how to link header files in c++

Question

I'm new to programming in C++ with header files. This is my current code:

//a.h
#ifndef a_H
#define a_H
namespace hello
{
  class A
  {
    int a;
    public:
      void setA(int x);
      int getA();
  };
} 
#endif

//a.cpp
#include "a.h"
namespace hello
{
   A::setA(int x)
  {
    a=x;
  }
  int A::getA()
  {
    return a;
  }
}

//ex2.cpp
#include "a.h"
#include<iostream>
using namespace std;

namespace hello
{
  A* a1;
}
using namespace hello;
int main()
{
  a1=new A();
  a1->setA(10);
  cout<<a1->getA();
  return 1;  
}

When I try to compile it with g++ ex2.cpp, I get this error:

In function `main':
ex2.cpp:(.text+0x33): undefined reference to `hello::A::setA(int)'
ex2.cpp:(.text+0x40): undefined reference to `hello::A::getA()'
collect2: ld returned 1 exit status

Why isn't it working, and how can I fix it?

Answer 1

You don't link header files. You link object files, which are created by compiling .cpp files. You need to compile all your source files and pass the resulting object files to the linker.

From the error message it seems you're using GCC. If so, I think you can do
g++ ex2.cpp a.cpp
to have it compile both .cpp files and invoke the linker with the resulting object files.

Answer 2

You need to compile and link both source files, e.g.:

g++ ex2.cpp a.cpp -o my_program

Answer 3

You need to compile and then link both source (.cpp) files:

g++ -Wall -pedantic -g -o your_exe a.cpp ex2.cpp