Multifile Programs

Separating interface from implementation

The #include directive

• In C++, it is usual for the code that makes up a program to be kept in multiple files.

• The compiler and/or the linker must be told about the files to be included in your program.

• The #include preprocessor directive acts like the paste function in a word processor, causing the text of one file to be inserted in another.

• The files to be included are called header files.

– They can be library files that come with the compiler.

– They can also be user written files.

Header files

• A header file contains basic information about a a file you want to include

– It is in source code and will be compiled with your source code

• The file it specifies to include is in object code (machine language) and is linked when your program is built.

• This means the included file can be compiled at any time (often before your own program)

• You can write your own header files directing the compiler to include code that has been separately compiled.

Projects (building and making)

• A project contains all the files necessary for an application to execute.

• Some compilers automatically recompile all the necessary files before they are linked together; this is called the build process.

• Other compiler require that you create a make file, which will compile only those source files that have changed since the last build.

Header file for class

• #ifndef TIME1_H // filename is time1.h
#define TIME1_H

• class Time {
public:
Time( ); // constructor
void setTime( int, int, int );
void printMilitary( ); // print military time
void printStandard( ); // print standard time
private:
int hour; // 0 – 23
int minute; // 0 – 59
int second; // 0 – 59
};
#endif

Definitions for class

• #include <iostream>
#include "time1.h " // filename is time1.cpp
using namespace std;
Time::Time( ) { hour = minute = second = 0; }

• void Time::setTime( int h, int m, int s )
{ implementation }

• void Time::printMilitary()
{ implementation }

• void Time::printStandard()
{ implementation }

Client of class Time

• #include <iostream>
#include "time1.h "
using namespace std;
// Driver to test simple class Time
int main( )
{ Time myTime; // instantiate object t of class time
cout << "The initial military time is ";
myTime.printMilitary( );
cout << "\nThe initial standard time is ";
myTime.printStandard( );

Creating the make file

• The purpose of the make file is to assure that all object files contain the most recent compilation of the source code.

– It keeps track of modifications to source code, and recompiles all necessary source code files.

– It avoids recompiling source code that does not need to be recompiled.

• Usually one copies a preexisting Makefile and then modifies it.

• We will do this in class, starting with the Makefile on the handout

The Makefile for the three files above is here.

A more complex Makefile, the one in the handout is here.