This document discusses C++ header files. It contains the following key points: - Header files are used to build libraries of classes and functions that can be reused across multiple programs. They are similar to predefined libraries. - There are three main rules for header file inclusion: 1) Be tolerant of duplicate inclusions, 2) Use forward declarations when possible instead of includes, 3) Header inclusion order is not important. - Forward declarations can be used when a class only uses references or pointers to another class, but includes are needed when a class derives from or uses objects of another class.

1. COMP 2710 Software Construction
Header Files
Dr. Xiao Qin
Auburn University
http://www.eng.auburn.edu/~xqin
xqin@auburn.edu

2. C++ Header Files
• Build libraries of classes or functions
• Re-used by many different programs
• Just like predefined libraries
1-2

3. Build Libraries of Classes or
Functions
1-3

4. Header File Inclusion Rules
• The inclusion mechanism should be tolerant to
duplicate header file inclusions.
• A header file should be included only when a
forward declaration would not do the job.
• The order of header file inclusion is not
important.
1-4

5. Rule 1: Be tolerant to duplicate
header file inclusions.
/*
* Header file name: estimate_function.h
*/
#ifndef ESTIMATE_FUNCTION_H
#define ESTIMATE_FUNCTION_H
#include <iostream>
using namespace std;
double estimateOfTotal(int minPeas, int maxPeas,
int podCount);
#endif
1-5

6. Rule 2: Use forward declaration
when possible.
Suppose we want to define a new class B that uses objects
of class A.
class B {
private:
A* fPtrA ;
public:
void myMethod(const& A) const ;
};
1-6
#include <A.h>;
B only uses references or pointers to A. Use forward
declaration then: you don't need to include <A.h>. This
will in turn speed a little bit the compilation.
class A;

7. When we need to include <A.h>?
• B derives from A or B explicitely (or implicitely) uses
objects of class A. You then need to include <A.h>
class B : public A {
} ;
class C {
private:
A fA ;
public:
void myMethod(A par) ;
};
1-7
#include <A.h>;

8. Exercise 1: Header Files
• Assume a class A with code stored in a.cpp and a.h.
#ifndef A_H
#define A_H
#include "abase.h"
#include "b.h"
// Forward Declarations/definition
class C;
class D;
class A : public ABase {
B m_b;
C *m_c;
D *m_d;
public:
void SetC(C *c);
C * GetC() const;
void ModifyD(D *d);
};
#endif 1-8