初始化
golang没有构造函数 变量声明后会零初始化(也算是RAII吧)
本文讲述不同类型变量零初始化的结果和其他初始化操作
零初始化
常量`const`
golang的常量存在一种特殊情况:无类型常量
const (
e = 2.71828182845904523536028747135266249775724709369995957496696763
pi = 3.14159265358979323846264338327950288419716939937510582097494459
)
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它可以赋值给有类型的变量 不过会折损精度
| 类型 | 初始化 | 备注 |
|---|---|---|
| 数值类型 | 0 | complex虚部和实部都初始化为0 |
| 字符串 | "" | 字符串不可变 |
| Slice/Map/func | nil | 只能和nil比较 |
| interface/channel/pointer | nil | |
| struct | 每个成员零初始化 | 按该表规则递归初始化各部分 |
new
为类型分配空间并进行零初始化 然后返回该空间地址
new 在 golang 中并不意味堆分配
func main() {
var local int
var nlocal = new(int)
fmt.Printf("%p,%p", &local, nlocal) // 0xc0000bc000,0xc0000bc008
}
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可以发现这里的地址是连续的 所以并不是new出来的在堆上 变量的存储地址由编译器决定
// 方式1
ptr := new(T)
// 方式2
var t T
ptr := &t
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反正变量都会零初始化 所以不知道new有什么意义
make
构造后底层数据后返回包含底层数据地址的复合结构
具体行为与传参时的复制类似
// 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 /*builtin for any type*/, size ...IntegerType /*builtin [IntergerType Int|Uint]*/) Type
var _map = make(map[string]int)
var _slice = make([]int, _len, _cap)
var _chan = make(chan in, _buffer)
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