Preprocessors in C++

Preprocessors in C++

The C++ preprocessor is a program that processes the source code before compilation. It handles macro substitution, file inclusion, and conditional compilation. Preprocessor commands are called directives and always begin with #.

1. #include
Used to include header files.

Types:

• System header files

            #include <iostream>

• User-defined header files

            #include "myfile.h"

2. #define
Used to define macros (constants or functions).

(a) Object-like macro
#define PI 3.14

(b) Function-like macro
#define SQUARE(x) ((x)*(x))

3. #undef
Used to undefine a macro.
#define MAX 100
#undef MAX

4. Conditional Compilation Directives
Used to compile code only if certain conditions are met.

#if, #elif, #else, #endif

• #define A 10

#if A > 5
    cout << "A is greater than 5";
#else
    cout << "A is small";
#endif

• #ifdef

#ifdef DEBUG
    cout << "Debug mode";
#endif

• #ifndef

#ifndef PI
    #define PI 3.14
#endif

Example Program Using Preprocessor
#include <iostream>
#define MAX 50
using namespace std;
int main()

{
    #if MAX > 40
        cout << "MAX is greater than 40";
    #else
        cout << "MAX is small";
    #endif
    return 0;
}

Example Program Using Header Files

#include <iostream>

#include <cmath>

using namespace std;

int main() 

{

    cout << sqrt(16);

    return 0;

}

Creating a User-Defined Header File

Step 1: Create myheader.h

int add(int a, int b);

Step 2: Include in main program

#include <iostream>

#include "myheader.h"

using namespace std;

int main() 

{

    cout << add(5, 3);

    return 0;

}

Include Guards

Prevent multiple inclusion of the same header file.

#ifndef MYHEADER_H

#define MYHEADER_H

int add(int a, int b);

#endif

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