go slice 的append的用法细节

append的时候，如果slice容量足够，不需要扩容，则slice底层数组的值会变动。如果需要扩容，会形成新的数组指针给到slice，原来的数组值是不会变的。对于想要在函数调用过程中改变上层传入的slice里的值的写法要注意！！！

容量足够

func main() {
    line := "123456789abcdefghijklmnopqrstuvw" // len 32
    l := []byte(line)                          // 默认cap 32
    aaa(l[27:30])
    logger.Infof("out : %v %v", len(l), cap(l))
    logger.Infof("l %v", string(l))
}

func aaa(line []byte) {
    logger.Infof("in 1: %v %v", len(line), cap(line))
    logger.Infof("in line: %v", string(line))
    line = append(line, []byte("AAA")...)
    logger.Infof("in 2: %v %v", len(line), cap(line))
    logger.Infof("in line2: %v", string(line))
}

输出

2022-08-31 15:38:58  file=TestThings/main.go:111 level=info in 1: 3 5
2022-08-31 15:38:58  file=TestThings/main.go:112 level=info in line: stu        
2022-08-31 15:38:58  file=TestThings/main.go:114 level=info in 2: 5 5           
2022-08-31 15:38:58  file=TestThings/main.go:115 level=info in line2: stuAA     
2022-08-31 15:38:58  file=TestThings/main.go:100 level=info out : 32 32         
2022-08-31 15:38:58  file=TestThings/main.go:101 level=info l 123456789abcdefghijklmnopqrstuAA

容量不够

func main() {
    line := "123456789abcdefghijklmnopqrstuvw" // len 32
    l := []byte(line)                          // 默认cap 32
    aaa(l[27:30])
    logger.Infof("out : %v %v", len(l), cap(l))
    logger.Infof("l %v", string(l))
}

func aaa(line []byte) {
    logger.Infof("in 1: %v %v", len(line), cap(line))
    logger.Infof("in line: %v", string(line))
    line = append(line, []byte("AAA")...)
    logger.Infof("in 2: %v %v", len(line), cap(line))
    logger.Infof("in line2: %v", string(line))
}

输出

2022-08-31 16:00:35  file=TestThings/main.go:105 level=info in 1: 3 5
2022-08-31 16:00:35  file=TestThings/main.go:106 level=info in line: stu        
2022-08-31 16:00:35  file=TestThings/main.go:108 level=info in 2: 6 16          
2022-08-31 16:00:35  file=TestThings/main.go:109 level=info in line2: stuAAA    
2022-08-31 16:00:35  file=TestThings/main.go:100 level=info out : 32 32         
2022-08-31 16:00:35  file=TestThings/main.go:101 level=info l 123456789abcdefghijklmnopqrstuvw