11.10. Header files

It might seem like a nuisance to declare functions inside the structure definition and then define the functions later. Any time you change the interface to a function, you have to change it in two places, even if it is a small change like declaring one of the parameters const.

There is a reason for the hassle, though, which is that it is now possible to separate the structure definition and the functions into two files: the header file, which contains the structure definition, and the implementation file, which contains the functions.

Header files usually have the same name as the implementation file, but with the suffix .h instead of .cpp. For the example we have been looking at, the header file is called Time.h, and it contains the following:

struct Time {
  // instance variables
  int hour, minute;
  double second;

  // constructors
  Time (int hour, int min, double secs);
  Time (double secs);

  // modifiers
  void increment (double secs);

  // functions
  void print () const;
  bool after (const Time& time2) const;
  Time add (const Time& t2) const;
  double convertToSeconds () const;

Notice that in the structure definition I don’t include the prefix Time:: at the beginning of every function name. The compiler knows that we are declaring functions that are members of the Time structure.

Time.cpp contains the definitions of the member functions (I have elided the function bodies to save space):

#include <iostream>
using namespace std;
#include "Time.h"

Time::Time (int h, int m, double s)  ...

Time::Time (double secs) ...

void Time::increment (double secs) ...

void Time::print () const ...

bool Time::after (const Time& time2) const ...

Time Time::add (const Time& t2) const ...

double Time::convertToSeconds () const ...

In this case the definitions in Time.cpp appear in the same order as the declarations in Time.h, although it is not necessary.

On the other hand, it is necessary to include the header file using an include statement. That way, while the compiler is reading the function definitions, it knows enough about the structure to check the code and catch errors.


Notice that outside of the structure definition, you do need to include the prefix Time:: at the beginning of each function name!

Finally, main.cpp contains the function main along with any functions we want that are not members of the Time structure (in this case there are none):

#include <iostream>
using namespace std;
#include "Time.h"

int main () {
  Time currentTime (9, 14, 30.0);
  currentTime.increment (500.0);
  currentTime.print ();

  Time breadTime (3, 35, 0.0);
  Time doneTime = currentTime.add (breadTime);
  doneTime.print ();

  if (doneTime.after (currentTime)) {
    cout << "The bread will be done after it starts." << endl;
  return 0;

Again, main.cpp has to include the header file.

It may not be obvious why it is useful to break such a small program into three pieces. In fact, most of the advantages come when we are working with larger programs:


Once you have written a structure like Time, you might find it useful in more than one program. By separating the definition of Time from main.cpp, you make is easy to include the Time structure in another program.

Managing interactions:

As systems become large, the number of interactions between components grows and quickly becomes unmanageable. It is often useful to minimize these interactions by separating modules like Time.cpp from the programs that use them.

Separate compilation:

Separate files can be compiled separately and then linked into a single program later. The details of how to do this depend on your programming environment. As the program gets large, separate compilation can save a lot of time, since you usually need to compile only a few files at a time.

For small programs like the ones in this book, there is no great advantage to splitting up programs. But it is good for you to know about this feature, especially since it explains one of the statements that appeared in the first program we wrote:

#include <iostream>
using namespace std;

iostream is the header file that contains declarations for cin and cout and the functions that operate on them. When you compile your program, you need the information in that header file.

The implementations of those functions are stored in a library, sometimes called the “Standard Library” that gets linked to your program automatically. The nice thing is that you don’t have to recompile the library every time you compile a program. For the most part the library doesn’t change, so there is no reason to recompile it.

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