golang中的cap vs len

Question

golang中一个切片的cap和len有什么区别？

根据定义:

切片具有长度和容量.

切片的长度是它包含的元素数.

切片的容量是基础数组中元素的数量,从切片中的第一个元素开始计算.

x := make([]int, 0, 5) // len(b)=0, cap(b)=5

len是否仅表示非空值？

Answer 1

切片是一种在封面下使用数组的抽象.

cap告诉你底层数组的容量.len告诉你数组中有多少项.

Go中的切片抽象非常好,因为它会为您调整底层数组的大小,而且Go数组中的大小不能调整大小,因此几乎总是使用切片.

例:

s := make([]int, 0, 3)
for i := 0; i < 5; i++ {
    s = append(s, i)
    fmt.Printf("cap %v, len %v, %p\n", cap(s), len(s), s)
}

会输出这样的东西:

cap 3, len 1, 0x1040e130
cap 3, len 2, 0x1040e130
cap 3, len 3, 0x1040e130
cap 8, len 4, 0x10432220
cap 8, len 5, 0x10432220

正如您所看到的那样,只要满足容量,append就会返回一个容量更大的新切片.在第4次迭代中,您将注意到更大的容量和新的指针地址.

播放示例

我意识到你没有问过数组和追加,但它们在理解切片和内置的原因方面非常基础.

Answer 2

从源代码：

// The len built-in function returns the length of v, according to its type:
//  Array: the number of elements in v.
//  Pointer to array: the number of elements in *v (even if v is nil).
//  Slice, or map: the number of elements in v; if v is nil, len(v) is zero.
//  String: the number of bytes in v.
//  Channel: the number of elements queued (unread) in the channel buffer;
//  if v is nil, len(v) is zero.
func len(v Type) int

// The cap built-in function returns the capacity of v, according to its type:
//  Array: the number of elements in v (same as len(v)).
//  Pointer to array: the number of elements in *v (same as len(v)).
//  Slice: the maximum length the slice can reach when resliced;
//  if v is nil, cap(v) is zero.
//  Channel: the channel buffer capacity, in units of elements;
//  if v is nil, cap(v) is zero.
func cap(v Type) int