static const string members of struct in class

main.cpp

#include <iostream>

class Character {
public:
    struct Direction {
        static const std::string Front;
        static const std::string Back;
        static const std::string Left;
        static const std::string Right;
    };
};

const std::string Character::Direction::Front = "front";
const std::string Character::Direction::Back = "back";
const std::string Character::Direction::Left = "left";
const std::string Character::Direction::Right = "right";


int main() {
    std::cout << Character::Direction::Front << std::endl;
    std::cout << Character::Direction::Back << std::endl;
    std::cout << Character::Direction::Left << std::endl;
    std::cout << Character::Direction::Right << std::endl;
    return 0;
}

clang++ -std=c++11 hoge.cpp でコンパイル