code is getting text from the tags and parsing it on "," to get strings values for inner loop.
This is the main part that should create the inner struct and add the parsed data to the string values.
t := reflect.TypeOf(Alias{})
alias = reflect.New(t)
for i := 0; i < alias.Elem().NumField(); i++ {
p := alias.Elem().Field(i)
p.SetString(params[i])
}
It looks like it is working when you look at the output from example, but after printing a value from outer struct it seems to be empty:
fmt.Println("Final01 = ", Attr.Final01.Command) // prints empty string
So not sure how to get values into the inner struct Alias{}
Thanks for any help.
Here's the working program. Explanation below.
package main
import "fmt"
import "strings"
import "reflect"
type Alias struct {
Name string
DevicePath string
GuiPath string
Setpoint string
Command string
Status string
FunctionCmds string
}
type Manifold struct {
Final01 Alias "Final01,/Gaspanel/Shared/Final01,,,wOpen,rIsOpen,"
Dump01 Alias "Dump01,/Gaspanel/Shared/Dump01,,,wOpen,rIsOpen,"
N2Vent01 Alias "N2Vent01,/Gaspanel/Shared/N2Vent01,,,wOpen,rIsOpen,"
N2Vent201 Alias "N2Vent201,/Gaspanel/Shared/N2Vent201,,,wOpen,rIsOpen,"
PurgeValve Alias "PurgeValve,/Gaspanel/Shared/Purge01,,,wOpen,rIsOpen,"
}
func MapTagedAliasToChamber(chamber string, struc interface{}) []string {
attributeStruct := reflect.ValueOf(struc).Elem()
typeAttributeStruct := attributeStruct.Type()
attributes := make([]string, attributeStruct.NumField(), attributeStruct.NumField())
for i := 0; i < attributeStruct.NumField(); i++ {
alias := attributeStruct.Field(i)
tag := string(typeAttributeStruct.Field(i).Tag)
name := typeAttributeStruct.Field(i).Name
params := strings.Split(tag, ",")
alias = reflect.New(reflect.TypeOf(Alias{})).Elem()
for i := 0; i < alias.NumField(); i++ {
alias.Field(i).SetString(params[i])
}
attributeStruct.Field(i).Set(alias)
fmt.Printf("%d: %s %s = %v\n", i, name, alias.Type(), alias.Interface())
}
return attributes
}
func main() {
Attr := Manifold{}
MapTagedAliasToChamber("A", &Attr)
fmt.Println("Final01 = ", Attr.Final01.Command)
}
The problem was on line 38 of your original program, where you created a new reflect.Value named alias representing a value of type *Alias, then filled it with your information but never wrote it back into your Manifold struct.
Additionally I suggest that you stick to the standard struct-tag format which can be parsed and used more easily through (reflect.StructTag).Get(key string). And don't use strings where you don't need them e.g. rIsOpen sounds like a boolean value to me.