Custom type to string

Getting Help
1

Hello guys,

I really really do not understand why fmt.Println(“hahoooExecuting SXtring() for CustomType!”) gets executed…

Looks like somehow, String() gets executed… but why? I have tried to look at Println documentation to see if anything implements them and using custom string as receiver but I did not see any… What am I missing? Please point me to right direction as I thought I start to understand golang but this just completely destroyed me and have me stuck here… Please help. I am desperate to understand this.

package main

import (
	"fmt"

)

type CustomType string

const (
	Foobar CustomType = "somestring"
)

func main() {
	fmt.Println("1")
	fmt.Println(Foobar, Foobar, "Hello, playground", Foobar)


		fmt.Println("2")

	//fmt.Printf("%s", Foobar)
	//fmt.Println("\n\n")
	//fmt.Println(SomeFunction())
}

func (c CustomType) String() string {
	fmt.Println("hahoooExecuting SXtring() for CustomType!")
	return string(c)
}

//func SomeFunction() string {
//	return Foobar.String()
//}
2

Take A Tour of Go. In particular, Stringers.

Package fmt

type Stringer

Stringer is implemented by any value that has a String method, which defines the “native” format for that value. The String method is used to print values passed as an operand to any format that accepts a string or to an unformatted printer such as Print.

type Stringer interface {
    String() string
}
3

Thanks for this and helped a lot. However, Having this knowledge, I still cannot connect to why Println is using String method… I do not see that in the doc

4

It is in the documentation.

5

I continue to read the documentation and still not clear(not your fault @petrus)… It is just my lack of understanding on golang but this is driving me crazy.

I get that Stringer is an interface which any that implements String func, it is eligible… But I fail to see how printer such as fmt.Println is suppose to implements func String() Where in the doc says that ? From this particular example, Is this because type is string and matches “String() string” part from the Stringer interface definition?

6

And I just tried below to see if the string theory that I am having it in my head is correct and it is not… (I somehow thought because return type is string, it’s matching it)

type Person struct {

	Age  int
}

func (p Person) String() string {
fmt.Println("this is crazy")
	return fmt.Sprintf(" (%v years)", p.Age)
}

func main() {
	a := Person{ 42}
	z := Person{9001}
	fmt.Println(a, z)
}
7

Finally understood… thank you

8

Follow the documentation:

Package fmt

type Stringer

Stringer is implemented by any value that has a String method, which defines the “native” format for that value. The String method is used to print values passed as an operand to any format that accepts a string or to an unformatted printer such as Print.

type Stringer interface {
    String() string
}

fmt.Println is an unformatted printer. Person satisfies the Stringer interface, it has a String() string method. Therefore, fmt calls the Person String() method with a Person receiver to return the string to print for any Println type Person operand (argument).

package main

import "fmt"

type Person struct {
    Age int
}

func (p Person) String() string {
    return fmt.Sprintf("%v years old", p.Age)
}

func main() {
    p := Person{42}
    fmt.Println(p)
}

42 years old

1 Like
9

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