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.
Related
I'm currently learning Go and am following a tutorial about how to use Go with Stripe. There is this example code:
package main
import (
"fmt"
"github.com/stripe/stripe-go"
"github.com/stripe-go/customer"
)
func main() {
sc := &client.API{}
sc.Init("somekey")
c, _ := sc.Customers.Get("customerid", nil)
// ...
}
What is/could be the reason that sc stores the pointer to the struct and not the struct itself?
[To supplement the comment you received]
While in this case with the small code sample it's hard to say, in most scenarios you'll see non-trivial types passed around by pointer to enable modification. As an anti-example, consider this code which uses a variable of a struct type by value:
type S struct {
ID int
}
func (s S) UpdateID(i int) {
s.ID = i
}
func main() {
s := S{}
s.UpdateID(99)
fmt.Println(s.ID)
}
What do you think this will print? It will print 0, because methods with value receivers cannot modify the underlying type.
There's much information about this in Go - read about pointers, and about how methods should be written. This is a good reference: https://golang.org/doc/faq#methods_on_values_or_pointers, and also https://golang.org/doc/effective_go#pointers_vs_values
Back to your example: typically non-trivial types such as those representing a "client" for some services will be using pointers because method calls on such types should be able to modify the types themselves.
I want to create cache with map. As map doesn't allow reference to its value, so it's not possible to change values in called functions.
After some search, I found, it's possible with creating map of pointer (of struct). It Almost solve problem and can work like variable by reference
But as i found a few using of this method for map. I worry about using it to be safe.
Is anyone has experience of using map of pointer? and is it right way to use it?
package main
import "fmt"
type Cache struct {
name string
counter int
}
func incr(c Cache) {
c.counter += 1
}
func incrp(c *Cache) {
c.counter += 2
}
func main() {
m := make(map[string]Cache)
m["james"] = Cache{name: "James", counter: 10}
c := m["james"]
incr(c)
fmt.Println(c.name, c.counter) // James 10
mp := make(map[string]*Cache)
mp["james"] = &Cache{name: "James", counter: 10}
cp := mp["james"]
incrp(cp)
fmt.Println(cp.name, cp.counter) // James 12
}
edited: My text had some confusing words and sentences, that caused to misunderstanding, so i tried to fixed it
You can accomplish this and still have a map of non-pointers, with a pointer receiver on the struct:
package main
import "fmt"
type Cache struct {
name string
counter int
}
func (c *Cache) incr() { // the '(c *Cache)' is the receiver;
c.counter += 1 // it makes incr() a method, not just a function
}
func main() {
m := make(map[string]Cache)
m["james"] = Cache{name: "James", counter: 10}
c := m["james"]
c.incr()
fmt.Println(c.name, c.counter)
}
Output:
James 11
If receivers and methods are new to you, here is where they are mentioned in the Tour of Go: https://tour.golang.org/methods/1
Note the page about pointer receivers a few steps later in the Tour: https://tour.golang.org/methods/4
in the main package i have:
var foo C.int
foo = 3
t := fastergo.Ctuner_new()
fastergo.Ctuner_register_parameter(t, &foo, 0, 100, 1)
in the fastergo package i have:
func Ctuner_register_parameter(tuner unsafe.Pointer, parameter *C.int, from C.int, to C.int, step C.int) C.int {
...
}
if i try to run it, i get:
demo.go:14[/tmp/go-build742221968/command-line-arguments/_obj/demo.cgo1.go:21]: cannot use &foo (type *_Ctype_int) as type *fastergo._Ctype_int in function argument
i am not really sure what go is trying to tell me here, but somehow i think it wants to tell me, that all C.int are not equal? why is this the case? how can i solve this / work around?
Since _Ctype_int doesn't begin with a Unicode upper case letter, the type is local to the package. Use Go types, except in the C wrapper package where you convert them to C types. The wrapper package should hide all the implementation details.
You don't provide sufficient information for us to create sample code which compiles and runs. Here's a rough outline of what I expected to see:
package main
import "tuner"
func main() {
var foo int
foo = 3
t := tuner.New()
t.RegisterParameter(&foo, 0, 100, 1)
}
.
package tuner
import (
"unsafe"
)
/*
#include "ctuner.h"
*/
import "C"
type Tuner struct {
ctuner uintptr
}
func New() *Tuner {
var t Tuner
t.ctuner = uintptr(unsafe.Pointer(C.ctuner_new()))
return &t
}
func (t *Tuner) RegisterParameter(parameter *int, from, to, step int) error {
var rv C.int
rv = C.ctuner_register_parameter(
(*C.ctuner)(unsafe.Pointer(t.ctuner)),
(*C.int)(unsafe.Pointer(parameter)),
C.int(from),
C.int(to),
C.int(step),
)
if rv != 0 {
// handle error
}
return nil
}
As explained by peterSO, you can't pass C.int between packages. However, you can pass pointers between packages by converting the pointer type. To do this, you would define a named type in the target package, import that type into the calling package and covert via unsafe.Pointer. There isn't any point in doing this with a single int.
However, it is helpful if you keep code to convert complex types in a package; for example an array of strings (or any sort of nested array).
The example below is for exporting a go function to be called in C, but this works in reverse, ie. if you want to call a C functions which a returns nested array.
package convert
import "C"
type PP_char **C.char
func From_c_to_go(arr_str PP_char, length int) []string {
// Some operation on the Ctype
var slice []string
for _, s := range unsafe.Slice(arr_str, length) {
if s == nil {
break
}
x := C.GoString(s)
slice = append(slice, x)
}
return slice
}
package main
import "C"
import "convert"
//export myFunc
func myFunc(arr_str **C.char, length int){
retyped_arr_str := convert.PP_char(unsafe.Pointer(arr_str))
slice := convert.From_c_to_go(retyped_arr_str, length)
// Do something with slice
}
You could instead decide to pass instance of unsafe.Pointer as an argument to the go function in the target package and perform the type conversion in that function.
In Go, is there a notable difference between the following two segments of code:
v := &Vector{}
as opposed to
v := new(Vector)
No. What they return is the same,
package main
import "fmt"
import "reflect"
type Vector struct {
x int
y int
}
func main() {
v := &Vector{}
x := new(Vector)
fmt.Println(reflect.TypeOf(v))
fmt.Println(reflect.TypeOf(x))
}
Result:
*main.Vector
*main.Vector
There is some contention on the mailing list that having both is confusing:
https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/GDXFDJgKKSs
One thing to note:
new() is the only way to get a pointer to an
unnamed integer or other basic type. You can write "p := new(int)" but
you can't write "p := &int{0}". Other than that, it's a matter of
preference.
Source : https://groups.google.com/d/msg/golang-nuts/793ZF_yeqbk/-zyUAPT-e4IJ
Yes, there is a fundamental difference between the two code fragments.
v := &Vector{}
Works only for Vector being a struct type, map type, array type or a slice type
v := new(Vector)
Works for Vector of any type.
Example: http://play.golang.org/p/nAHjL1ZEuu
Here is a difference: for a Person struct, the JSON string marshalled from &[]*Person{} is [] and from new([]*Person) is null using json.Marshal.
Check out the sample here: https://play.golang.org/p/xKkFLoMXX1s
https://golang.org/doc/effective_go.html#allocation_new
https://golang.org/doc/effective_go.html#composite_literals
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