substr

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<iostream>
#include<algorithm>
using namespace std;
int main()
{
    string s("0123456789");
    string s1=s.substr(0);//只有一个数字0表示从下标为0开始一直到结尾：sub1 = "0123456789"
    string s2=s.substr(5,3);//从下标为5开始截取长度为3位：sub2 = "567"
    cout<<s1<<endl<<s2<<endl;
    return 0;
}