C++ Multiple Classes Cause Architecture Error - c++

Question

I am working on a project that requires me to use a variety of classes.
For some reason, whenever I try compiling, I get an error that says:
Undefined symbols for architecture x86_64:
I originally was working on a class we can call User that had a declaration like this in user.h:
#ifndef USER_H
#define USER_H
...other include statements
class User
{
public:
User();
public and private members and functions
};
#endif
Definitions of the methods was housed in a separate file in the same directory, user.cpp.
Whenever I would try to compile, I would get an error saying there were undefined symbols for the constructor and destructor. I had both of these defined in the .cpp file, but I couldn't get the error to go away.
I include the User class in main with the following:
#include "user.h"
I managed to fix this by pasting all code from the user.cpp file into user.h, but now I am having issues with other classes whose code I am not allowed to modify.
Right now, I have a class called GMLReader in gmlreader.h with a method I am calling statically. However, even though I am importing it for my main, I am getting the following error code whenever I try to call it:
Undefined symbols for architecture x86_64:
"GMLReader::read(char const*, std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >&, std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >&)", referenced from:
_main in cchnzOEh.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status
I am using the following to compile with sn.cpp housing my main:
g++ -g -Wall -o sn sn.cpp
Is there any reason an error like this would have occurred and how can I go about fixing it?

Answer 1

You need to compile and link user.cpp as well:
g++ -g -Wall -c user.cpp -o user.o
g++ -g -Wall -o sn sn.cpp user.o