defer ：
1 函数离开前执行
2 FILO 栈
package main
import "fmt"
func trace(s string) {
fmt.Println("entering:", s);
}
func untrace(s string) {
fmt.Println("leaving:", s);
}
func a() {
trace("a");
defer untrace("a");
fmt.Println("in a")
}
func b() {
trace("b");
defer untrace("b");
fmt.Println("in b");
a()
}
func trace2(s string) string {
fmt.Println("entering:",s)
return s
}
func un(s string) {
fmt.Println("leaving:",s)
}
func a2() {
defer un(trace2("a"))
fmt.Println("in a")
}
func b2() {
defer un(trace2("b"))
fmt.Println("in b")
a2()
}
func main() {
for i:=0;i<5;i++{
defer fmt.Println(i)
}
b2();
}