Below is slightly modified example from the go laws of reflection http://blog.golang.org/laws-of-reflection. The 2nd code section uses a pointer from a map[string]interface{} and it does not work, what am I doing wrong?
Thanks
//http://play.golang.org/p/LuMBUWLVT6
package main
import (
"fmt"
"reflect"
)
type T struct {
x float64
}
func (x T) RowMap() map[string]interface{} {
return map[string]interface{}{
"x": &x.x,
}
}
func main() {
// this section works as expected, x.x will be 7.1 when done
var x = T{3.4}
p := reflect.ValueOf(&x.x) // Note: take the address of x.
v := p.Elem()
v.SetFloat(7.1)
fmt.Println(x.x, x) // 7.1 {7.1}
// this section I do not understand why x.x is not being set to 7.1
x = T{3.4}
rowmap := x.RowMap()
p = reflect.ValueOf(rowmap["x"]) // rowmap["x"] => &x.x just like above, but is containted in interface{}
v = p.Elem()
v.SetFloat(7.1)
fmt.Println(x.x, x) // 3.4 {3.4} ?? huh, should be // 7.1 {7.1}
}
Elem returns the value that the interface v contains or that the pointer v points to.
Try printing the following and you'll see what you want to see but x does not change meaning it's never being updated.
fmt.Println(v.Float()) // 7.1
You need to pass a pointer to your method. Change your method signature to look like this
func (x *T) RowMap() map[string]interface{} {
Pass a pointer instead of a copy.
I've added some print statements that I think will help clear things up http://play.golang.org/p/xcFMicIPcP
Look at the address of x inside and outside of your method and see how they're different.
Related
Although I think the problem I have is not correctly described in the heading it is the only way I am able to describe it now.
I have a struct Mini which is defined in another file. A set of Minis should be described as a slice. As I want to change some fields of the Mini struct when it is appended to the slice custom functions for append are needed.
Until now I got the following code:
import (
"fmt"
"reflect"
)
//Minis is a slice of all Minis
type Minis struct {
AllMinis []*Mini
}
//Append adds a new Mini to the Minis slice
func (m *Minis) Append(n *Mini) {
m.AllMinis = append(m.AllMinis, n)
}
This code works totally fine.
But in my opinion a struct with just one field is kind of witless.
Is there any way to make a method on a struct or a more elegant solution in general?
Thanks!
Declare the type as a slice:
//Minis is a slice of all Minis
type Minis []*Mini
//Append adds a new Mini to the Minis slice
func (m *Minis) Append(n *Mini) {
*m = append(*m, n)
}
Run it on the playground.
concat panics because QForename passes a nil slice pointer as the receiver to concat. Fix by using a non-nil pointer:
func (m *Minis) QForename(q string) *Minis {
var matches Minis
for _, n := range *m {
if n.Forename == q {
matches.concat(n)
}
}
return &matches
}
I've had difficulty learning the basics of reflect, pointers and interface in go, so here's another entry level question I can't seem to figure out.
This code does what I want it to do - I'm using reflect to add another record to a slice that's typed as an interface.
package main
import (
"reflect"
"log"
)
type Person struct {
Name string
}
func Add(slice interface{}) {
s := reflect.ValueOf(slice).Elem()
// in my actual code, p is declared via the use of reflect.New([Type])
p := Person{Name:"Sam"}
s.Set(reflect.Append(s,reflect.ValueOf(p)))
}
func main() {
p := []Person{}
Add(&p)
log.Println(p)
}
If I changed the Add and main function to this, things don't work the way I want it to.
func Add(slice interface{}) {
s := reflect.ValueOf(&slice).Elem()
p := Person{Name:"Sam"}
s.Set(reflect.Append(reflect.ValueOf(slice),reflect.ValueOf(p)))
log.Println(s)
}
func main() {
p := []Person{}
Add(p)
log.Println(p)
}
That is, the log.Println(p) at the end doesn't show a slice with the record Sam in it like the way I had hoped. So my question is whether it's possible for me to have Add() receive a slice that is not a pointer, and for me to still write some code in Add() that will produce the outcome shown in my first scenario?
A lot of my recent questions dance around this kind of subject, so it's still taking me a while to figure out how to use the reflect package effectively.
