Include a header file multiple times in a source code file, What will be?

If you include a header file multiple times in a source code file (such as a C or C++ file), the compiler will process the contents of the header file each time it encounters the #include directive for that file. Here's what happens when you include a header file multiple times:

1. Inclusion Guards: Most header files include what are called "inclusion guards" to prevent multiple inclusions. These guards typically use preprocessor directives like #ifndef, #define, and #endif to ensure that the header file is only processed once, even if it's included multiple times.

For example:

// example.h
#ifndef EXAMPLE_H
#define EXAMPLE_H

// Header contents go here

#endif        

When the header file is first included, EXAMPLE_H is not defined, so the contents of the header file are processed, and EXAMPLE_H is defined. On subsequent inclusions of the same header file, the preprocessor checks whether EXAMPLE_H is already defined and skips the contents if it is.

2. Multiple Inclusions without Guards: If a header file doesn't use inclusion guards and is included multiple times in a source file, you may encounter errors and unexpected behavior. The compiler will process the same declarations, definitions, and code multiple times, which can lead to redefinition errors, duplicate symbol errors, or other issues. This can happen when you include a header file directly and indirectly through other included files.

To avoid problems when including header files, it's a good practice to:

• Use inclusion guards in your header files to prevent multiple inclusions.
• Include necessary header files only once in your source files.
• Ensure that your header files are self-contained, meaning they include all the necessary dependencies, so you don't rely on the order of inclusion.
• Use forward declarations when possible to minimize the need for including headers in other headers.

By following these practices, you can help ensure that your code compiles correctly and efficiently even when header files are included multiple times.