undefined reference to '...'

#include <curses.h>
#include <string>
using std::string;

class Console
{
private:
    Console();
    ~Console();
   
    static WINDOW * cmd;
    static WINDOW * con;
   
    typedef void( * handler )( string );
    static handler command; // hook to interpreter, should return asap
   
public:
    static string set_prompt( string );
    static Console & get()
    {
        static Console singleton;
        return singleton;
    }
   
    static void run( handler ); // starts command interpreting session
    static void log( string ); // makes output to console
    static string ask( string ); // takes input from user on demand
    static string ask_for_password( string ); // stars instead of chars
};

#include "console.hpp"

Console::Console()
{
    initscr();
    cmd = newwin( 2 * LINES, COLS, - LINES - 1, 0 );
    con = newwin( 1, COLS, LINES - 1, 0 );
    wmove( cmd, 0, 0 );
    wmove( con, 2 * LINES, 0 );
}

Console::~Console()
{
    delwin( cmd );
    delwin( con );
    endwin();
}

void Console::log( string str )
{
    printw( str.c_str() );
    refresh();
}

#include "console.hpp"

int main()
{
    Console & console = Console::get();
    console.log( "Hello World !!!" );
    return 0;
}

WINDOW Console::* con, Console::* cmd;

WINDOW * Console::cmd;
WINDOW * Console::con;

typedef void( * handler )( string );
handler Console::command;