Golang - Asterisk and Ampersand Cheatsheet

values_pointers.go

/*
********************************************************************************
Golang - Asterisk and Ampersand Cheatsheet
********************************************************************************

Also available at: https://play.golang.org/p/lNpnS9j1ma

Allowed:
--------
p := Person{"Steve", 28} 	stores the value
p := &Person{"Steve", 28} 	stores the pointer address (reference)
PrintPerson(p) 			passes either the value or pointer address (reference)
PrintPerson(*p) 		passes the value
PrintPerson(&p) 		passes the pointer address (reference)
func PrintPerson(p Person)	ONLY receives the value
func PrintPerson(p *Person)	ONLY receives the pointer address (reference)

Not Allowed:
--------
p := *Person{"Steve", 28} 	illegal
func PrintPerson(p &Person)	illegal

*/

package main

import (
	"fmt"
	"reflect"
)

type Person struct {
	Name string
	Age int
}

// This only works with *Person, does not work with Person
// Only works with Test 2 and Test 3
func (p *Person) String() string {
	return fmt.Sprintf("%s is %d", p.Name, p.Age)
}

// This works with both *Person and Person, BUT you can't modiy the value and
// it takes up more space
// Works with Test 1, Test 2, Test 3, and Test 4
/*func (p Person) String() string {
	return fmt.Sprintf("%s is %d", p.Name, p.Age)
}*/

// *****************************************************************************
// Test 1 - Pass by Value
// *****************************************************************************

func test1() {
	p := Person{"Steve", 28}
	printPerson1(p)
	updatePerson1(p)
	printPerson1(p)
}

func updatePerson1(p Person) {
	p.Age = 32
	printPerson1(p)
}

func printPerson1(p Person) {
	fmt.Printf("String: %v | Name: %v | Age: %d\n",
        p,
        p.Name,
        p.Age)
}

// *****************************************************************************
// Test 2 - Pass by Reference
// *****************************************************************************

func test2() {
	p := &Person{"Steve", 28}
	printPerson2(p)
	updatePerson2(p)
	printPerson2(p)
}

func updatePerson2(p *Person) {
	p.Age = 32
	printPerson2(p)
}

func printPerson2(p *Person) {
	fmt.Printf("String: %v | Name: %v | Age: %d\n",
        p,
        p.Name,
        p.Age)
}

// *****************************************************************************
// Test 3 - Pass by Reference (requires more typing)
// *****************************************************************************

func test3() {
	p := Person{"Steve", 28}
	printPerson3(&p)
	updatePerson3(&p)
	printPerson3(&p)
}

func updatePerson3(p *Person) {
	p.Age = 32
	printPerson3(p)
}

func printPerson3(p *Person) {
	fmt.Printf("String: %v | Name: %v | Age: %d\n",
        p,
        p.Name,
        p.Age)
}

// *****************************************************************************
// Test 4 - Pass by Value (requires more typing)
// *****************************************************************************

func test4() {
	p := &Person{"Steve", 28}
	printPerson4(*p)
	updatePerson4(*p)
	printPerson4(*p)
}

func updatePerson4(p Person) {
	p.Age = 32
	printPerson4(p)
}

func printPerson4(p Person) {
	fmt.Printf("String: %v | Name: %v | Age: %d\n",
        p,
        p.Name,
        p.Age)
}

// *****************************************************************************
// Main
// *****************************************************************************

/*
Outputs:
String: {Steve 28} | Name: Steve | Age: 28
String: {Steve 32} | Name: Steve | Age: 32
String: {Steve 28} | Name: Steve | Age: 28
String: Steve is 28 | Name: Steve | Age: 28
String: Steve is 32 | Name: Steve | Age: 32
String: Steve is 32 | Name: Steve | Age: 32
String: Steve is 28 | Name: Steve | Age: 28
String: Steve is 32 | Name: Steve | Age: 32
String: Steve is 32 | Name: Steve | Age: 32
String: {Steve 28} | Name: Steve | Age: 28
String: {Steve 32} | Name: Steve | Age: 32
String: {Steve 28} | Name: Steve | Age: 28
*/
func main() {	
	test1()
	test2()
	test3()
	test4()
	
	//fmt.Println("Option 1: ", Concat1("", 0))
	//fmt.Println("Option 2: ", Concat2("not-the-default"))
	fmt.Println("Option 3: ", Concat3(Parameters{}))
	fmt.Println("Option 4: ", Concat4(10))
}

// Both parameters are optional, use "" for default value
func Concat1(a string, b int) string {
  if a == "" {
    a = "default-a"
  }
  if b == 0 {
    b = 5
  }

  return fmt.Sprintf("%s%d", a, b)
}

// a is required, b is optional.
// Only the first value in b_optional will be used.
func Concat2(a string, b_optional ...int) string {
  b := 5
  if len(b_optional) > 0 {
    b = b_optional[0]
  }
  
  return fmt.Sprintf("%s%d", a, b)
}


// A declarative default value syntax
// Empty values will be replaced with defaults
type Parameters struct {
  A string `default:"default-a"` // this only works with strings
  B int // default is 5
}

func Concat3(prm Parameters) string {
  typ := reflect.TypeOf(prm)

  if prm.A == "" {
    f, _ := typ.FieldByName("A")
    prm.A = f.Tag.Get("default")
  }

  if prm.B == 0 {
    prm.B = 5
  }

  return fmt.Sprintf("%s%d", prm.A, prm.B)
}


func Concat4(args ...interface{}) string {
  a := "default-a"
  b := 5

  for _, arg := range args {
    switch t := arg.(type) {
      case string:
        a = t
      case int:
        b = t
      default:
        panic("Unknown argument")
    }
  }

  return fmt.Sprintf("%s%d", a, b)
}

func main() {
	//fmt.Println("Option 1: ", Concat1("", 0))
	//fmt.Println("Option 2: ", Concat2("not-the-default"))
	fmt.Println("Option 3: ", Concat3(Parameters{}))
	fmt.Println("Option 4: ", Concat4(10))
}