c++ compile error Undefined symbols for architecture x86_64 - c++

Question

I wrote a class named UserScore:
//header
using namespace std;
class UserScore{
public:
UserScore(const int &user_id, const int &rating);
private:
int _user_id;
int _rating;
};
//cpp
#include "UserScore.h"
UserScore::UserScore (const int &user_id, const int &rating):
_user_id(user_id),
_rating(rating)
{
}
The compile command is:
g++ src/UserScore.cpp -o obj/UserScore.o
But why this simple thing won't compile?
The error is:
Undefined symbols for architecture x86_64:
"_main", referenced from:
start in crt1.10.6.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

Answer 1

You need to include the -c option to the compilation line if you want to generate an object file, otherwise the compiler assumes you are trying to build an executable and will complain if you don't have a main() method (which is happening here).
To make an object file which you will link later to a code file that has a "main()" method in it you need to use
g++ -c src/UserScore.cpp -o obj/UserScore.o
Which is what I think you are trying to do.
Alternatively you just need to add a main function to your code and then you can make an executable.
//header
using namespace std;
class UserScore{
public:
UserScore(const int &user_id, const int &rating);
private:
int _user_id;
int _rating;
};
//cpp
#include "UserScore.h"
UserScore::UserScore (const int &user_id, const int &rating):
_user_id(user_id),
_rating(rating)
{
}
int main()
{
return 0;
}