After creating a struct like this:
type Foo struct {
name string
}
func (f Foo) SetName(name string) {
f.name = name
}
func (f Foo) GetName() string {
return f.name
}
How do I create a new instance of Foo and set and get the name?
I tried the following:
p := new(Foo)
p.SetName("Abc")
name := p.GetName()
fmt.Println(name)
Nothing gets printed, because name is empty. So how do I set and get a field inside a struct?
Working playground
Commentary (and working) example:
package main
import "fmt"
type Foo struct {
name string
}
// SetName receives a pointer to Foo so it can modify it.
func (f *Foo) SetName(name string) {
f.name = name
}
// Name receives a copy of Foo since it doesn't need to modify it.
func (f Foo) Name() string {
return f.name
}
func main() {
// Notice the Foo{}. The new(Foo) was just a syntactic sugar for &Foo{}
// and we don't need a pointer to the Foo, so I replaced it.
// Not relevant to the problem, though.
p := Foo{}
p.SetName("Abc")
name := p.Name()
fmt.Println(name)
}
Test it and take A Tour of Go to learn more about methods and pointers, and the basics of Go at all.
Setters and getters are not that idiomatic to Go.
Especially the getter for a field x is not named GetX
but just X.
See http://golang.org/doc/effective_go.html#Getters
If the setter does not provide special logic, e.g.
validation logic, there is nothing wrong with exporting
the field and neither providing a setter nor a getter
method. (This just feels wrong for someone with a
Java background. But it is not.)
For example,
package main
import "fmt"
type Foo struct {
name string
}
func (f *Foo) SetName(name string) {
f.name = name
}
func (f *Foo) Name() string {
return f.name
}
func main() {
p := new(Foo)
p.SetName("Abc")
name := p.Name()
fmt.Println(name)
}
Output:
Abc
Related
I have a type with three fields
type Person struct {
FirstName string
LastName string
Age int
}
Creating an instance and using default fmt.Sprint() returns {John Smith 45}. However for my use case I need a string of format John, Smith, 45. A comma delimited list without being surrounded by curly braces. Is there a more reusable and effective way than:
fmt.Sprintf("%s, %s, %d", x.FirstName, x.LastName, x.Age)
I will be using this method alot with other types and I would prefer a generic method rather than having to type out a format for each type I use:
func asFields(data interface{}) string {
// TODO logic here
}
That exact format is not supported by verbs of the fmt package.
The closest would be
s := fmt.Sprintf("%#v", p)
Which generates a string like:
main.Person{FirstName:"John", LastName:"Smith", Age:45}
If you exactly need what you posted in the question, you may use reflection to iterate over the fields and build the result like this:
func asFields(data interface{}) string {
v := reflect.ValueOf(data)
b := &strings.Builder{}
for i := 0; i < v.NumField(); i++ {
if i > 0 {
b.WriteString(", ")
}
b.WriteString(fmt.Sprint(v.Field(i).Interface()))
}
return b.String()
}
This indeed gives:
John, Smith, 45
Try the examples on the Go Playground.
Note that this asFields() function handles all struct types of course not just your Person. Adjustment would be needed to handle pointers and struct of structs of course.
Also note that alternatively to fmt.Sprint() you may also use fmt.Fprint() directed to the buffer in which we're assembling the string:
func asFields(data interface{}) string {
v := reflect.ValueOf(data)
b := &strings.Builder{}
for i := 0; i < v.NumField(); i++ {
if i > 0 {
b.WriteString(", ")
}
fmt.Fprint(b, v.Field(i).Interface())
}
return b.String()
}
Which of course gives the same result (and may or may not be faster, benchmark it). Try it on the Go Playground.
Can we use this approach, it will be applicable only to Person type struct though ?
https://play.golang.org/p/YI2Nu0q51ls
package main
import (
"fmt"
)
type Person struct {
FirstName string
LastName string
Age int
}
func (p Person) String() string {
return fmt.Sprintf("%s, %s, %d", p.FirstName, p.LastName, p.Age)
}
func main() {
p := Person{FirstName: "John",LastName: "Doe", Age: 25}
fmt.Printf("%v", p)
}
Output:
John, Doe, 25
Similar SO question:
ToString() function in Go
The Golang "fmt" package has a dump method called Printf("%+v", anyStruct). I'm looking for any method to dump a struct and its methods too.
For example:
type Foo struct {
Prop string
}
func (f Foo)Bar() string {
return f.Prop
}
I want to check the existence of the Bar() method in an initialized instance of type Foo (not only properties).
Is there any good way to do this?
You can list the methods of a type using the reflect package. For example:
fooType := reflect.TypeOf(&Foo{})
for i := 0; i < fooType.NumMethod(); i++ {
method := fooType.Method(i)
fmt.Println(method.Name)
}
You can play around with this here: http://play.golang.org/p/wNuwVJM6vr
With that in mind, if you want to check whether a type implements a certain method set, you might find it easier to use interfaces and a type assertion. For instance:
func implementsBar(v interface{}) bool {
type Barer interface {
Bar() string
}
_, ok := v.(Barer)
return ok
}
...
fmt.Println("Foo implements the Bar method:", implementsBar(Foo{}))
Or if you just want what amounts to a compile time assertion that a particular type has the methods, you could simply include the following somewhere:
var _ Barer = Foo{}
In this bizarre example, someone has created a new type which is really just a string:
type CustomType string
const (
Foobar CustomType = "somestring"
)
func SomeFunction() string {
return Foobar
}
However, this code fails to compile:
cannot use Foobar (type CustomType) as type string in return argument
How would you fix SomeFunction so that it is able to return the string value of Foobar ("somestring") ?
