为什么substr函数不能用

substr函数用不了

#include<bits/stdc++.h>
using namespace std;
string n,a,s; 
int m,x,y,k,b;
char w;
int main(){
cin>>n>>m;
for(int i=1;i<=m;i++)
{
    cin>>x;
    if(x==1)
    {
        cin>>y>>w;
        for(int j=1;j<=y;j++) n+=w;    
    } 
    if(x==2)
    {
        cin>>a;
        n=n+a;
    }
    if(x==3)
    {
        cin>>k>>b;
        n.substr(k,b);    
    }    
}
cout<<n;
    return 0;
}

没有取子串

substr要有返回值啊。

// string::substr
#include <iostream>
#include <string>

int main ()
{
  std::string str="We think in generalities, but we live in details.";
                                           // (quoting Alfred N. Whitehead)

  std::string str2 = str.substr (3,5);     // "think"

  std::size_t pos = str.find("live");      // position of "live" in str

  std::string str3 = str.substr (pos);     // get from "live" to the end

  std::cout << str2 << ' ' << str3 << '\n';

  return 0;
}

怎么不能用？你想达到什么结果？