​一​

#include<iostream>
#include<string>
using namespace std;
int main()
{
  string s1(8, 'x');
  cout << s1 << endl;
  string s2;
  cin >> s2;
  s1.assign(s1, 0, 1);
  cout << s1 << endl;
  system("pause");
  return 0;
}

assign(0,1)函数可以实现从下标为0开始往后1个字符赋值

#include<iostream>
#include<string>
using namespace std;
int main()
{
  string s1("hello");
  string s2("good");
  s1.append(s2, 1, s1.size());
  cout << s1;
  system("pause");
  return 0;
}

append()函数实现部分字符串的拼接，如若长度不够，访问到s2的末尾

#include <iostream>
#include<string>
#include<cstring> 
using namespace std;
int main(int argc, char *argv[]) 
{
  string s1,s2;
  s1="1234";
  s2="56";
  swap(s1,s2);
  cout<<s1<<" "<<s2<<endl;
  swap(s1,s2);
  s1=s2;
  cout<<s1;
  return 0;
}