In C++, `#ifndef`, `#define`, and `#endif` (5) These are preprocessor directives used to prevent multiple inclusions of the same header file, which is often referred to as "include guards." 1. #ifndef: This directive checks if a particular identifier (usually a macro) has not been defined yet. 2. #define: If the identifier has not been defined, this directive defines it. 3. #endif: This directive marks the end of the conditional compilation block started by #ifndef. Here’s an example: ```cpp #ifndef MYHEADER_H // If MYHEADER_H is not defined #define MYHEADER_H // Define MYHEADER_H // Code or declarations go here #endif // End of the conditional compilation block ``` The number "(5)" in your question might refer to a specific context, such as an example number or section, but without additional details, it can be interpreted as part of the labeling or enumeration of examples or sections related to these directives.

by stilling2006 on 2009-09-22 17:32:44

Although I have always known the functions of these three things:

The **#ifndef** in the header file is a very important mechanism. For example, if you have two C files and both of them include the same header file. During compilation, these two C files need to be compiled together into one executable file, and thus problems arise—numerous declaration conflicts.

So it's better to place all the content of the header file between **#ifndef** and **#endif**. Regardless of whether your header file will be referenced by multiple files, you should always add this. The general format is as follows:

```c

#ifndef

#define

...... (code of the header file)

#endif

```

However, what I've never understood is: how should this **** be determined?!

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