C++ class method not found when compiled

Question

I created a simple class 'Hello' in C++ using header(.h) and definition(.cpp) files. This is the header file content:

#ifndef HELLO_H
#define HELLO_H

#include <string>

namespace test
{
    class Hello
    {
    private:
        std::string name;

    public:
        Hello();
        void say_hello();
    };
}

#endif

And the definition file content is just as you expected:

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

using namespace test;

Hello::Hello()
{
    this->name = "Yoppy Yunhasnawa";
}

void Hello::say_hello()
{
    string message = "Hello, " + this->name + ".. Have nice day!";

    cout << message << "\n";
}

I included this class to a main.cpp file and use it like this:

#include "Hello.h"

using namespace test;

int main(int argc, char *argv[])
{   
    Hello* hello = new Hello;

    hello->say_hello();
}

When I compiled the main.cpp file with g++ like this,

g++ main.cpp

I got following annoying error:

Undefined symbols for architecture x86_64:
  "test::Hello::say_hello()", referenced from:
      _main in ccsaoOZa.o
  "test::Hello::Hello()", referenced from:
      _main in ccsaoOZa.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status

However, that error does not appear when I don't call both constructor and say_hello method:

int main(int argc, char *argv[])
{   
    Hello* hello;// = new Hello;

    //hello->say_hello();
}

I use macport GCC 4.7 and I am very sure that my method is there but why this symbol(s) not found error keep appearing? Please show me my mistake. Thank you.

Answer 1

When you invoke g++ main.cpp, compiler performs both compiling AND linking. But the code cannot be linked without Hello.cpp file. So, you have two options: either compile and link separately:

g++ -c main.cpp
g++ -c hello.cpp
gcc main.o hello.o

or compile and link everything at the same time:

g++ main.cpp hello.cpp