A basic C++ file with a header

What follows is a basic C++ file with a Header file

 -- taken from https://www.learncpp.com/cpp-tutorial/header-files/ --




There are three files;

1. main.cpp - the main file.
2. add.cpp - the function that we want to include or import.
3. add.h - The header for for add.cpp

main.cpp

#include <iostream>

#include "add.h"

int main()

{

std::cout<< "The sum of 3 and 4 is " << add(3,4) <<std::endl;

return 0;

}

add.h

int  add (int x, int y);

add.cpp

int add(int x, int y)

{

return x + y;

}

---

Now compiling the above with g++ main.cpp produces the following error;

Undefined symbols for architecture x86_64:

  "add(int, int)", referenced from:

      _main in main-4335f8.o

ld: symbol(s) not found for architecture x86_64

clang: error: linker command failed with exit code 1 (use -v to see invocation)

But if we do g++ *.cpp OR g++ main.cpp add.cpp it compiles just fine and gives us;

a.out

which of course we can run with ./a.out (we may need to make it executable first of course with chmod +x a.out)

Note: we can of course do g++ *.cpp -o output.o which will produce a binary output.o

---

What if we were to have our Header(s) in another directory?

The common sense thing would be to have out import/include looking like this:

#include "myLibs/add.h" OR #include "../add.h" or similar, while this will work, it is considered bad practice and we should do as below;


g++ myLibs/add.cpp main.cpp -I myLibs

Which is basically compiling myLibs/add.cpp and then main.cpp

Finally we use the -I option to tell the compiler that there is another directory, in our case myLibs and I believe this is so that when main.cpp has an include file, the compiler knows where to look to find said included file.