20位的数组，为什么输出是27（31）呢？

程序清单：
//strtype4.cpp--line input
#include<iostream>
#include<string>  //make string class available
#include<cstring>  //C-style string library
int main()
{
    using namespace std;
    char charr[20];
    string str;
    cout<<"Length of string in charr before input: "
        <<strlen(charr)<<endl;
    cout<<"Length of string in str before input: "
        <<str.size()<<endl;
    cout<<"Enter a line of text:\n";
    cin.getline(charr,20);  //indicate maximum length
    cout<<"You entered: "<<charr<<endl;
    cout<<"Enter another line of text:\n";
    getline(cin,str);  //cin now an argument;no length specifier
    cout<<"Length of string in charr after input: "
        <<strlen(charr)<<endl;
    cout<<"Length of string in str after input: "
        <<str.size()<<endl;
    return 0;
}
请问输出结果“Length of string in charr before input”是什么（我觉得应该是20）呢？
书上给出的结果是27
而我敲进编译器给出的结果是31
求大神指导！！！