Header File

Preprocessor

The C Preprocessor is is just a text substitution tool to allow extra pre-processing before the actual compilation.

#ifndef MOTION2D_H
#define MOTION2D_H
 
...
#endif 

• The above are Preprocessor directives - commands that allow transformations of the source code before it is compiled.

For C++

C++ has many tools such as inline functions that may be better suited compared to a preprocessor definition such as a macro.

Example

Seen in Lecture 7 of CS247. Consider an example: Linear Algebra modules.

// vec.h
struct Vec {
	int x,y;
};

// Matrix.h
#include "Vec.h"
struct Matrix {
	Vec e1;
	Vec e2;
};

// main.cc
#include "vec.h"
#include "Matrix.h"
 
int main() {
	Vec v1{1,2};
	vec v2{3,4};
	Matrix m{v1, v2};
}
 

main.cc includes vec.h main.cc includes matrix.h, includes vec.h

• Without the #ifndef guard, there are two definitions of struct vec in main.cc, which is not allowed

To fix this issue of multiple definitions, use “include guards”:

#ifndef 
#define
...
#endif

• #ifndef VARIABLE: The code between #ifndef and #endif will only be seen by the compiler if VARIABLE is not defined.
• #define VARIABLE defines a preprocessor variable.

Include Guard

#ifndef FILE_H
#define FILE_H
...
#endif

Works because once File.h is included once, the variable becomes defined, so in all future includes the block is omitted.

This doesn't fix all issues

There is an issue with circular dependencies. Consider the following example:

class A {
	B* myB;
}
class B {
	A* myA;
}

The issue is that each class needs to know the other exists before it can be created.

Solution: Break circular dependencies chain using Forward Declaration.

Preprocessor Directives

All preprocessor commands begin with a hash symbol (#).

Directive	Description
#define	Substitutes a preprocessor macro.
#include	Inserts a particular header from another file.
#undef	Undefines a preprocessor macro.
ifdef	Returns true if this macro is defined.
#ifndef	Returns true if this macro is not defined.
#if	Tests if a compile time condition is true.
#else	The alternative for #if.
#elif	#else and #if in one statement.
#endif	Ends preprocessor conditional.
#error	Prints error message on stderr.
#pragma	Issues special commands to the compiler, using a standardized method.