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"}
Related
I've been reading about how Go passes arguments to functions via pointer vs. value. I've been reading about the interface type. And I've been tampering with the reflect package. But clearly, I still don't understand how it all works because of this example code here:
package main
import (
"reflect"
"fmt"
)
type Business struct {
Name string
}
func DoSomething(b []Business) {
var i interface{}
i = &b
v := reflect.ValueOf(i).Elem()
for c:=0 ;c<10; c++ {
z := reflect.New(v.Type().Elem())
s := reflect.ValueOf(z.Interface()).Elem()
s.Field(0).SetString("Pizza Store "+ fmt.Sprintf("%v",c))
v.Set(reflect.Append(v, z.Elem()))
}
fmt.Println(b)
}
func main() {
business := []Business{}
DoSomething(business)
}
When I run this code, it will print a list of ten Business structs with the Business.Name of Pizza 0 to 9. I understand that in my example, that my DoSomething function received a copy of the slice of business, and hence, the business variable in my main function remains unaffected by whatever DoSomething does.
What I did next was change my func DoSomething(b []Business) to func DoSomething(b interface{}). Now when I try to run my script, I get the run time error of panic: reflect: Elem of invalid type on on the line z := reflect.New(v.Type().Elem())
I noticed that with DoSomething(b []Business), the variable i == &[]. But with DoSomething(b interface{}), the variable i == 0xc42000e1d0. Why is the variable i different under these two circumstances?
Your debugger most likely uses (or at least follows) the default formatting rules of the fmt package:
For compound objects, the elements are printed using these rules, recursively, laid out like this:
struct: {field0 field1 ...}
array, slice: [elem0 elem1 ...]
maps: map[key1:value1 key2:value2 ...]
pointer to above: &{}, &[], &map[]
In your first case i holds a value of type *[]Business. So if a value being printed (or inspected) is a pointer to slice, it is printed as &[values].
In your second case i holds a pointer to an interface{} value, which is of type *interface{}. When printing a value of this type, the default %p format is used which simply prints the memory address as a hexadecimal value prefixed with 0x.
I have managed to do this, but it does not look efficient:
var t reflect.Type
switch t {
case reflect.TypeOf(([]uint8)(nil)):
// handle []uint8 array type
}
First question, are you sure you want to switch on reflect.Type and not use a type switch? Example:
switch x := y.(type) {
case []uint8:
// x is now a []uint8
}
Assuming that will not work for your situation, my recommendation is to make those package variables. Example:
var uint8SliceType = reflect.TypeOf(([]uint8)(nil))
func Foo() {
var t reflect.Type
switch t {
case uint8SliceType:
// handle []uint8 array type
}
}
you may not need reflect if you are just trying to detect type.
switch t := myVar.(type){
case []uint8:
// t is []uint8
case *Foo:
// t is *Foo
default:
panic("unknown type")
}
What are you actually trying to accomplish?
The answer to the initial question How to switch on reflect.Type? is: You can’t. However, you can do it with reflect.Value.
Given a variable v interface{} you can call reflect.TypeOf(v) and reflect.ValueOf(v), which return a reflect.Type or reflect.Value, resp.
If the type of v is not interface{} then these function calls will convert it to interface{}.
reflect.Type contains various run-time information about the type, but it does not contain anything usable to retrieve the type of v itself as needed in a type switch.
Hovewer, reflect.Value provides it through its Interface() method, which returns the underlying value as interface{}. This you can use in a type switch or type assertion.
import "fmt"
import "reflect"
var v int
var rt reflect.Type = reflect.TypeOf(v)
fmt.Println(rt.String(), " has awesome properties: Its alignment is",
rt.Align(), ", it has", rt.Size(), "bytes, is it even comparable?",
rt.Comparable())
// … but reflect.Type won’t tell us what the real type is :(
// Let’s see if reflect.Value can help us.
var rv reflect.Value = reflect.ValueOf(v)
// Here we go:
vi := rv.Interface()
switch vi.(type) {
// Mission accomplished.
}
Perhaps it helps to clarify a few points which may cause confusion about dynamic typing in Go. At least I was confused by this for quite some time.
reflect vs. interface{}
In Go there are two systems of run-time generics:
In the language: interface{}, useful for type switches/assertions,
In the library: The reflect package, useful for inspection of run-time generic types and values of such.