No, it's not possible to append to a slice in a function without passing in a pointer to the slice. This isn't related to reflection, but to how variables are passed in to functions. Here's the same code, modified to not use reflection:
package main
import (
"log"
)
type Person struct {
Name string
}
func AddWithPtr(slicep interface{}) {
sp := slicep.(*[]Person)
// This modifies p1 itself, since *sp IS p1
*sp = append(*sp, Person{"Sam"})
}
func Add(slice interface{}) {
// s is now a copy of p2
s := slice.([]Person)
sp := &s
// This modifies a copy of p2 (i.e. s), not p2 itself
*sp = append(*sp, Person{"Sam"})
}
func main() {
p1 := []Person{}
// This passes a reference to p1
AddWithPtr(&p1)
log.Println("Add with pointer: ", p1)
p2 := []Person{}
// This passes a copy of p2
Add(p2)
log.Println("Add without pointer:", p2)
}
(Above, when it says 'copy' of the slice, it doesn't mean the copy of the underlying data - just the slice)
When you pass in a slice, the function effectively gets a new slice that refers to the same data as the original. Appending to the slice in the function increases the length of the new slice, but doesn't change the length of the original slice that was passed in. That's why the original slice remains unchanged.
a := []int{1,2,3}
x, a := a[len(a)-1], a[:len(a)-1]
fmt.Println(a,x)
How to create a pop() function that will do the same for any type of an array?
Here is what I came up with so far:
func pop(a []*interface{}) interface{}{
x := a[len(a)-1]
a = a[:len(a)-1]
return x
}
func main(){
a := []int{1,2,3}
x = pop(a)
fmt.Println(a,x) // -> [1,2] 3
}
But I get cannot use a (type []int) as type []interface {}or other error messages if I try to tweak the code by trial and error.
package main
import (
"fmt"
"reflect"
)
func pop(a interface{}) interface{} {
v := reflect.ValueOf(a).Elem()
x := v.Index(v.Len() - 1)
v.SetLen(v.Len() - 1)
return x
}
func main() {
a := []int{1, 2, 3}
x := pop(&a)
fmt.Println(a, x) // -> [1,2] 3
}
Though this can be implemented, I still think that x, a = a[len(a)-1], a[:len(a)-1] should be better than a pop function.
The go type system doesn't allow you to cast from []type1 -> []type2. Even if it did interfaces are a struct containing a type id and pointer to the object, where normally you would just have the object. Because of this you need to take a interface{} and use reflect to do the slicing.
func pop(slice interface{}) (interface{}, interface{}) {
v := reflect.ValueOf(slice)
return v.Slice(0,v.Len()-1).Interface(), v.Index(v.Len()-1).Interface()
}
Go Playground
Note that this loses compile time type safety, because it must use an interface. Additionally, due to using interfaces the poped value may be allocated, creating extra GC pressure.
Common Go style typically recommends not writing a function like this, and just inlining the small amount of code manually.
After all that really good anwers using reflection I also want to add one answer which offers a more idiomatic Go solution. Like Rob Pike said in his great talk about Go Proverbs
interface{} says nothing
Reflection is never clear
So there should be also one answer showing the idiomatic Go way. This solution does not work for slices of standard types. But there the answer of cshu shows the best solution: x, a = a[len(a)-1], a[:len(a)-1]
For own defined types we have to define a Poper interface and the Pop function takes that as input and returns an empty interface.
type Poper interface {
Pop() interface{}
}
type MyType struct {
a []int
}
func (mt *MyType) Pop() interface{} {
x := mt.a[len(mt.a)-1]
mt.a = mt.a[:len(mt.a)-1]
return x
}
func Pop(p Poper) interface{} {
return p.Pop()
}
func main() {
a := &MyType{[]int{1, 2, 3}}
fmt.Println(Pop(a), a)
}
https://play.golang.org/p/UbDkoVYSMA
At all it is not a good idea to return an empty interface, because all following code has to support the interface{}.
The following code example does not work:
func main() {
a := &MyType{[]int{1, 2, 3}}
fmt.Println(Pop(a), a)
var b int
b = Pop(a)
}
https://play.golang.org/p/wg9__O44A8
The error says everything about that problem: cannot use Pop(a) (type interface {}) as type int in assignment: need type assertion
So the Pop() function does work by returning interface{} but the rest of the code using the result of that function needs to make a type assertion. So if you can avoid it you should search for another solution using types.
They seem to be the same:
package main
import "fmt"
type S struct {
i int
}
func main() {
var s1 *S = new(S)
fmt.Println(s1)
var s2 *S = &S{}
fmt.Println(s2) // Prints the same thing.
}
Update:
Hm. I just realized that there's no obvious way to initialize S.i using new. Is there a way to do that? new(S{i:1}) does not seem to work :/
From Effective Go:
As a limiting case, if a composite literal contains no fields at all, it creates a zero value for the type. The expressions new(File) and &File{} are equivalent.
Not only do they give the same resulting value, but if we allocate something both ways and look at their values...
// Adapted from http://tour.golang.org/#30
package main
import "fmt"
type Vertex struct {
X, Y int
}
func main() {
v := &Vertex{}
v2 := new(Vertex)
fmt.Printf("%p %p", v, v2)
}
...we'll see that they are in fact allocated in consecutive memory slots. Typical output: 0x10328100 0x10328108. I'm not sure if this is an implementation detail or part of the specification, but it does demonstrate that they're both being allocated from the same pool.
Play around with the code here.
