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.