Why I cannot run this simple c++ code by using g++ on mac?

Question

I am studying the C++ now and I am trying to learn how to use the head file. However, there is always giving a mistake when I run the program.

/* File add.h */
#ifndef ADD_H
#define ADD_H

int add(int, int);

#endif
/* ADD_H */


/* File add.cpp */
#include "add.h"

int add(int a, int b)
{
    return a + b;
}

/* File triple.cpp */
#include<iostream>
#include "add.h"
using namespace std;

int triple(int);
int triple(int x)
{
    return add(x, add(x, x));
}

int main()
{
  int i=0;
  int j;
  while (i<=5)
  {     j=triple(i);
        cout<<j<<endl;
       //cout<<triple(j)<<endl;
      i++;
   }
 return 0;
}

These are the 3 files I used. When I run: g++ triple.cpp on mac, the error is given as below:

Undefined symbols for architecture x86_64:
 "add(int, int)", referenced from:
   triple(int) in triple-e0558f.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Who can give me some hints about this mistake. Thanks a lot! By the way, the gcc version info as below:

gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include/c++/4.2.1
Apple LLVM version 6.0 (clang-600.0.51) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin13.4.0
Thread model: posix

Answer 1

Two way to compile:

a. Compile one-by-one

g++ -c add.cpp     -o add.o
g++ -c triple.cpp  -o triple.o
g++ add.o triple.o -o triple

b. Compile everything at once

g++ add.cpp triple.cpp -o triple