Public vs. private

A package is the smallest unit of private encap­sulation in Go.

• All identifiers defined within a package are visible throughout that package.
• When importing a package you can access only its exported identifiers.
• An identifier is exported if it begins with a capital letter.

Exported and unexported identifiers are used to describe the public interface of a package and to guard against certain programming errors.

Warning: Unexported identifiers is not a security measure and it does not hide or protect any information.

Example

In this package, the only exported identifiers are StopWatch and Start.

package timer

import "time"

// A StopWatch is a simple clock utility.
// Its zero value is an idle clock with 0 total time.
type StopWatch struct {
	start   time.Time
	total   time.Duration
	running bool
}

// Start turns the clock on.
func (s *StopWatch) Start() {
	if !s.running {
		s.start = time.Now()
		s.running = true
	}
}

The StopWatch and its exported methods can be imported and used in a different package.

package main

import "timer"

func main() {
	clock := new(timer.StopWatch)
	clock.Start()
	if clock.running { // ILLEGAL
		// …
	}
}

../main.go:8:15: clock.running undefined (cannot refer to unexported field or method clock.running)

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