Convert the value to a string:
func SomeFunction() string {
return string(Foobar)
}
Better to define a String function for the Customtype - it can make your life easier over time - you have better control over things as and if the structure evolves. If you really need SomeFunction then let it return Foobar.String()
package main
import (
"fmt"
)
type CustomType string
const (
Foobar CustomType = "somestring"
)
func main() {
fmt.Println("Hello, playground", Foobar)
fmt.Printf("%s", Foobar)
fmt.Println("\n\n")
fmt.Println(SomeFunction())
}
func (c CustomType) String() string {
fmt.Println("Executing String() for CustomType!")
return string(c)
}
func SomeFunction() string {
return Foobar.String()
}
https://play.golang.org/p/jMKMcQjQj3
For every type T, there is a corresponding conversion operation T(x)
that converts the value x to type T. A conversion from one type to
another is allowed if both have the same underlying type, or if both
are unnamed pointer types that point to variables of the same
underlying type; these conversions change the type but not the
representation of the value. If x is assignable to T, a conversion
is permitted but is usually redundant. - Taken from The Go
Programming Language - by Alan A. A. Donovan
As per your example here are some of the different examples which will return the value.
package main
import "fmt"
type CustomType string
const (
Foobar CustomType = "somestring"
)
func SomeFunction() CustomType {
return Foobar
}
func SomeOtherFunction() string {
return string(Foobar)
}
func SomeOtherFunction2() CustomType {
return CustomType("somestring") // Here value is a static string.
}
func main() {
fmt.Println(SomeFunction())
fmt.Println(SomeOtherFunction())
fmt.Println(SomeOtherFunction2())
}
It will output:
somestring
somestring
somestring
The Go Playground link
You can convert like this:
var i int = 42
var f float64 = float64(i)
check here
you can return like this:
return string(Foobar)
Say for example you have something like this, trying to make the example as simple as possible.
type Home struct {
Bedroom string
Bathroom string
}
How do you pass the field name, or can you, to a function?
func (this *Home) AddRoomName(fieldname, value string) {
this.fieldname = value
}
Obviously that does not work... The only way I can see to do this is to use two functions which adds a lot of extra code when the struct gets really big and has a lot of similar code.
func (this *Home) AddBedroomName(value string) {
this.Bedroom = value
}
func (this *Home) AddBathroomName(value string) {
this.Bathroom = value
}
The only way that I am aware of is to use reflection:
func (this *Home) AddRoomName(fieldname, value string) {
h := reflect.ValueOf(this).Elem()
h.FieldByName(fieldname).Set(reflect.ValueOf(value))
return
}
http://play.golang.org/p/ZvtF_05CE_
One more idea that comes to my mind is like this, not sure if it makes sense in your case though:
func Set(field *string, value string) {
*field = value
}
home := &Home{"asd", "zxc"}
fmt.Println(home)
Set(&home.Bedroom, "bedroom")
Set(&home.Bathroom, "bathroom")
fmt.Println(home)
http://play.golang.org/p/VGb69OLX-X
Use type assertions on an interface value:
package main
import "fmt"
type Test struct {
S string
I int
}
func (t *Test) setField(name string, value interface{}) {
switch name {
case "S":
t.S = value.(string)
case "I":
t.I = value.(int)
}
}
func main() {
t := &Test{"Hello", 0}
fmt.Println(t.S, t.I)
t.setField("S", "Goodbye")
t.setField("I", 1)
fmt.Println(t.S, t.I)
}
Take this sample of code (playground):
package main
import (
"fmt"
)
type Foo struct {
Name string
}
var data = make(map[string]interface{})
func main() {
data["foo"] = &Foo{"John"}
foo := data["foo"].(*Foo)
fmt.Println(foo.Name)
}
When I add something to data, the type turns into an interface{}, so when I later retrieve that value I have to assert the original type back onto it. Is there a way to, for example, define a getter function for data which will automagically assert the type?
Not really, unless you turn to reflect and try to get the type of the interface that way.
But the idiomatic (and faster) way remains the type assertion (a "type conversion" which must be checked at runtime, since data only contains interface{} values).
If data were to reference a specific interface (instead of the generic interface{} one), like I mentioned here, then you could use a Name() method defined directly on it.
You can do something like this, but you might want to think about your design.. It is very rare that you need to do this kind of things.
http://play.golang.org/p/qPSxRoozaM
package main
import (
"fmt"
)
type GenericMap map[string]interface{}
func (gm GenericMap) GetString(key string) string {
return gm[key].(string)
}
func (gm GenericMap) GetFoo(key string) *Foo {
return gm[key].(*Foo)
}
func (gm GenericMap) GetInt(key string) int {
return gm[key].(int)
}
var data = make(GenericMap)
type Foo struct {
Name string
}
func main() {
data["foo"] = &Foo{"John"}
foo := data.GetFoo("foo")
fmt.Println(foo.Name)
}
You might want to add error checking, in case the key does not exists or is not the expected type.