stoi undeclared even when using -std=c++11 flag with both g++ and clang++

Question

I have a very simple main.cpp file:

#include <iostream>
#include <string>
using namespace std;
int main() {
    string s = "194";
    int i = stoi(s);
    cout << i << endl;
}

In the command line clang++ main.cpp -std=c++11 outputs:

main.cpp:6:10: error: use of undeclared identifier 'stoi'
        int i = stoi(s);
                ^
1 error generated.

and clang++ --version outputs:

clang version 3.6.1 (tags/RELEASE_361/final)
Target: i686-pc-windows-gnu
Thread model: posix

And with GCC, g++ main.cpp -std=c++11 outputs:

main.cpp: In function 'int main()':
main.cpp:6:16: error: 'stoi' was not declared in this scope
  int i = stoi(s);
                ^

with g++ --version outputting:

g++ (GCC) 4.8.1
Copyright (C) 2013 Free Software Foundation, Inc.

I've tried looking at answers to similar questions, but none of the solutions seem to work.

What could be the problem?