Exported and UnExported fields of a struct in Go (Golang)

Go doesn’t have any public,  private or protected keyword. The only mechanism to control the visibility outside the package is using the capitalized and non-capitalized formats

• Capitalized Identifiers are exported. The capital letter indicates that this is an exported identifier and is available outside the package.

• Non-capitalized identifiers are not exported. The lowercase indicates that the identifier is not exported and will only be accessed from within the same package.

So any struct which starts with a capital letter is exported to other packages.  Similarly, any struct field which starts with capital is exported otherwise not. And also similarly any struct method which starts with a capital letter is exported. Let’s see an example that shows exporting and non-exporting of structs, struct fields, and methods. See model.go and test.go below. Both belong to the main package.

• Structure
  • Struct Person is exported
  • Struct company is non-exported

• Structure’s Field
  • Person struct field Name is exported
  • Person struct field age is not exported but Name is exported

• Structure’s Method
  • Person Struct’s Method GetAge() is exported
  • Person Struct’s Method getName() is not exported

model.go

package main

import "fmt"

//Person struct
type Person struct {
    Name string
    age  int
}

//GetAge of person
func (p *Person) GetAge() int {
    return p.age
}

func (p *Person) getName() string {
    return p.Name
}

type company struct {
}

Let’s write a file test.go in same main package. See below.

test.go

package main

import "fmt"

//Test function
func Test() {
    //STRUCTURE IDENTIFIER
    p := &Person{
        Name: "test",
        age:  21,
    }
    fmt.Println(p)
    c := &company{}
    fmt.Println(c)
    
    //STRUCTURE'S FIELDS
    fmt.Println(p.Name)
    fmt.Println(p.age)

    
    //STRUCTURE'S METHOD
    fmt.Println(p.GetAge())
    fmt.Println(p.getName())

}

On running this file, it is able to access all exported and un-exported fields in model.go as both lies in the same package main. There is no compilation error and it gives below output

Output:

&{test 21}
&{}
test
21
21
test

Let’s move the above file model.go to a different package named model. Now notice the output on running ‘go build’. It gives compilation errors. All the compilation error are because test.go in main package to not able to refer to un-exported fields of model.go in model package

model.go

package model

//Person struct
type Person struct {
	Name string
	age  int
}

func (p *Person) GetAge() int {
    return p.age
}

func (p *Person) getName() string {
    return p.Name
}

type company struct {
}

test.go

package main

import (
	"fmt"
        //This will path of your model package
	"/model"
)

//Test function
func main() {
	//STRUCTURE IDENTIFIER
	p := &model.Person{
		Name: "test",
		age:  21,
	}
	fmt.Println(p)
	c := &model.company{}
	fmt.Println(c)

	//STRUCTURE'S FIELDS
	fmt.Println(p.Name)
	fmt.Println(p.age)
          
        //STRUCTURE'S METHOD
        fmt.Println(p.GetAge())
        fmt.Println(p.getName())

}

Output:

unknown field 'age' in struct literal of type model.Person
cannot refer to unexported name model.company
undefined: model.company
p.age undefined (cannot refer to unexported field or method age)
p.getName undefined (cannot refer to unexported field or method model.(*Person).getName)