Golang分享（二）：new和make的区别

var i int
var f float64
var b bool
var s string

var ip *int

// The new built-in function allocates memory. The first argument is a type,
// not a value, and the value returned is a pointer to a newly
// allocated zero value of that type.
func new(Type) *Type

func main() {
  ip := new(int)
  fmt.Println("指向int的指针:", ip) //0xc00001a098
  fmt.Println("int的零值:", *ip)  //0

  sp := new(string)
  fmt.Println("指向string的指针:", sp) //0xc00004e260
  fmt.Println("string的零值:", *sp)  //""

  pp := new(Person)
  fmt.Printf("指向person的指针: %pn", pp)          //0xc0000503e0
  fmt.Println("person.name的零值:", (*pp).name)   //""
  fmt.Println("person.phone的零值:", (*pp).phone) //0
  fmt.Println("person.id的零值:", (*pp).address)  //nil

  ipp := new(*int)
  fmt.Println("指向int指针的指针:", ipp) //0xc00000a030
  fmt.Println("int指针的零值:", *ipp)  //nil

  mp := new(map[string]string)
  fmt.Printf("指向map的指针: %pn", mp)               //0xc0000ca028
  fmt.Printf("指向map的指针是否为nil: %t", (*mp) == nil) //true
}

type Person struct {
  name    string
  phone   int
  address *string
}

(*m) = map[string]string{"张三丰": "武当山", "乔峰": "少林寺"}
  fmt.Printf("%pn", (*m))  //0xc00007e4b0
  fmt.Printf("%pn", &(*m)) //0xc0000ca028

// The make built-in function allocates and initializes an object of type
// slice, map, or chan (only). Like new, the first argument is a type, not a
// value. Unlike new, make's return type is the same as the type of its
// argument, not a pointer to it. The specification of the result depends on
// the type:
//
//  Slice: The size specifies the length. The capacity of the slice is
//  equal to its length. A second integer argument may be provided to
//  specify a different capacity; it must be no smaller than the
//  length. For example, make([]int, 0, 10) allocates an underlying array
//  of size 10 and returns a slice of length 0 and capacity 10 that is
//  backed by this underlying array.
//  Map: An empty map is allocated with enough space to hold the
//  specified number of elements. The size may be omitted, in which case
//  a small starting size is allocated.
//  Channel: The channel's buffer is initialized with the specified
//  buffer capacity. If zero, or the size is omitted, the channel is
//  unbuffered.
func make(t Type, size ...IntegerType) Type

func main() {
  mapDemo := make(map[string]string)
  fmt.Println("mapDemo: ", mapDemo) //demo:  map[]

  chanDemo := make(chan int, 10)
  fmt.Println("chanDemo: ", chanDemo) //chanDemo:  0xc000116000

  sliceDemo := make([]int, 10)
  fmt.Println("sliceDemo: ", sliceDemo) //sliceDemo:  [0 0 0 0 0 0 0 0 0 0]
}

func main() {
  //先看make的初始化
  mp1 := make(map[string]string)
  mp1["乔峰"] = "少林寺"
  fmt.Println(mp1)
  
  //再看new的置零值
  mp := new(map[string]string)
  fmt.Printf("指向map的指针是否为nil: %tn", (*mp) == nil) //true
  //以下这行会报错，因为*mp被置为零值nil，没有初始化，
  (*mp)["张三丰"] = "武当山"
}