Error message appearing on compiling g++ [duplicate] - c++

Question

This question already has answers here:
What is an undefined reference/unresolved external symbol error and how do I fix it?
(39 answers)
Closed 4 years ago.
So for class we are learning about OOP in C++, and I built my class but every time I try to compile it I get this error message:
Undefined symbols for architecture x86_64:
"Player::set_assits(int)", referenced from:
_main in playerDataBase-666bbb.o
"Player::set_last_name(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >)", referenced from:
_main in playerDataBase-666bbb.o
"Player::set_team_name(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >)", referenced from:
_main in playerDataBase-666bbb.o
"Player::set_first_name(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >)", referenced from:
_main in playerDataBase-666bbb.o
"Player::set_year_of_birth(int)", referenced from:
_main in playerDataBase-666bbb.o
"Player::set_goals(int)", referenced from:
_main in playerDataBase-666bbb.o
"Player::Player()", referenced from:
_main in playerDataBase-666bbb.o
"Player::~Player()", referenced from:
_main in playerDataBase-666bbb.o
ld: symbol(s) not found for architecture x86_64
I can't understand why this happens, I have tried to run this by compiling separate .h, and .cpp files as well as by putting the class, and main function inside of the same .cpp file. Any help would be appreciated as well here is my code.
#include <iostream>
#include <string>
using namespace std;
class Player{
public:
Player();
~Player();
// accessors and mutators
void set_first_name(string in_first_name);
string first_name();
void set_last_name(string in_last_name);
string last_name();
void set_team_name(string in_team_name);
string team_name();
void set_year_of_birth(int in_year_of_birth);
int year_of_birth;
void set_goals(int in_goals);
int goals;
void set_assits(int in_assists);
int assists;
//methods
void display();
private:
string first_name_;
string last_name_;
string team_name_;
int year_of_birth_;
int goals_;
int assits_;
};
void Player::display(){
cout << first_name_ << last_name_ << endl;
}
int main(){
Player player;
player.set_first_name("John");
player.set_last_name("Tedesco");
player.set_team_name("Blyth Warriors");
player.set_year_of_birth(2002);
player.set_goals(2);
player.set_assits(7);
player.display();
}
Thanks for any help and sorry again for messy / poor code.
Thanks,
John

Answer 1

None of these methods are defined, only declared:
Player();
~Player();
// accessors and mutators
void set_first_name(string in_first_name);
string first_name();
void set_last_name(string in_last_name);
string last_name();
void set_team_name(string in_team_name);
string team_name();
void set_year_of_birth(int in_year_of_birth);
void set_goals(int in_goals);
void set_assits(int in_assists);
You need to provide reasonable definitions for these methods. Right now, you tell the compiler you will provide a definition for each of these methods, and never do. For default constructors, you may use Player() = default;, if you want the compiler to generate a constructor.
For example, to define the first_name property, you may do:
void Player::set_first_name(const string& in_first_name)
{
first_name_ = in_first_name;
}
const string& first_name() const
{
return first_name_;
}
Please note I've changed this to take value by constant reference, and return a value by constant reference, rather than by value, for efficiency reasons.

Answer 2

You declared all those fancy methods, e.g:
void set_first_name(string in_first_name);
string first_name();
But you never defined them!
Implement them, and it will work, e.g:
void set_first_name(string in_first_name) {
first_name_ = in_first_name;
}
string first_name() {
return first_name_;
}