These two systems are separated worlds, and things that are possible with one are impossible with the other. For example, Given an interface{}, it is in plain Go (with safe code) impossible to, say, if the value is an array or slice, regardless of its element type, then get the value of the i-th element. One needs to use reflect in order to do that. Conversely, with reflect it is impossible to make a type switch or assertion: convert it to interface{}, then you can do that.
There are only very few points of an interface between these systems. In one direction it is the TypeOf() and ValueOf() functions which accept interface{} and return a reflect struct. In the other direction it is Value.Interface().
It is a bit counter-intuitive that one needs a Value, not a Type, to do a type switch. At least this is somewhat consistent with the fact that one needs a value construct a Type by calling TypeOf().
reflect.Kind
Both reflect.Type and reflect.Value have a Kind() method. Some suggest using the value these methods return, of type reflect.Kind, to imitate a type switch.
While this may be useful in certain situations, it is not a replacement for a type switch. For example, using Kind one cannot distinguish between int64 and time.Duration because the latter is defined as
type Duration int64
Kind is useful to tell if a type is any kind of struct, array, slice etc., regardless of the types it is composed of. This is not possible to find out with a type switch.
(Side note. I had the same question and found no answer here helpful so I went to figure it out myself. The repeated counter-question “why are you doing this?”, followed by unrelated answers did not help me either. I have a good reason why I want to do it precisely this way.)
This might work.
switch t := reflect.TypeOf(a).String() {
case "[]uint8":
default:
}
As others have said, it's not clear what you are trying to achieve by switching on reflect.Type However, I came across this question when probably trying to do something similar, so I will give you my solution in case it answers your question.
As captncraig said, a simple type switch could be done on a interface{} variable without needing to use reflect.
func TypeSwitch(val interface{}) {
switch val.(type) {
case int:
fmt.Println("int with value", val)
case string:
fmt.Println("string with value ", val)
case []uint8:
fmt.Println("Slice of uint8 with value", val)
default:
fmt.Println("Unhandled", "with value", val)
}
}
However, going beyond this, the usefulness of reflection in the context of the original question could be in a function that accepts a struct with arbitrarily typed fields, and then uses a type switch to process the field according to its type. It is not necessary to switch directly on reflect.Type, as the type can be extracted by reflect and then a standard type switch will work. For example:
type test struct {
I int
S string
Us []uint8
}
func (t *test) SetIndexedField(index int, value interface{}) {
e := reflect.ValueOf(t).Elem()
p := e.Field(index)
v := p.Interface()
typeOfF := e.Field(index).Type()
switch v.(type) {
case int:
p.SetInt(int64(value.(int)))
case string:
p.SetString(value.(string))
case []uint8:
p.SetBytes(value.([]uint8))
default:
fmt.Println("Unsupported", typeOfF, v, value)
}
}
The following examples demonstrate the use of this function:
var t = test{10, "test string", []uint8 {1, 2, 3, 4}}
fmt.Println(t)
(&t).SetIndexedField(0, 5)
(&t).SetIndexedField(1, "new string")
(&t).SetIndexedField(2, []uint8 {8, 9})
fmt.Println(t)
(A few points on reflection in go:
It is necessary to export the struct fields for reflect to be able to use them, hence the capitalisation of the field names
In order to modify the field values, it would be necessary to use a pointer to the struct as in this example function
Elem() is used to "dereference" the pointer in reflect
)
Well, I did this by first transfer it to interface and then use the.(type)
ty := reflect.TypeOf(*c)
vl := reflect.ValueOf(*c)
for i:=0;i<ty.NumField();i++{
switch vl.Field(i).Interface().(type) {
case string:
fmt.Printf("Type: %s Value: %s \n",ty.Field(i).Name,vl.Field(i).String())
case int:
fmt.Printf("Type: %s Value: %d \n",ty.Field(i).Name,vl.Field(i).Int())
}
}
I just need a pointer to time.Time, so the code below seems invalid:
./c.go:5: cannot take the address of time.Now()
I just wonder why? Is there any way to do that except to do assignment to a variable first and take the pointer of the variable?
package main
import "time"
func main() {
_ = &time.Now()
}
The probably unsatisfying answer is "you can't do it because the spec says so." The spec says that to use & on something it has to be addressable or a compound literal, and to be addressable it has to be "a variable, pointer indirection, or slice indexing operation; or a a field selector of an addressable struct operand; or an array indexing operation of an addressable array." Function calls and method calls are definitely not on the list.
