undefined reference error, assigning a value to a vector of char vectors by passing a classes function? c++

Question

I'm not even sure if that's the right way to phrase my question. basically I have a class Game, and I'm trying to do something like:

Game game;
std::vector< std::vector<char> > gameBoard;
gameBoard = game.initVector(rows,rowSize);

and in game.hpp:

class Game{
public:
Game();
//method to initialize the start vector
std::vector< std::vector<char> > initVector(int rows, int rowSize);
...}

and when I try to link I'm getting the following error:

...test-game.cpp:15: undefined reference to 'Game::initVector(int,int)'

What's happening here? How do I eliminate this?