Returning a reference to a local object is a mistake.

console output

>  make mistake && ./mistake
c++ -Wall -Wextra -std=c++14 -O0    mistake.cpp   -o mistake
mistake.cpp:8:10: warning: reference to stack memory associated with local variable 's' returned [-Wreturn-stack-address]
  return s;
         ^
1 warning generated.
This is a mista��8

mistake.cpp

#include <iostream>
#include <string>

using namespace std;

string & mistake() {
  string s("This is a mistake.");
  return s;
}

int main() {

  string s = mistake();

  cout << s << endl;

  return 0;
}

Yeah, so changing line 6 to:

string mistake() {

Will fix that problem. And should be perfectly performant in any modern C++ implementation as well.