As for initializing with new, according to the language spec: The built-in function new takes a type T and returns a value of type *T. The memory [pointed to] is initialized as described in the section on initial values. Because functions in go can't be overloaded, and this isn't a variadic function, there's no way to pass in any initialization data. Instead, go will initialize it with whatever version of 0 makes sense for the type and any member fields, as appropriate.
Case 1: package main
import (
"fmt"
)
type Drink struct {
Name string
Flavour string
}
func main() {
a := new(Drink)
a.Name = "Maaza"
a.Flavour = "Mango"
b := a
fmt.Println(&a)
fmt.Println(&b)
b.Name = "Frooti"
fmt.Println(a.Name)
}//This will output Frooti for a.Name, even though the addresses for a and b are different.
Case 2:
package main
import (
"fmt"
)
type Drink struct {
Name string
Flavour string
}
func main() {
a := Drink{
Name: "Maaza",
Flavour: "Mango",
}
b := a
fmt.Println(&a)
fmt.Println(&b)
b.Name = "Froti"
fmt.Println(a.Name)
}//This will output Maaza for a.Name. To get Frooti in this case assign b:=&a.
having a rough time working with struct fields using reflect package. in particular, have not figured out how to set the field value.
type t struct { fi int; fs string }
var r t = t{ 123, "jblow" }
var i64 int64 = 456
getting Name of field i - this seems to work
var field = reflect.TypeOf(r).Field(i).Name
getting value of field i as a) interface{}, b) int - this seems to work
var iface interface{} = reflect.ValueOf(r).Field(i).Interface()
var i int = int(reflect.ValueOf(r).Field(i).Int())
setting value of field i - try one - panic
reflect.ValueOf(r).Field(i).SetInt( i64 )
panic: reflect.Value·SetInt using value obtained using unexported field
assuming it did not like field names "id" and "name", so renamed to "Id" and "Name"
a) is this assumption correct?
b) if correct, thought not necessary since in same file / package
setting value of field i - try two (with field names capitalized ) - panic
reflect.ValueOf(r).Field(i).SetInt( 465 )
reflect.ValueOf(r).Field(i).SetInt( i64 )
panic: reflect.Value·SetInt using unaddressable value
Instructions below by #peterSO are thorough and high quality
Four. this works:
reflect.ValueOf(&r).Elem().Field(i).SetInt( i64 )
he documents as well that the field names must be exportable (begin with capital letter)
The Go json package marshals and unmarshals JSON from and to Go structures.
Here's a step-by-step example which sets the value of a struct field while carefully avoiding errors.
The Go reflect package has a CanAddr function.
func (v Value) CanAddr() bool
CanAddr returns true if the value's
address can be obtained with Addr.
Such values are called addressable. A
value is addressable if it is an
element of a slice, an element of an
addressable array, a field of an
addressable struct, or the result of
dereferencing a pointer. If CanAddr
returns false, calling Addr will
panic.
The Go reflect package has a CanSet function, which, if true, implies that CanAddr is also true.
func (v Value) CanSet() bool
CanSet returns true if the value of v
can be changed. A Value can be changed
only if it is addressable and was not
obtained by the use of unexported
struct fields. If CanSet returns
false, calling Set or any
type-specific setter (e.g., SetBool,
SetInt64) will panic.
We need to make sure we can Set the struct field. For example,
package main
import (
"fmt"
"reflect"
)
func main() {
type t struct {
N int
}
var n = t{42}
// N at start
fmt.Println(n.N)
// pointer to struct - addressable
ps := reflect.ValueOf(&n)
// struct
s := ps.Elem()
if s.Kind() == reflect.Struct {
// exported field
f := s.FieldByName("N")
if f.IsValid() {
// A Value can be changed only if it is
// addressable and was not obtained by
// the use of unexported struct fields.
if f.CanSet() {
// change value of N
if f.Kind() == reflect.Int {
x := int64(7)
if !f.OverflowInt(x) {
f.SetInt(x)
}
}
}
}
}
// N at end
fmt.Println(n.N)
}
Output:
42
7
If we can be certain that all the error checks are unnecessary, the example simplifies to,
package main
import (
"fmt"
"reflect"
)
func main() {
type t struct {
N int
}
var n = t{42}
fmt.Println(n.N)
reflect.ValueOf(&n).Elem().FieldByName("N").SetInt(7)
fmt.Println(n.N)
}
BTW, Go is available as open source code. A good way to learn about reflection is to see how the core Go developers use it. For example, the Go fmt and json packages. The package documentation has links to the source code files under the heading Package files.
This seems to work:
package main
import (
"fmt"
"reflect"
)
type Foo struct {
Number int
Text string
}
func main() {
foo := Foo{123, "Hello"}
fmt.Println(int(reflect.ValueOf(foo).Field(0).Int()))
reflect.ValueOf(&foo).Elem().Field(0).SetInt(321)
fmt.Println(int(reflect.ValueOf(foo).Field(0).Int()))
}
Prints:
123
321