Practically speaking, it's probably because the return value of a function may not have a usable address; it may be in a register (in which case it's definitely not addressable) or on the stack (in which case it has an address, but one that won't be valid if it's put in a pointer that escapes the current scope. To guarantee addressability, Go would have to do pretty much the exact equivalent of assigning it to a variable. But Go is the kind of language that figures that if it's going to allocate storage for a variable it's going to be because you said to, not because the compiler magically decided to. So it doesn't make the result of a function addressable.
Or I could be over-thinking it and they simply didn't want to have a special case for functions that return one value versus functions that return multiple :)
You can't directly take the address of a function call (or more precisely the return value(s) of the function) as described by hobbs.
There is another way but it is ugly:
p := &[]time.Time{time.Now()}[0]
fmt.Printf("%T %p\n%v", p, p, *p)
Output (Go Playground):
*time.Time 0x10438180
2009-11-10 23:00:00 +0000 UTC
What happens here is a struct is created with a literal, containing one element (the return value of time.Now()), the slice is indexed (0th element) and the address of the 0th element is taken.
So rather just use a local variable:
t := time.Now()
p := &t
Or a helper function:
func ptr(t time.Time) *time.Time {
return &t
}
p := ptr(time.Now())
Which can also be a one-liner anonymous function:
p := func() *time.Time { t := time.Now(); return &t }()
Or as an alternative:
p := func(t time.Time) *time.Time { return &t }(time.Now())
For even more alternatives, see:
How do I do a literal *int64 in Go?
Also see related question: How can I store reference to the result of an operation in Go?
Fortunately, generics now offer quite a clean solution by defining a function only one time, that can be used on any type:
package main
func ptr[T any](x T) *T {
return &x
}
func main() {
print(ptr("foo"))
print(ptr(42))
}
Playground: https://go.dev/play/p/TgpEPKjpXX7
However, this will work only starting from Golang 1.18. For previous versions, you'll need a function for each type, as other answers suggested.
If you are having this trouble with a function you wrote, change your function to return a pointer. Even though you can't take the address of a return value, you can dereference a return value, so it will be suitable whether you want the pointer or the object.
func Add(x, y int) *int {
tmp := x + y
return &tmp
}
func main() {
fmt.Println("I want the pointer: ", Add(3, 4))
fmt.Println("I want the object: ", *Add(3, 4))
}
https://play.golang.org/p/RogRZDNGdmY
I have the function below which accepts a bool pointer. I'm wondering if there is any notation which allows me to set the value of the is field to true in the struct literal; basically without to define a new identifier (i.e. var x := true ; handler{is: &x} )
package main
import "fmt"
func main() {
fmt.Println("Hello, playground")
check(handler{is: new(bool) })
}
type handler struct{
is *bool
}
func check(is handler){}
You can do that but it's not optimal:
h := handler{is: &[]bool{true}[0]}
fmt.Println(*h.is) // Prints true
Basically it creates a slice with one bool of value true, indexes its first element and takes its address. No new variable is created, but there is a lot of boilerplate (and backing array will remain in memory until the address to its first element exists).
A better solution would be to write a helper function:
func newTrue() *bool {
b := true
return &b
}
And using it:
h := handler{is: newTrue()}
fmt.Println(*h.is) // Prints true
You can also do it with a one-liner anonymous function:
h := handler{is: func() *bool { b := true; return &b }()}
fmt.Println(*h.is) // Prints true
Or a variant:
h := handler{is: func(b bool) *bool { return &b }(true)}
To see all your options, check out my other answer: How do I do a literal *int64 in Go?
This simplest way is to write a short function to turn a bool into a *bool.
func BoolPointer(b bool) *bool {
return &b
}
h := handler{is: BoolPointer(true)}
No.
There is no syntax to define a pointer to a primitive type, other than the zero value returned by new. The same goes for numeric types, and strings.
