golang中，new和make的区别

在golang中,make和new都是分配内存的，但是它们之间还是有些区别的，只有理解了它们之间的不同，才能在合适的场合使用。

简单来说,new只是分配内存，不初始化内存； 而make即分配又初始化内存。所谓的初始化就是给类型赋初值，比如字符为空，整型为0, 逻辑值为false等。

new

先看下new函数的定义

// 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

可以看出，它的参数是一个类型，返回值为指向该类型内存地址的指针，同时会把分配的内存置为零，也就是类型的零值, 即字符为空，整型为0, 逻辑值为false

看几个new的示例

   type p struct{
        name string
        age int
    }
    var a *[2]int
    var s *string
    var b *bool
    var i *int
    var ps *p

    a = new([2]int)
    s = new(string)
    b = new(bool)
    i = new(int)
    ps = new(p) //结构

    fmt.println(a, " ", *a)
    fmt.println(s,  " ",*s)
    fmt.println(b,  " ",*b)
    fmt.println(i,  " ",*i)
    fmt.println(ps, " ", *ps)

输出结果如下

&[0 0]   [0 0]
0xc00000e1e0   
0xc00001a07a   false
0xc00001a090   0
&{ 0}   { 0}

上面示例是基本的类型，再看下slice, map,chan这些用new咋操作

    map操作
    var mp *map[string]string
    mp = new(map[string]string)
    //*mp = make(map[string]string)  //这行注掉会panic "panic: assignment to entry in nil map""
    (*mp)["name"] = "lc"
    fmt.println((*mp)["name"])
    
    slice操作
    var ms *[]string
    ms = new([]string)
    //*ms = make([]string,5) //这行注掉会pance "panic: runtime error: index out of range"
    (*ms)[0] = "lc"
    fmt.println((*ms)[0])
    

上面可以看出，silce、map、channel等类型属于引用类型，引用类型初始化为nil，nil是不能直接赋值的，也不能用new分配内存，还需要使用make来分配。

make

看下make的函数声明

/ 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

可以看出，它返回的就是类型本身，而不是指针类型，因为make只能给slice,map,channel等初始化内存，它们返回的就是引用类型，那么就没必要返回指针了

看下make的一些示例

    mm :=make(map[string]string)
    mm["name"] = "lc"
    fmt.println(mm["name"])

    mss :=make([]int,2)
    mss[0] = 100
    fmt.println(mss[0])

    ch :=make(chan int,1)
    ch <-100

    fmt.println(<-ch)

小结

make 仅用来分配及初始化类型为 slice、map、chan 的数据。new 可分配任意类型的数据.
new 分配返回的是指针，即类型 *type。make 返回引用，即 type.
new 分配的空间被清零, make 分配空间后，会进行初始化.