C++11 incompatible with strings?

It seems that I need to choose one or the other: C++11 or strings.

Player.h

#ifndef PLAYER_H_
#define PLAYER_H_

#include <string>

class Player {
public:
    Player();
    Player(char symbol);
    int promptMove();
    virtual ~Player();
    bool validMoveFormat(std::string input); // !!!!!!!!
private:
    char symbol;
};

#endif /* HW1_PLAYER_H_ */

And Playercpp:

#include "Player.h"
#include <iostream>

using namespace std;

Player::Player() {}

Player::Player(char symbol) {
    this->symbol = symbol;
}

Player::~Player() { }

bool validMoveFormat(string move) {
   ...
   ...
   ...
}

int Player::promptMove() {
  ...
  ...
  ...
  string move = "hello";
  if (!validMoveFormat(move)) { // ERROR!!!!
     ...
     ...    
  }
  ...
  ...
}

In promptMove, when I call "validMoveFormat(move)" I get the debilitating error:

undefined reference to `Player::validMoveFormat(std::__cxx11::basic_string, std::allocator >)'

My impression is that this has something to do with GCC 5 versus older versions.

But it is unfathomable to me that such a simple program could not be run without plugging in some esoteric macro to the top of my header files.

Can I just change my compiler? Is there no sensible way to solve this?

Or should I just program without strings?