golang提供內建函式cap用於檢視channel緩衝區長度。

cap的定義如下：

func cap(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)).等同於len

- Pointer to array: the number of elements in *v (same as len(v)).等同於len

- Slice: the maximum length the slice can reach when resliced;
if v is nil, cap(v) is zero.對於slice，表示在不重新分配空間的情況下，可以達到的切片的最大長度。如果切片是nil， 則長度為0.

-  Channel: the channel buffer capacity, in units of elements;表示緩衝區的長度。
if v is nil, cap(v) is zero. 如果通道是nil，則緩衝區長度為0。
 

Example

package main

import ("fmt")

func main(){

    ch1 := make(chan int)
    ch2 := make(chan int, 2)//緩衝區長度為2

    fmt.Println("ch1 buffer len:", cap(ch1))
    fmt.Println("ch2 buffer len:", cap(ch2))
}

output:

ch1 buffer len:0
ch2 buffer len:2