c++ 错误： reference to local variable ‘...’ returned

This happens when returning a zero-hour reference to a function.

3.cc: In function ‘const string& add_(const string&, const string&, const string&)’:
3.cc:6:12: warning: reference to local variable ‘s’ returned [-Wreturn-local-addr]
     string s = s1;
例如：

#include <iostream>
#include <string>
using namespace std;
const string &  add_(const string &s1,const string & pre,const string & behind)
  {
    string s = s1;    
    s.insert(0,pre);
    s.append(behind);
    return s;
  }
int main()
{
  string name  = "robert";
  string pre = "Mr.";
  string hi = "Jr.";
  string new_name = add_(name,pre,hi);
  cout << new_name << endl;

  return 0;
}

The result of the compilation is:

r@r-Sys:~/c++/ex$ g++ 3.cc -o 123
3.cc: In function ‘const string& add_(const string&, const string&, const string&)’:
3.cc:6:12: warning: reference to local variable ‘s’ returned [-Wreturn-local-addr]
     string s = s1;
            ^

Error analysis:

The c++ function will destroy the zero-hour amount at the end, and it is of course wrong for the function to return the zero-hour amount. Because it has been destroyed, its reference has no meaning.