Go-make和new的区别

2021-03-23

字数统计: 587字 | 阅读时长: 2分

阅读量

值类型和引用类型

值类型：int，float，bool，string，struct和array。变量直接存储值，分配栈区的内存空间，这些变量所占据的空间在函数被调用完后会自动释放
引用类型：slice，map，chan和值类型对应的指针。变量存储的是一个地址（或者理解为指针），指针指向内存中真正存储数据的首地址。内存通常在堆上分配，通过GC回收。

对于引用类型的变量，不仅要声明这个变量，还要手动为其分配空间。make和new都在堆上分配内存。

New

// 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

该方法的参数要求传入一个类型，而不是一个值，它会申请一个该类型大小的内存空间，并会初始化为对应的零值，返回该类型的指针类型。

Make

// 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

make入参指定slice, map, chan类型，只为这些类型申请内存。可以看到返回的是该类型的实例，而不是指针。

切片：第二个参数len，第三个是cap，cap不能小于len

map：会根据size大小分配资源，以足够存储size个元素。如果入参没有写size，会默认分配一个小的起始size

通道：对于chan，size表示缓冲区的大小。如果入参没有写size，channel为无缓冲channel

总而言之：new是申请一块内存，而make返回一个类型的初始值。