"undefined reference to" in G++ Cpp

Question

I can't seem to get the errors to go away. The errors are below. I have looked on Google Search and still I can't figure it out. It is not like I am new to C++, but I have not fooled around with it in a while.

The weird thing is it worked with g++ on Windows...

Errors using:

g++ main.cpp

Output:

/tmp/ccJL2ZHE.o: In function main': \ main.cpp:(.text+0x11): undefined reference to Help::Help()'
main.cpp:(.text+0x1d): undefined reference to Help::sayName()' \ main.cpp:(.text+0x2e): undefined reference to Help::~Help()'
main.cpp:(.text+0x46): undefined reference to `Help::~Help()'
collect2: ld returned 1 exit status

File main.cpp

#include <iostream>
#include "Help.h"

using namespace std;

int main () {

    Help h;
    h.sayName();

    // ***

    // ***

    // ***
    return 0;

}

File Help.h

#ifndef HELP_H
#define HELP_H

class Help {
    public:
        Help();
        ~Help();
        void sayName();
    protected:
    private:
};

#endif // HELP_H

File Help.cpp

#include <iostream>
#include "Help.h"

using namespace std;

Help::Help() { // Constructor
}

Help::~Help() { // Destructor
}

void Help::sayName() {
    cout << "            ***************" << endl;
    cout << "   ************************************" << endl;
    cout << "              ************" << endl;
    cout << "         *********************" << endl;
}

Answer 1

Use

g++ main.cpp Help.cpp

You have to tell the compiler all the files that you want it to compile, not just the first one.

Answer 2

You should add help.o to your g++ line:

g++ -c help.cpp -o help.o
g++ help.o main.cpp

By splitting it to two lines you can save compilation time (in case of larger projects), because you can compile help.cpp only when it was changed. make and Makefile used well will save you a lot of headache:

#Makefile
all: main

main: help main.cpp
    g++ -o main help.o main.cpp

help: help.cpp
    g++ -c -o help.o help.cpp