11.exporting 导出

Declare and access exported identifiers - Pkg

// Package counters provides alert counter support.
package counters

// AlertCounter is an exported named type that
// contains an integer counter for alerts.
type AlertCounter int

Declare and access exported identifiers - Main

// Sample program to show how to access an exported identifier.
package main

import (
	"fmt"

	"github.com/ardanlabs/gotraining/topics/go/language/exporting/example1/counters"
)

func main() {

	// Create a variable of the exported type and initialize the value to 10.
	counter := counters.AlertCounter(10)

	fmt.Printf("Counter: %d
", counter)
}

Declare unexported identifiers and restrictions - Pkg (

// Package counters provides alert counter support.
package counters

// alertCounter is an unexported named type that
// contains an integer counter for alerts.
type alertCounter int

Declare unexported identifiers and restrictions - Main

// Sample program to show how the program can't access an
// unexported identifier from another package.
package main

import (
	"fmt"

	"github.com/ardanlabs/gotraining/topics/go/language/exporting/example2/counters"
)

func main() {

	// Create a variable of the unexported type and initialize the value to 10.
	counter := counters.alertCounter(10)

	// ./example2.go:17: cannot refer to unexported name counters.alertCounter
	// ./example2.go:17: undefined: counters.alertCounter

	fmt.Printf("Counter: %d
", counter)
}

Access values of unexported identifiers - Pkg

// Package counters provides alert counter support.
package counters

// alertCounter is an unexported named type that
// contains an integer counter for alerts.
type alertCounter int

// New creates and returns values of the unexported type alertCounter.
func New(value int) alertCounter {
	return alertCounter(value)
}

Access values of unexported identifiers - Main

// Sample program to show how the program can access a value
// of an unexported identifier from another package.
package main

import (
	"fmt"

	"github.com/ardanlabs/gotraining/topics/go/language/exporting/example3/counters"
)

func main() {

	// Create a variable of the unexported type using the exported
	// New function from the package counters.
	counter := counters.New(10)

	fmt.Printf("Counter: %d
", counter)
}

Unexported struct type fields - Pkg

// Package users provides support for user management.
package users

// User represents information about a user.
type User struct {
	Name string
	ID   int

	password string
}

Unexported struct type fields - Main

// Sample program to show how unexported fields from an exported struct
// type can't be accessed directly.
package main

import (
	"fmt"

	"github.com/ardanlabs/gotraining/topics/go/language/exporting/example4/users"
)

func main() {

	// Create a value of type User from the users package.
	u := users.User{
		Name: "Chole",
		ID:   10,

		password: "xxxx",
	}

	// ./example4.go:21: unknown users.User field 'password' in struct literal

	fmt.Printf("User: %#v
", u)
}

Unexported embedded types - Pkg

// Package users provides support for user management.
package users

// user represents information about a user.
type user struct {
	Name string
	ID   int
}

// Manager represents information about a manager.
type Manager struct {
	Title string

	user
}

Unexported embedded types - Main

// Sample program to show how to create values from exported types with
// embedded unexported types.
package main

import (
	"fmt"

	"github.com/ardanlabs/gotraining/topics/go/language/exporting/example5/users"
)

func main() {

	// Create a value of type Manager from the users package.
	u := users.Manager{
		Title: "Dev Manager",
	}

	// Set the exported fields from the unexported user inner type.
	u.Name = "Chole"
	u.ID = 10

	fmt.Printf("User: %#v
", u)
}