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I want to call a function with a int variable that can have a value or can be nil. In the main function i'm doing this call.
doDomething(10) // does not work?
doDomething(nil) // works
func doSomething(parent_id *int) {
fmt.Print(parent_id)
}
I'm getting the following error:
cannot use a (type int) as type *int in argument to doSomething
I use the *int pointer, therefore the nil works but not if it is a value. Then I'm getting the type mismatch.
doSomething expects a *intparameter. 10 is of type int, so this is he source of the error.
This is a valid usage:
i := 10
doDomething(&i)
This is how doSomething should look like:
func doSomething(parent_id *int) {
if parent_id == nil{
//nil
fmt.Println("nil")
return
}
//valid int
fmt.Printf("%d", *parent_id)
}
Related
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i wanna know if we are able to use inbuild data types as a method for a func in golang, cause whenever I use it as such, it shows an error
You can define methods on built-in types by first wrapping them with your own types, like this:
type MyInteger int
func (my MyInteger) Tell() {
fmt.Println("I'm MyInteger with value", my)
}
func main() {
var my MyInteger = 42
my.Tell()
}
You can try this on the Go Playground, it will print:
I'm MyInteger with value 42
This can be useful if you want to make a built-in type implement an interface. For example, here's how MyInteger would implement the fmt.Stringer interface:
type MyInteger int
func (my MyInteger) String() string {
return "MyInteger " + strconv.Itoa(int(my))
}
func main() {
var my MyInteger = 42
fmt.Println(my)
}
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When I try to assign a map key to a function it executes the function. How can I do this without executing the function?
var myslice []int
myslice = append(myslice, 1)
func RemoveIndex(s []string, index int) []string {
return append(s[:index], s[index+1:]...)
}
a["removeIndex"] = removeIndex(myslice, 0)
Just assign the function to the map[string]func([]string,int)[]string's key
func RemoveIndex(s []string, index int) []string {
return append(s[:index], s[index+1:]...)
}
a["removeIndex"] = RemoveIndex
rifn := a["removeIndex"]
rifn(myslice, 0)
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An error in Go can be nil. The following compiles:
var err error
err = nil
err = errors.New("hello")
Yet errors are values, and value types are not nullable in Go.
Looking at error, it is an ordinary interface:
type error interface {
Error() string
}
implemented by this struct:
type errorString struct {
s string
}
No pointers here. There is a method that takes a pointer receiver,
func (e *errorString) Error() string {
return e.s
}
Yet that doesn't explain why error behaves as a pointer rather than a value.
errors.New() does return a pointer,
func New(text string) error {
return &errorString{text}
}
which makes my third code line above more puzzling -- we are assigning the result of New to a value variable.
How does this work?
I think you may be conflating some concepts. Yes, "errors are values", in that they are not exceptions that can be thrown, but rather they are just regular values returned from a function like any other return value. The Go quality "errors are values" has nothing to do with value vs reference semantics.
The built-in type error is an interface, and all interface values are nilable, so error is nilable.
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I am trying to find if a variable is of type float64:
package main
import ("fmt")
func main() {
myvar := 12.34
if myvar.(type) == float64 {
fmt.Println("Type is float64.")
}
}
However, it is not working and giving following error:
./rnFindType.go:6:10: use of .(type) outside type switch
./rnFindType.go:6:21: type float64 is not an expression
What is the problem and how can it be solved?
You know that myvar is a float64 because the variable is declared with the concrete type float64.
If myvar is an interface type, then you can use a type assertion to determine if the concrete value is some type.
var myvar interface{} = 12.34
if _, ok := myvar.(float64); ok {
fmt.Println("Type is float64.")
}
Try this program at https://play.golang.org/p/n5ftbp5V2Sx
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How do you construct an interface as a parameter to a function?
type blahinterface interface {
method1()
method2()
method3()
}
func blah (i blahinterface) {
}
blah(?) < what goes in here
Actually, if you try to put anything "in here", the compiler will tell you precisely what is missing:
type S struct{}
func main() {
fmt.Println("Hello, playground")
s := &S{}
blah(s)
}
A go build on this example would tell you:
prog.go:20: cannot use s (type *S) as type blahinterface in argument to blah:
*S does not implement blahinterface (missing method1 method)
[process exited with non-zero status]
But with:
func (s *S) method1(){}
func (s *S) method2(){}
func (s *S) method3(){}
The program do compile just fine.
So even without reading about interfaces, you are guided and can guess what is missing.