string类中有没有定位函数？

#include<iostream>
#include<string>
using namespace std;
int main()
{
    string str="asdfghjkkasdf";
    int i, n;
    cin >> i >> n;
    string str2(str,i-1,n);//string::string(const string&,int index,int size)
    cout << str2 << endl;


    char cstr3[100];
    //int string::copy(char* buf,int size, int index)
    //copy函数不在字符串结尾处写上'\0'
    str.copy(cstr3,n,i-1);
    cstr3[n] = '\0';
    cout << cstr3 << endl;
    return 0;
}