You either need to create a value before hand to take the address of, or you create the pointer with a zero value, and assign a new value after the fact.
I used a function similar to #icza but in a way more convenient (for me)
I created a BoolAddr function in my utils package
package utils
func BoolAddr(b bool) *bool {
boolVar := b
return &boolVar
}
For me it's easier to use
package main
import "example.com/example/utils"
...
type Example struct {
isActive *bool
}
ex := Expample {
isActive: utils.BoolAddr(true)
}
...
One of the reasons why pointers are helpful in go or any language for that matter, is they help us to "pass by reference". So if we pass anything by reference we can then "change" that thing. A function which takes a pointer to bool, can change the bool's value effectively even after the function returns. This is the very thing we do not want with constants, ie. their values should not change. Hence this restriction makes sense.
Apart from the tricks mentioned by icza above, would want to add a point here. Mostly we use pointers to bools rather than bools directly in order to use the nil value of pointers effectively, which otherwise have to be either true or false. If that IS the case, then you might want to use optional bool flags directly in the functions, rather than have pointers to bool or even a struct wrapping the single bool pointer as shown in your example, doing away with the complete requirement of a struct even.. Now, of course if the struct is reqd for any other reason, you can very well use any of the tricks by icza above.
Btw, you can directly have a copy of the bool value for using the adress of as below as well.
const check = true
chk := check
fmt.Println(&chk) // will give you the address of chk
chk = false
fmt.Println(chk) // will print false
fmt.Println(check) // will print true
So let's say that we have a function of the following form:
func WorkMagic(obj interface{}) interface{} {
switch t := obj.(type) {
case string:
// Do string magic
default:
// Do slice magic
}
...
}
I am expecting obj to be either a string or a slice, which I can ascertain via the switch. In the case of a slice, I want to be able to do ordering work on any arbitrary slice, regardless of type. Seems like the best way to accomplish this is using the unsafe package in a similar fashion to that discussed in this article.
Here however, the function accepts a specific type of slice ([]string), whereas I would like to be able to work on any slice. So the question is, given that I am accepting an empty interface as input, how might I access the underlying slice / array using unsafe.Pointer so as to be able to loop through and modify which value is associate with which index?
You'll want to use reflection. It enables you to work generically without giving up type and memory safety like unsafe would. Read the Go blog's Laws of Reflection.
func actOnSlices(i interface{}) {
v := reflect.ValueOf(i)
for v.Kind() == reflect.Ptr { // dereference pointers
v = v.Elem()
}
if v.Kind() != reflect.Slice { // ensure you actually got a slice
panic("given argument is not a slice")
}
// do slice stuff
}
Edit to answer your second question:
Yes – this can be done: elements of a slice are adressable and hence settable. See the following working example:
package main
import (
"fmt"
"reflect"
)
func main() {
s := []string{"foo", "bar"}
fmt.Println(swapIndexes(s, 0, 1)) // prints [bar foo]
}
func swapIndexes(i interface{}, x, y int) interface{} {
v := reflect.ValueOf(i)
for v.Kind() == reflect.Ptr { // dereference pointers
v = v.Elem()
}
if v.Kind() != reflect.Slice { // ensure you actually got a slice
panic("given argument is not a slice")
}
t := v.Index(x).Interface()
v.Index(x).Set(v.Index(y))
v.Index(y).Set(reflect.ValueOf(t))
return v.Interface()
}
Edit to answer your third question:
The unsafe package is not something you'll encounter much in user-land code. It exists to implement certain features (e.g. reflection, C interaction) that need to circumvent Go's safety guarantees to work. Using unsafe is unsafe, as the name suggests, because you can mess up big time without even realizing. By using unsafe, you're incurring in a big trade-off, so it better be worth it. Quoting #twotwotwo:
The downside of unsafe is that if you mess up you're in the old days of segfaults, memory corruption, and buffer-overflow security holes.
Also, as #twotwotwo suggested; it's more "Go-like" to repeat code than using reflection to achieve genericity.
To Go's type-system, []string and []int are two completely separate and unrelated types. just as int and string would be. The relation (both are slices) is obvious only to the programmer. There is no way of expressing "a slice" without saying a slice of what.