C / C++ / MFC

printf("%d+%d+%d=%d",number1,number2,number3,number1+number2+number3);

g++ proj01.cpp
Undefined                       first referenced
 symbol                             in file
Player::SetFirst(std::string)       /var/tmp/ccZLDByc.o
ld: fatal: Symbol referencing errors. No output written to a.out
collect2: ld returned 1 exit status

#include <string>
#include <vector>
//#include "playerstat.h"

using namespace std;

class Player
{
	public:
	string GetFirst();
	void SetFirst(string first);



	private:
	string FirstName;
				
};

#include "player.h"
#include <string>
using namespace std;

string Player::GetFirst()
{

	return FirstName;

};
void Player::SetFirst(string first)
{
	FirstName = first;

};

#include "player.h"

int main(int argc,char **args) {

    Player player;
    player.SetFirst("foo!");

    return 0;
}

Undefined                       first referenced
 symbol                             in file
Player::SetFirst(std::string)       /var/tmp/ccF4AOmw.o
ld: fatal: Symbol referencing errors. No output written to a.out
collect2: ld returned 1 exit status