What are #ifndef and #define Directives?

In C and C++ programming, #ifndef and #define are preprocessor directives used for conditional compilation. They are typically used together with #endif to include or exclude a block of code from compilation based on whether a certain condition is met.

The #ifndef directive checks if the given token has been #defined earlier in the C code. If the token has not been defined earlier then it includes the code between #ifndef and #else. If no #else is present then code between #ifndef and #endif is included.

The both #ifndef and #define directives are known as header guards or #include guard in C/C++. They are used to prevent multiple declarations of variables and other data types and header files from being included multiple times. The #ifndef also prevents recursive inclusions of header files. Another non-standard preprocessor directive named #pragma once is widely used to force the inclusion of a header file only once.

The syntax of using #ifndef is as below:

#ifndef MY_HEADER_FILE
#define MY_HEADER_FILE

// Your code here

#endif

In this example, the code between #ifndef MY_HEADER_FILE and #endif will be included only if the identifier MY_HEADER_FILE has not been defined before in the code.

The #ifndef Directive:

• This directive checks if a particular identifier (usually a macro or a symbol) has not been defined previously in the code.
• If the identifier has not been defined, the code following #ifndef will be included in the compilation; otherwise, it will be excluded.

The #define Directive:

• This directive is used to define a macro or a symbol in the code.
• In the context of conditional compilation, it is commonly used with #ifndef to create include guards to prevent header files from being included multiple times.

Use of #ifndef and #define

Here is a rather complete C program to demonstrate the use of #ifndef and #define.

#include <stdio.h>

#ifndef INTEREST_RATE
#define INTEREST_RATE 8
#endif

int main()
{
   printf("Current Interest Rate is %d percent.\n", INTEREST_RATE );

   return 0;
}

The output of the above C program is:

Current Interest Rate is 8 percent.

C program to show the use of #ifndef and #define

Here is a another C program to demonstrate the use of #ifndef and #define. As INTEREST_RATE is already defined so #ifndef does not execute.

#include <stdio.h>

#define INTEREST_RATE 9
#ifndef INTEREST_RATE
#define INTEREST_RATE 8
#endif

int main()
{
   printf("Current Interest Rate is %d percent.\n", INTEREST_RATE );

   return 0;
}

The output of the above program is:

Current Interest Rate is 9 percent.

The #ifndef and #define are commonly used in C and C++ to ensure that header files are included only once in a translation unit.