You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: main.go
    path of file: ./repos/learning/golang/source_code/Go语言核心编程代码/chapter20/circelqueue
    
    the code of the file until where you have to start completion: package main
import (
	"fmt"
	"errors"
	"os"
)

//使用一个结构体管理环形队列
type CircleQueue struct {
	maxSize int // 4
	array
    
func (this *CircleQueue) Pop() (val int, err error) {

	if this.IsEmpty() {
		return 0, errors.New("queue empty")
	}
	//取出,head 是指向队首，并且含队首元素
	val = this.array[this.head]
	this.head = (this.head + 1) % this.maxSize
	return 
}