C++ Function was not declared in this scope error

Question

I am compiling a code, but I seem to get an error, even as I examine my code, I find it weird that I'm getting this error.

My code:

#include <iostream>
#include <string>

using namespace std;

int main() {
    string s = "2A";
    int n = stoul(s, nullptr, 16);
    cout << n << endl;

    return 0;
}

and in the command line, I'm typing g++ -std=c++11 main.cpp, I'm using -std=c++11 because the function stoul is from C++11 according to this page stoul.

The error I'm getting is the following:

main.cpp: In function 'int main()':
main.cpp:8:30: error: 'stoul' was not declared in this scope
  int n = stoul(s, nullptr, 16);
                              ^

Why am I getting this error?