1.3: Header Files

A Bit More About Header Files

Just wanted to provide a bit more clarification about header files - to show what actually happens with the preprocessor.

Let's say we have some code for our big project, and we have some customized functions that we declare in the file foo-1.h. In the code example below we call the function testFoo(), which is is defined in the file foo-1.

// file: main.cpp 
#include <iostream> 
#include "foo-1"
int main() { 
  int myFoo;
  
  myFoo = testFoo();
  cout << "Returned from testFoo: " << myFoo << endl;
}

So, the foo-1 file looks like and simply returns the value defined by our macro which is 99. 

#define FOOVAL 99

int testFoo() { 
  return FOOVAL;
}

The main.cpp outputs:

Returned from testFoo: 99

The preprocessor then actually passes the following code to the compiler to convert into an executable program.

// file: main.cpp 
// The iostream file code has been ommitted in this example but it would be here.

#define FOOVAL 99

int testFoo() {
  return FOOVAL;
}


int main() { 
  int myFoo;
  
  myFoo = testFoo();
  cout << "Returned from testFoo: " << myFoo << endl;
}

This is a simple example and there is a bit more detail that is left out of this example, but hopefully this calrifies what the preprocessor actually does with header files.